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Russian Billionaire Continues the Search for Extraterrestrial Intelligence

Russian Billionaire Continues the Search for Extraterrestrial Intelligence


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It hasn’t been widely publicized, but the most extensive search for extraterrestrial intelligence ever undertaken has been up and running for more than four years.

Breakthrough Listen is a 10-year, $100 million space scanning project sponsored the Berkeley (California) SETI Research Center and funded by Russian billionaire Yuri Milner. Breakthrough Listen is the latest manifestation of the SETI (search for extraterrestrial intelligence) program, which has passed through multiple phases since its inception in the late 1970s. This privatized version of SETI will eventually aim its detecting devices at more than one million nearby star systems and 100 neighboring galaxies, searching for optical and radio signals that might indicate the presence of technologically advanced life.

Current sky monitoring operations are being carried out using two of the world’s most powerful telescopes: the 100-metre Robert C. Byrd Green Bank Telescope in Green Bank, West Virginia (USA) and the 64-metre Parkes Observatory Telescope in Parkes, New South Wales (Australia). Eventually, the new MeerKAT observatory telescope array in South Africa will join the project as well, and its 864 metres of cumulative magnifying capacity will dramatically expand Breakthrough Listen’s star scanning abilities.

So far, scientists associated with this new SETI iteration have completed analysis on data collected from 1,327 stars, all located within 160 light years of Earth. As of now, they’ve come up completely empty.

Not a single indicator of life has been found. No radio signals, laser traces, or anomalous activity covering any part of the electromagnetic spectrum.

“There’s certainly nothing out there glaringly obvious,” says Danny Price, an astrophysicist affiliated with the Berkeley SETI Research Center. Price was the lead author of an article printed in the June 17, 2019 edition of The Astrophysical Journal , which revealed the unremarkable results. “There’s no amazingly advanced civilizations trying to contact us with incredibly powerful transmitters.”

Overall, researchers analysed more than one petabyte, or one million gigabytes, of data collected in radio and optical wavelengths. A few possible hits were detected, but further checking revealed all to be false alarms.

While SETI searchers were undoubtedly disappointed to hear this news, its unlikely they were surprised. At different times SETI projects have been funded by governments, educational institutions, nonprofit foundations and rich individual investors. In each of these instances, no meaningful results have been obtained.

It’s reasonable to ask why this is so. Obviously, if humans are the only intelligent life form in the universe that would explain it. But that answer seems unlikely.

The universe is simply too vast to believe that we’re alone. Billions of Earth-like planets exist in other solar systems, and if evolution has occurred here there’s no reason to think it hasn’t happened elsewhere. So the lack of evidence for alien life calls out for a different explanation.

Some of the possibilities that have been raised include:

Interstellar interference

Radio or optical signals traveling astronomical distances might be reflected, deflected, disrupted or otherwise weakened or redirected by objects, forces, space distortions or other interstellar phenomenon we don’t yet understand.

Small sample size

SETI has been active for decades. But the universe has been active for billions of years, and its size after post-Big Bang expansion is beyond our capacity to comprehend.

American astronomer Jill Tarter, who has been involved in the SETI project since the beginning, puts the challenge into perspective .

“If you build a mathematical model, the amount of searching that we’ve done in 50 years is equivalent to scooping one eight-ounce glass out of the ocean, and then looking to see if you caught a fish … We just haven’t searched very well yet.”

Land-based intelligent life is rare

In the best of circumstances, looking for life using the methods of SETI would be like searching for a needle in a haystack. But if, say, only one percent of planets capable of producing life actually do so, and if only a fraction of those planets develop advanced intelligence, the odds of SETI succeeding might be vanishingly small.

And what if most of the intelligent life in the universe is water-based? Limited by environmental circumstances, intelligent oceanic life forms wouldn’t be sending out or receiving radio signals.

Star Trek fans will certainly remember the plot of “Star Trek IV: The Voyage Home,” when the safety of Earth was inadvertently threatened by an alien probe seeking contact with humpback whales. It apparently never occurred to these aliens that intelligent beings might be based on land rather than water.

Other advanced civilizations may have moved beyond radio technology

Radio is a 20 th century technology on the verge of being phased out in favor of digital communications. If the use of radio represents a brief interlude for advanced civilizations, it might not occur to alien civilizations to advertise their presence using radio frequencies. SETI was itself conceived in the 20 th century, and its radio-centered approach may be a product of its times.

“In a lot of ways, SETI is a bit of a mirror back on ourselves and our own technology and our understanding of physics,” Dennis Price admits .

The development of human society is out of sync with the rest of the universe

Even if we aren’t unusual, perhaps the pace of our technological development is.

Perhaps our civilization rose to prominence either long before or long after other advanced civilizations developed (or will develop) in the Milky Way. If either is true, we may be looking for signals that were sent out eons ago and have long since faded; or conversely, we may be looking for signals that won’t be sent until a few thousand years in the future.

Fear of the unknown

A society that beams optical or radio signals into space would be announcing its existence and location to any and all parties intercepting the message.

In a benign universe, this might be perfectly okay. But in a universe filled with violent predators, sending out such signals could be an open invitation to invasion and enslavement. Other advanced civilizations might not be willing to take that risk, and that may be why the universe is currently observing radio silence.

Government cover-ups

If SETI ever did find confirmation of intelligent life, how can we be sure they’d tell us?

In 1960, a U.S.-based think tank called the Brookings Institute issued a report that discussed the impact alien visitation or detection might have on human society.

“Anthropological files contain many examples of societies, sure of their place in the universe, which have disintegrated when they have had to associate with previously unfamiliar societies espousing different ideas and different ways of life,” the report cautioned. “Others that survived such an experience usually did so by paying the price of changes in values and attitudes and behaviour.”

For years, rumors about SETI cover-ups have run rampant in conspiracy and UFOlogical circles. Supposedly, SETI has found proof of alien life numerous times, but has not been allowed to reveal the truth to the public. Shadowy government agencies are usually responsible for issuing these gag orders, according to conspiracy theorists, who often attribute their claims to mysterious inside sources.

SERENDIP and the Wow! Signal

For a SETI-detected signal to be accepted as legitimate, it must be both anomalous and repeatable. No signal that meets the second requirement has ever been found. However, something extremely mysterious and anomalous was discovered on one well-documented occasion.

The Berkeley SETI Research Center, which is sponsored by the Astronomy Department at the University of California-Berkeley, is involved with another SETI initiative that traces its origins back to the 1970s. This program is called SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations), and it is run by researchers who analyse radio data collected by astronomers at telescopes all across the United States.

SERENDIP continues to this day, and while it has not found any definitive evidence of anything it did have one apparently remarkable “hit” back in 1977. On August 15, a 72-second radio emission with highly unusual characteristics was picked up by astronomers working at Ohio State University in Columbus, Ohio. This so-called “Wow!” signal emerged from the constellation Sagittarius, and its content carried several markers that seemed to suggest intelligent design.

The Wow! signal was unexplained at the time and remains so today. However, all attempts to find it again have failed, and that lack of repeatability has kept it from being accepted as definitive proof of alien contact.

Alternative explanations have been found wanting as well, and that makes the Wow! Signal a riddle that defies logical explanation—which makes it a good metaphor for the SETI quest as a whole.

There’s good reason to believe intelligent life is out there, most likely in abundance. But our long search for conclusive evidence of its existence has proven futile. Why it should be that way is the riddle, and until ET lands on the White House lawn or on the grounds of Buckingham Palace all we can do is wait and wonder.


Building a Language to Communicate With Extraterrestrials

If SETI researchers ever do find intelligent life on other planets, how will they know what to say?

Last July, the Russian billionaire Yuri Milner launched a campaign called the Breakthrough Initiatives, a $100-million dollar donation to be doled out to scientists working on the Search for Extra Terrestrial Intelligence (SETI). This donation didn’t just provide some much-needed legitimacy to a field that has, since its inception, often struggled to be taken seriously. It also helped to alleviate one of SETI’s largest problems, second only to actually locating aliens: finding the funding to continue the search. Scanning the cosmos for intelligent life isn’t cheap, and there’s a lot of work to be done to prepare for first contact—which some scientists, like Seth Shostak, the director of the non-profit SETI Institute in California, expect to happen within our lifetime.

Milner’s donation will go a long way in footing the bill as SETI scientists work to make this contact a reality, yet there are some SETI problems money alone can’t solve. In this sense, perhaps the most pressing issue faced by SETI researchers is figuring out not only what we would say when ET calls, but more fundamentally, how we would go about saying it.

Astronomers and SETI scientists have been mulling over this particular communication problem for a while and have come up with a few ideas, some more outlandish than others. One of the earliest solutions was suggested in the early 19th century by the Austrian astronomer Joseph Johann Von Littrow, who proposed digging massive trenches in the Sahara desert, filling them with water, pouring kerosene on top, and then setting the kerosene alight in order to send flaming messages to our planetary neighbors.

Vittrow’s scheme never came to fruition, but for about 150 years it was the best idea around. It wasn’t until 1960 that another proposal for ET communication came along, one that continues to shape the exolinguistic field that it founded. The proposal took the form of a book titled Lincos: Design of a Language for Cosmic Intercourse, and its publication marked the first artificial language constructed for the sole purpose of communicating with extraterrestrial life.

Lincos, a portmanteau of lingua cosmica, was the brainchild of the German mathematician Hans Freudenthal. Born in 1905, Freudenthal began his academic career as a lecturer at the University of Amsterdam in 1930, a position he held until the Nazi invasion of the Netherlands in 1940. After losing his job, Freudenthal kept a low profile for several years of the occupation. He was sent to the labor camp Havelte in 1944, but managed to escape a few months later to Amsterdam, where he would watch the Allied liberation in May of 1945. Shortly thereafter, Freudenthal was offered a full professorship in geometry at the University of Utrecht.

A fierce proponent of educational reform in the post-war years, Freudenthal vehemently opposed the introduction of “New Math,” a rigorously logic-based teaching methodology, into the Dutch curriculum. Freudenthal favored pedagogical methods that connected mathematic principles with daily life, something at which New Math failed dismally. It was the combination of these two interests—logic and applied math—which would ultimately provide Freudenthal with the intellectual fodder necessary to create Lincos, a language that used math to communicate both universal truths and the particulars of daily life on Earth.

Despite its status as a milestone for the SETI community, Lincos never really garnered much attention from the general public. This is probably because the text is mostly comprised of dense, technical jargon and mathematical formulas that are all but unintelligible to the lay reader.

“It’s a groundbreaking work,” said Yvan Dutil, an astrophysicist with the University of Québec’s Télé-université, “but Freudenthal’s book is the most boring I have ever read. Logarithm tables are cool compared to it.”

In the introduction to Lincos, Freudenthal announced that his primary purpose “is to design a language that can be understood by a person not acquainted with any of our natural languages, or even their syntactic structures … The messages communicated by means of this language [containing] not only mathematics, but in principle the whole bulk of our knowledge.”

To this end, Freudenthal developed Lincos as a spoken language, rather than a written one—it’s made up of phonemes, not letters, and governed by phonetics, not spelling. The speech is itself made up of unmodulated radio waves of varying length and duration, encoded with a hodgepodge of symbols borrowed from mathematics, science, symbolic logic, and Latin. In their various combinations, these waves can be used to communicate anything from basic mathematical equations to explanations for abstract concepts like death and love.

The very first message sent in Lincos, Freudenthal wrote, should contain numerals that introduce the receiver to mathematics. This would consist of short, regular pulses or “peeps,” the number of pulses corresponding to a particular numeralone peep for 1, two peeps for 2, and so on. The next step, he wrote, would be to transmit basic formulas, using symbols such as =, +, or > to demonstrate properties of human notation and mathematical knowledge (for example: . . . . . > . . . . to show that 5 is greater than 4). Each successive message would increase in complexity, moving from numerals and basic formulas to complex subjects like human behavior.

Despite its rigorous methodology and logical coherence, one of the primary criticisms of Lincos was that Freudenthal had failed to consider that extraterrestrials might not think like us at all, in which case our logic would be lost on them. Freudenthal acknowledges this limitation, writing that he must “suppose that the person who is to receive my messages is human or at least humanlike as to his mental state…[because] I should not know how to communicate with an individual who does not fulfill these requirements.”

According to Dutil, this also seems to be the general consensus of other SETI scientists. If there is intelligent life in the universe, some have argued, odds are it will in fact think like us—or will at least be familiar with our mathematics. According to this train of thought, if the civilization is capable of building a receiver for our message, this implies that they are capable of understanding the mathematics and science necessary to build such a machine. Another argument in favor of like-minded aliens was put forth by the renowned cognitive scientist Marvin Minsky, who noted in an essay dedicated to Freudenthal’s memory that aliens are subject to the “same ultimate constraints – limitations on space, time, and materials.”

On October 13, 1990, Freudenthal was found by school children on a bench in Utrecht, having succumbed to humanity’s time-constraint while taking his daily walk. He had died without finishing his lingua cosmica—the 1960 Lincos text was meant to be the first of two volumes, the latter to include means for sending messages on the subjects of “Matter,” “Life,” and “Earth.”

There’s no evidence that Freudenthal ever began work on this second volume, but the fact that Lincos is technically an uncompleted work has failed to temper its enduring legacy within the SETI community. In 1999, his mathematical language was given new life by Dutil and his colleague Stéphane Dumas when the two astrophysicists used Lincos as a starting point for a series of messages they sent out into the cosmos from a radio telescope in Ukraine. Known as the Evaptoria Messages, these transmissions were the third to ever be sent out to potential alien civilizations, and the first to draw from Freudenthal’s Lincos protocols.

“Lincos was the starting point of our language and to my knowledge, there are not many messages that have been sent using that approach,” said Dumas. ”Most messages sent to the stars are series of pictures describing simple ideas, similar to the Arecibo message in 1974.”

Unlike the Arecibo message, which was the first message intentionally sent into the cosmos for the purpose of communication, Dutil and Dumas’ goal was to send an encyclopedic message, one containing as much information about life on Earth as possible. The duo designed a simple character alphabet that would allow for the maximum amount of information to be transmitted during their limited use of the Ukrainian radar transmitter. In keeping with Freudenthal’s dictum that interstellar messages should begin with topics that would conceivably be known to any intelligence in the universe, the 1999 message began by teaching the recipient how to count, before moving on to more complex subjects such as physics and biology. The final message consisted of 23 pages (one page was 127x127 pixels) and was sent to its targets three times each to ensure sufficient redundancy.

Importantly, the message also included a formal request for a reply.

The targets for the message these messages were selected from SETI’s long list of potential solar systems that might harbor intelligent life. The candidates, all somewhere between 50 and 70 light-years away, were chosen based on a number of criteria, such as the age of the star and its position in the galaxy. The first message can be expected to arrive at its cosmic address (Hip4872 in Cassiopeia) sometime around 2035.

Page 1 of the 1999 Evaptoria Message (Yvan Dutil and Stéphane Dumas)

In more recent years, the SETI community has refined Lincos to develop more advanced options for interstellar communication. For example, the language CosmicOS was designed by MIT’s Paul Fitz as a computer program that could be run by extraterrestrials once they had received the message. Then there’s a second-generation lingua cosmica designed by the Dutch mathematician Alexander Ollongren (described by Dutil as “the only human to master Freudenthal”) which largely relies on constructive logic to compose the message.

While a standard language for interstellar communication doesn’t yet exist within the SETI community, CosmicOS and the second generation of Lincos neatly encapsulate the two ends of the spectrum: On one end is CosmicOS, which has the capability to transmit more information, but also has more logistical problems as a result on the other is the new version of Lincos, with a smaller data package and a more limited amount of information that can be sent.

Yet according to Duntil and Dumas, these two disparate approaches to cosmic communications need not be mutually exclusive. In fact, they see them as two potential steps in a much larger program, where interstellar communication begin with a more simple message of greeting in one language and then proceeds to send increasingly complex messages in another. Yet regardless of the package, the duo agrees that any messages sent or received by extraterrestrial civilizations are going to be strongly rooted in mathematics.

“The creation of any language to communicate with an alien civilization is very difficult, because you need something called a metalanguage to make the bridge between the artificial language and the alien's language,” said Dumas. “The metalanguage par excellence is mathematics because it is the base of science, and any civilization that has built a device to listen to radio waves knows science.”

According to Dutil and Dumas, some more recent language designed for interstellar communication have incorporated more artistic elements such as music into the message—but at their core, most are still using Lincos as a starting point, at least in part.

Regardless of whether Lincos is totally abandoned in the future or continues to be a source of inspiration for our cosmic messages, Freudenthal’s tome will always be remembered among SETI scientists as a manual for uniting species light-years apart, species that will in all probability have little but mathematics in common. It was a Dutch mathematician’s attempt to make the alien familiar by rendering the most profound human emotions in mathematical equations.

Indeed, as Dumas was quick to point out, in many ways, Lincos was written more for Earthlings than ET.

“The creation of an interstellar language to communicate with an alien civilization is interesting, but in the end not very practical since the communication would occur over decades,” said Dumas. “The exercise of creating a message is more of a benefit to us than the alien because it shows how we want to portray ourselves. What do we want to give ET — the truth, or a beautiful image? Will the message contain our history of war, famine, and ecological disaster, or only the nice things? Ultimately, an interstellar message is a reflection of humankind to itself.”


[email protected] Is Over. But the Search for Alien Life Continues

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In 1995, the computer scientist David Gedye had an idea that could only originate at a cocktail party. What if the world’s personal computers were linked together on the internet to create a virtual supercomputer that could help with SETI, the search for extraterrestrial intelligence? The network would be able to sort through the massive amounts of data being collected by radio telescopes, seeking signals that might point to an alien civilization around another star. A distributed supercomputer sounded outlandish at the time, but within four years, Gedye and his collaborator, computer scientist David Anderson, had built the software to make it a reality. They called it [email protected]

On Tuesday, researchers at the Berkeley SETI Research Center announced they would stop distributing new data to [email protected] users at the end of March. It marks the culmination of an unprecedented 20-year experiment that engaged millions of people from almost every country on earth. But all experiments must come to an end, and [email protected] is no exception. So far, the researchers at Berkeley have only been able to analyze small portions of the [email protected] data. They had to hit pause on the public-facing part of the experiment to analyze the full two decades of radio astronomy data they’ve collected to see what they might find.

“For 20 years, there’s been this fight between keeping the project running and getting the results out to the scientific community,” says Eric Korpela, the director of [email protected] “At this point, we can’t even be sure that we haven’t found anything because we’ve been doing most of our data analysis on small test databases rather than the whole sky.”

Officially launched at Berkeley on May 17, 1999, the [email protected] initiative helped address one of the biggest challenges in the search for extraterrestrial intelligence: noise. Professional alien hunters are in the business of searching for weak radio signals in a vast sky washed out by interference from satellites, TV stations, and astrophysical phenomena like pulsars. This means they are fundamentally grappling with a big data problem—they’re looking for a single signal sent by ET floating on a vast ocean of radio flotsam.

Filtering through all this data requires computing power—and lots of it. More processors crunching data from outer space means a more sensitive analysis of more signals. By borrowing unused processing power from personal computers around the world, [email protected] could plow through radio telescope data faster than ever before. When a computer was idle, the [email protected] program launched a screensaver that showed a field of colorful spikes that represented signals collected at the Arecibo radio telescope in Puerto Rico as it scanned the cosmos. And for anyone who downloaded the software, it meant that if ET called Earth, it could very well be your own CPU that picked up the phone.

It didn’t take long for the idea to catch on. [email protected] quickly grew into what its collaborator, the nonprofit Planetary Society, has called its “most successful public participation project ever undertaken.” As WIRED reported in 2000, within months of [email protected]’s launch, more than 2.6 million people in 226 countries were volunteering their spare processing power to parse the mounds of data generated by alien-hunting radio telescopes. Together, they ran about 25 trillion calculations per second, which made [email protected] more than twice as powerful as the best supercomputer in the world at that time.

“We didn’t anticipate how fast it would grow,” says Dan Werthimer, who helped create [email protected] and now serves as its chief scientist. “It grew exponentially and I think it’s because people are really excited about the question of whether we’re alone. It’s not very often that people can participate in a science project that has these sorts of profound implications.”

Over the last 20 years, the army of [email protected] screensavers has parsed billions of signals collected at Arecibo and selected those that seemed the most likely to have been generated by an extraterrestrial intelligence. Once the program parsed this data, it was shipped off to Berkeley where the data was further processed to filter out signals from satellites, TV stations, and other sources of interference, to match the data with historical observations, and then to determine if a followup was warranted.

In the early days of the [email protected] program, the internet connection at Arecibo wasn’t fast enough to push out data onto the internet directly, so the [email protected] team had to record the data on 35 gigabyte tapes that were mailed to Berkeley and then uploaded to the internet. Today, the data is piped over the internet to [email protected]’s servers in California, which are equipped with terabytes of storage to handle the data for processing.

When the software stops pushing out new data to users at the end of March, the Berkeley [email protected] team will continue to work through the backlog of data generated by the program over the next few months. The team is small—there are only four full-time employees—and it has struggled to stay on top of managing the public-facing part of the [email protected] program while also publishing research on the data that has been collected. So far, the team has only been able to deeply analyze portions of the dataset. Getting a solid understanding of what it contains will require looking at all the data in aggregate.

[email protected] volunteers only have access to 100 seconds of data from the telescope, so they can’t see this global picture over 20 years,” says Werthimer. “If you see an interesting signal in the sky, it needs to be there when you go back and look again. That’s what we’re going to be looking for.”

Although the public-facing portion of the [email protected] experiment may be coming to a close, Korpela says the project isn’t dead it’s hibernating. After the data analysis is wrapped up, he says, [email protected] could possibly be relaunched using data from other telescopes like the MeerKAT array in South Africa or the FAST telescope in China. Korpela says it would probably take a year or more to stand up a successor to the program’s first iteration, but he hasn’t ruled it out as a possibility.

In the meantime, Breakthrough Listen will be carrying the torch for massive public-facing SETI projects. Founded in 2015 with a $100 million donation from the Russian-born billionaire Yuri Milner, Breakthrough Listen is dedicated to collecting and analyzing massive amounts of radio data to search for signs of extraterrestrial intelligence. Like [email protected], Breakthrough is also being shepherded by the Berkeley SETI Research Center, but its data firehose would overwhelm a distributed computing program like [email protected] to search through it all. Instead, to parse through the data it uses massive banks of GPUs at the Green Bank Telescope in West Virginia running advanced search algorithms.

“Developing these new algorithms and bringing them on site is really the way to crack this problem today,” says Steve Croft, Breakthrough Listen’s project scientist at the Green Bank Telescope. “It’s just not feasible anymore to go over the internet to individual users.”

Each day, the telescopes around the world that contribute to Breakthrough Listen generate more than 100 terabytes of raw data. Even if there were enough people volunteering their computers to analyze it, the internet connections at the telescopes can’t push the data onto the net fast enough. As Croft says, it was time to “bring the computers to the data” and do as much processing of radio signals on site as possible.


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The SETI Institute — an organization devoted to space education and exploration — spent two days pointing the Allen Telescope Array in the direction of the mysterious signal.

The array is located 470 kilometres north of San Francisco and is made up of 42 six-metre wide radio antennae capable of detecting centimetre wavelength signals, just like the ones the Russian scientists located. SETI astronomer Seth Shostak said that the search turned up nothing.

“We spent two nights looking for it…We couldn’t find the signal, and now the Russian Academy of Science says it’s a military satellite,” he said.

It is unclear whether the Director of the Russian Academy of Sciences, Alexander Ipatov actually said that the 2015 signal came from a military satellite. However, he recently told a Russian state news agency that he was once involved with a special Soviet observatory searching for signals from extraterrestrials, and discovered a strange signal that later turned out to be coming from Earth.


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The new search for intelligent life, which promises to cover 10 times more of the sky than previous attempts, is backed by Russian billionaire entrepreneur Yuri Milner, who set up the Breakthrough Prize for scientific endeavours.

WHAT THE STARS WOULD HEAR FROM US ON EARTH

We might be listening to the talents of vocal artists like Taylor Swift, but distant stars are only now just starting to hear jazz from the 1940s.

The Whitburn Project has mapped how far radio waves from Earth have travelled since they started being broadcast more than 100 years ago.

These transmissions have not only beamed around the world but also out into space at the speed of light.

Around five light years from Earth, the red dwarf Proxima Centauri, is just starting to receive hits from 2011, including Lady Gaga's Born This Way.

Nearing 10 light-years away, around the star Ross 154 or V1216 Sagittarii, music from the likes of Justin Timberlake and Fergie would be heard.

Mariah Carey's 1990 hit Vision of Love is likely to be playing as the animation reaches 25 light-years from Earth, near the star Steph 538.

At 31 light-years from Earth, when songs from the mid-1980s including Wham's Wake Me UP Before You Go-Go would only just be arriving at the constellation Ursa Major.

As the animation reaches 75 light-years away in the Comae Berenices constellation, the sounds of Glenn Miller Orchestra and jazz tracks from Tommy Dorsey and his Orchestra ring out from the 1930s and 1940s.

And when it reaches 105 and 110 light-years away, By The Light Of The Silvery Moon by Billy Murray and the Haydn Quartets would be playing.

The attempt to find signs of alien life, which has been named the Breakthrough Listen Initiative, will draw on the expertise of leading scientists, physicists and astronomers.

Professor Stephen Hawking, who has in the past said that there is certainly alien life out there but has warned humanity against trying to contact them, was among those to back the project.

He said: 'We believe that life arose spontaneously on Earth. So in an infinite Universe, there must be other occurrences of life.

'Somewhere in the cosmos, perhaps, intelligent life may be watching these lights of ours, aware of what they mean.

'Or do our lights wander a lifeless cosmos - unseen beacons, announcing that here, on one rock, the Universe discovered its existence.

'Either way, there is no bigger question. It’s time to commit to finding the answer -- to search for life beyond Earth. The Breakthrough Initiatives are making that commitment.

'We are alive. We are intelligent. We must know.'

The new projects were launched at a special event at the Royal Society in London.

The 10-year Listen initiative will use the 100 metre (328 feet) Robert C Byrd Green Bank Telescope in West Virginia and the 64 metre (210 feet) Parkes Telescope in New South Wales, Australia, to search for radio signals.

It will also conduct the world's most extensive search for optical laser transmissions from outer space using the Automated Planet Finder Telescope at Lick Observatory in California, USA.

Professor Lord Martin Rees, Astronomer Royal and a cosmologist at the University of Cambridge, is among those heading up the project.

He will join Dr Frank Drake, the astrophysicist who was one of the founders of the Seti Institute, which searches for extraterrestrial life.

Professor Stephen Hawking, shown above, has previously said he is convinced there is alien life in the universe waiting to be found but has questioned the wisdom of trying to contact them. He has now offered his support to the new Breakthrough Initiative search for extraterrestrial life beyond our own solar system

Dr Drake said the Breakthrough Listen program promised to be 50 times more sensitive than previous attempts to listen for life amoung the stars and will cover 10 times more of the sky.

Dr Drake said: 'Right now there could be messages from the stars flying right through the room, through us all. That still sends a shiver down my spine.

'The search for intelligent life is a great adventure. And Breakthrough Listen is giving it a huge lift.'

'We've learned a lot in the last fifty years about how to look for signals from space. With the Breakthrough Initiatives, the learning curve is likely to bend upward significantly.'

The 100 metre Green Bank radio telescope in West Virginia (above) will help the scientists make the most extensive serach of the universe for signs of intelligent life yet conducted. They hope to examine one million of the closest stars, 100 galaxies and peer into the middle of our own galaxy The Milky Way

The Breakthrough Listen and Message Initiative was launched at the Royal Society by Yuri Milner (far left) and a panel of leading scientists including (L-R) Professor Stephen Hawking, Lord Martin Rees, Seti director Dr Frank Drake, Ann Druyan and Professor Geoff Marcy, an astronomer at University of California

Experts say that if a civilisation was broadcasting signals with the power of common aircraft radar from one of the 1,000 nearest stars to Earth, the telescopes used in the project could detect them.

The closest star to Earth – Proxima Centauri – is 24 trillion miles away and it takes light from it 4.2 years to reach our planet.

Breakthough Listen's optical search could also detect a 100 watt laser from some of the nearest stars.

Lord Rees said: 'The search for extra-terrestrial life is the most exciting quest in 21st-century science.

'The Breakthrough Initiatives aim to put it on the same level as the other ultimate scientific questions.'

Announcing the initiative, Yuri Milner said: 'With Breakthrough Listen, we're committed to bringing the Silicon Valley approach to the search for intelligent life in the Universe.

'Our approach to data will be open and taking advantage of the problem-solving power of social networks.'

The Breakthrough Message initiative will also offer prizes totalling $1 million for digital messages designed to convey information about our planet to alien life forms.

Yuri Milner (pictured above) is a Russian venture capitalist and entrepreneur who established the Breakthrough Prize Awards in 2012. He has now teamed up with some of the world's leading astronomers and physicists to back a new search of the universe for signs of intelligent life on other planets

Ann Druyan, the producer of the science fiction film Contact and a science writer who helped draw up the interplanetary message carried on the Voyager Spacecraft, said: 'The Breakthrough Message competition is designed to spark the imaginations of millions, and to generate conversation about who we really are in the universe and what it is that we wish to share about the nature of being alive on Earth.

'Even if we don't send a single message, the act of conceptualizing one can be transformative.

'In creating the Voyager Interstellar Message, we strived to attain a cosmic perspective on our planet, our species and our time.

'It was intended for two distinct kinds of recipients - the putative extraterrestrials of distant worlds in the remote future and our human contemporaries.

'As we approach the Message's fortieth anniversary, I am deeply grateful for the chance to collaborate on the Breakthrough Message, for what we might discover together and in the hope that it might inform our outlook and even our conduct on this world.'

The new search for alien life will look into the centre of the Milky Way (pictured arcing over Flagstaff in Arizona above) and also towards 100 of the closest galaxies to our own in the hope of detecting alien radio signals

THE FRUITLESS SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE

Scientists have been searching for signs of intelligent life in the cosmos under the Search for Extraterrestrial Intelligence (Seti) programme since the 1960s.

Initially it was conducted on the fringe of radio astronomy, with just short amounts of time obtained on relatively small radio telescopes.

However, in 1984 the Seti Institute was established to provide a coordinated approach to the search, using radio telescopes as permanent ‘ears’ to listen for alien signals.

The project however suffered a set back in 1994 when Nasa funding to Seti was cut and it now seeks support from private sources instead.

The project has yet to detect any positive signs of signals from intelligent life, but some scientists have predicted it could happen within the next 20 years.

However, the project has also been criticised for being overly optimistic despite not receiving any signals in the past 30 years.

Recently scientists proposed taking a more active approach by broadcasting signals to nearby stars in the hope of getting a response.


Why We’ll Have Evidence of Aliens—If They Exist—By 2035

The search for alien technology is about to get much more efficient.

I ’ve bet a cup of coffee to any and all that by 2035 we’ll have evidence of E.T. To many of my colleagues, that sounds like a losing proposition. For more than a half-century, a small coterie of scientists has been pursuing the Search for Extraterrestrial Intelligence, or SETI. And we haven’t found a thing.

I’m optimistic by nature—as a scientist, you have to be. But my hopeful feeling is not wishful thinking it is firmly grounded in the logic of SETI. Half a century sounds like a long time, but the search is truly in its early days. Given the current state of SETI efforts and abilities, I feel that we’re on the cusp of learning something truly revolutionary.

Most of our experiments so far have used large radio antennas in an effort to eavesdrop on radio signals transmitted by other societies, an approach that was dramatized by Jodie Foster in the 1997 movie Contact. Unlike other alien potboilers, Contact’s portrayal of how we might search for extraterrestrials was reasonably accurate. Nonetheless, that film reinforced the common belief that SETI practitioners paw through cosmic static looking for unusual patterns, such as a string of prime numbers. The truth is simpler: We have been searching for narrow-band signals. “Narrow-band” means that a large fraction of the transmitter power is squeezed into a tiny part of the radio dial, making the transmission easier to find. This is analogous to the way a laser pointer, despite having only a few milliwatts of power, nonetheless looks bright because the energy is concentrated into a narrow wavelength range.

Anybody out there: Jodie Foster as Ellie Arroway in the 1997 movie “Contact,” which was based on the bestseller by Carl Sagan. Getty Images

A modern SETI receiver simultaneously examines tens or even hundreds of millions of channels, each having a cramped 1-hertz bandwidth. That bandwidth is 5 million times narrower than a TV signal and lacks the capacity to carry information—a message. But the idea is to first discover aliens that are on the air, after which a far larger instrument would be built to dig out any modulation.

To aim our antennas, SETI has traditionally used two approaches. One is to scan as much of the sky as possible the other is to zero in on nearby star systems. You might think that the former would have an edge, since it makes no assumptions about where the aliens might be hanging out. But a sky survey spends most of its time looking at empty space. If you subscribe to the conventional view that extraterrestrials will most likely be ensconced on planets or moons, then it’s better to devote precious telescope time to examining nearby star systems.

It’s hard to imagine that aliens would go to the trouble of smashing together two black holes.

One current targeted search is the SETI Institute’s red dwarf survey, which takes place at the Allen Telescope Array, an ensemble of 42 antennas hunkered down in the California Cascades. We are going down a list of 20,000 small stars that are prime candidates for hosting habitable planets. These ruddy runts are both numerous and, on average, old. Most have been around for billions of years, the time it took life on Earth to evolve from microscopic slime to high-tech hominids. Astronomers estimate that roughly one-half of all red dwarfs might have a rocky world in the habitable zone, where temperatures would abide liquid water.

The SETI Institute is not the only band of alien hunters. Buoyed by a large infusion of money from the Russian billionaire Yuri Milner, the SETI group at the University of California, Berkeley, is renting time on the Green Bank Telescope in West Virginia and the Parkes Radio Telescope in the sheep country west of Sydney, Australia. Their decade-long project, known as Breakthrough Listen, also homes in on individual star systems.

While these efforts are broadly similar to what’s been done for decades, they are not your daddy’s SETI. The rapid growth in digital processing means that far larger swaths of the radio dial can be examined at one go and—in the case of the Allen array—many star systems can be checked out simultaneously. The array now examines three stars at once, but additional computer power could boost that to more than 100. Within two decades, SETI experiments will be able to complete a reconnaissance of 1 million star systems, which is hundreds of times more than have been carefully examined so far. SETI practitioners from Frank Drake to Carl Sagan have estimated that the galaxy currently houses somewhere between 10,000 and a few million broadcasting societies. If these estimates are right, then examining 1 million star systems could well lead to a discovery. So, if the premise of SETI has merit, we should find a broadcast from E.T. within a generation. That would spare me the expense of buying you a cup of coffee.

Furthermore, scientists have been diversifying. For two decades, some SETI researchers have used conventional optical telescopes to look for extremely brief laser flashes coming from the stars. In many ways, aliens might be more likely to communicate by pulsed light than radio signals, for the same reason that people are turning to fiber optics for Internet access: It can, at least in principle, send 100,000 times as many bits per second as radio can. These so-called optical SETI experiments have been limited to looking at one star system at a time. But like their radio cousins, they’re poised to become speedier as new technology allows them to survey ever-wider tracts of sky.

NEUTRINOS IN THE ICE: The IceCube neutrino observatory in Antarctica has been searching for energetic cosmic neutrinos, which some astronomers have proposed—probably quixotically—as a medium for extraterrestrial communications. NSF/B. Gudbjartsson

Physicists have also proposed wholly new modes of communications, such as neutrinos and gravitational waves. Some of my SETI colleagues have mulled these options, but we don’t see much merit in them at the moment. Both neutrinos and gravitational waves are inherently hard to create and detect. In nature, it takes the collapse of a star or the merger of black holes to produce them in any quantity. The total energy required to send “Hello, Earth” would be daunting, even for a civilization that could command the resources of a galaxy.

IceCube, the University of Wisconsin’s big neutrino detector in Antarctica, is sensitive only to very high-energy particles, which are precisely those that would be costliest to produce. In all the years it has been operating, the instrument has detected a total of a few dozen of these particles, even though it is a cubic kilometer in size. As for gravitational waves, the Laser Interferometric Gravitational-Wave Observatory has been able to detect colliding black holes over the final second of their infall. It is hard to imagine that aliens would go to the trouble of smashing together two huge black holes for a second’s worth of signal.

B ut there is a completely different approach that has yet to be explored in much detail: to look for artifacts—engineering projects of an advanced society. Some astronomers have suggested an alien megastructure, possibly an energy-collecting Dyson sphere, as the explanation for the mysterious dimming of Tabby’s star (officially known as KIC 8462852). It is a serious possibility, but no evidence has yet been found to support it.

We can never prove that aliens are not out there, only that they are.

It’s also conceivable that extraterrestrials could have left time capsules in our own solar system, perhaps millions or billions of years ago, on the assumption that our planet might eventually evolve a species able to find them. The Lagrange points in the Earth-moon system—locations where the gravity of Earth, moon, and sun are balanced, so that an object placed there will stay there—have been suggested as good hunting grounds for alien artifacts, as has the moon itself.

Another idea is that we should search for the high-energy exhausts of interstellar rockets. The fastest spacecraft would presumably use the most efficient fuel: matter combining with antimatter. Their destructive “combustion” would not only shoot the craft through space at a fair fraction of the speed of light, but would produce a gamma-ray exhaust, which we might detect. Rockets could be sorted out from natural gamma ray sources by their relatively quick motion across the sky.

The appealing thing about artifacts is that finding them is not time-critical. In contrast, to search for signals, you need to activate your instruments at the right time. It doesn’t help to look for radio pings, laser flashes, or neutrino bursts if E.T. reached out to touch us during the reign of the dinosaurs or will do so a hundred million years from now. Artifacts have no such synchronicity problem. That said, looking for artifacts has its own bummer factors. Anything beyond our solar system would need be truly huge to be visible cousins of the starship Enterprise would be very difficult to find.

SETI is not a traditional science problem in which a hypothesis can be falsified. We can never prove that the aliens are not out there, only that they are. But our ability to search improves with every technological innovation. I compare the situation to the year 1491. European civilization had been around for 2,500 years, yet the Americas were not on any map. Mesoamerican civilization, for its part, had been around for about as long, but also was ignorant of what lay over the oceans. With a glimpse and a shout from a sailor on the Pinta, everything changed.

Seth Shostak is the senior astronomer at the SETI Institute. He chaired the International Academy of Astronautics’s SETI Permanent Study Group for a decade and hosts the SETI Institute’s weekly hour-long science radio show, “Big Picture Science.” He is the co-author of a textbook on astrobiology and of Confessions of an Alien Hunter: A Scientist’s Search for Extraterrestrial Intelligence. Follow him on Twitter @SethShostak.

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This article was originally published on Nautilus Cosmos in November 2016.


How Would You React If We Discovered Alien Life?

For more than a century, from George Melies’ A Trip to the Moon to Stephen Spielberg’s E.T. and Close Encounters to this summer’s blockbuster sequel to Independence Day, mass media, and the general public, have pondered what will happen if we ever came into contact with extraterrestrial life forms. Carl Sagan’s book Contact, and Jodie Foster’s movie of the same name, explores one possible scenario in which a Search for Extraterrestrial Intelligence (SETI) scientist (played by Foster) discovers a signal repeating a sequence of prime numbers originating from star system Vega, the 5th brightest star visible from Earth. Even if Contact’s version of an alien encounter is more likely than that presented in Spielberg’s E.T., the possibilities are worth pondering.

And yet experts believe that the odds of receiving a radio transmission composed of prime numbers or encountering intelligent extraterrestrial life in the near future are "astronomical." even with Hillary Clinton's promise that if elected President, she would open up the “X-files” (Area 51).

But the odds may be increasing due to continuing advances in technology and money. At a press conference held in April in New York City, Russian billionaire and Breakthrough Prize co-founder Yuri Milner, along with famed physicist Stephen Hawking, announced Breakthrough Starshot, a 20-year voyage to the Alpha Centauri star system. Should the existence of planets in the Alpha Centauri system be confirmed, Starshot could provide us with the best measurements of an exoplanet atmosphere we could ever hope to get this century. Milner will spend $100 million dollars to fund the project. Facebook's founder and CEO, Mark Zuckenberg, is on the project’s board of directors.

The goal of NASA's Kepler Mission was to find terrestrial planets in the habitable zone of stars both near and far where liquid water and possibly life might exist. To date, Kepler has confirmed the existence of 2,337 exoplanets, including 1,284 new planets announced as of this writing. In a press release issued by NASA, chief scientist Ellen Stofan, said, "This announcement more than doubles the number of confirmed planets from Kepler. This gives us hope that somewhere out there, around a star much like ours, we can eventually discover another Earth."

But what would happen if we discovered life beyond Earth?

Christof Koch, president and chief scientific officer of the Allen Institute for Brain Science, believes most people will be excited to learn that there is intelligent life out there. "For some 'contact" would be a wish come true and fill us with awe. But for others it would raise concerns. One can't assume that alien cultures are by definition benevolent," Koch says. "If we look at the history of our world, lesser civilizations were often destroyed by more advanced ones. Would the same happen to us if we encountered an advanced alien civilization?" Hawking has warned against sending messages out into space for this very reason.

Koch has devoted his life to defining what consciousness is whether it be the internet, robots, animals, etc. Since it is doubtful that our first contact will be with humans from another planet it is important for us to understand what consciousness is so we can better understand what we do discover as we explore space. "The first discovery would probably be bacteria which might excite some scientists but not the general public. Another scenario might be a radio signal whose origin would be questioned. Was it a deliberate signal sent to us or is it random noise that can be explained scientifically? I am not holding my breath for a signal that includes prime numbers," Koch says.

Mary A. Voytek is the senior scientist and head of NASA's Astrobiology Program who started Nexus for Exoplanet System Science to search for life on exoplanets. She notes that NASA scientists are currently looking at the most extreme conditions on Earth to better understand what conditions can support life throughout the universe.  "If we can determine what makes a habitable planet on Earth it will help guide us to look for conditions in the universe” she says.

Voytek notes that NASA acknowledges that the discovery of life has significance beyond science: "In order to fully understand the societal implications, we must talk to the experts-scholars in sociology and the humanities as well as theologians."

"When I give lectures about my work ,most people are excited about the possibility of the discovery of extraterrestrial life," Voytek says. "This is nothing new… The ancient Greek atomists in the fourth century B.C. wrote about it. There is a quote by Democritus that I like to cite. ‘To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet only one grain will grow.’"

Douglas Vakoch, president of Messaging Extraterrestrial Intelligence (METI) has devoted much of his career with SETI to exploring what would happen on first contact and how we could even initiate it through interstellar messages. He says the majority of people believe that intelligent life is widespread in the cosmos.


Contents

Alien life, such as microorganisms, has been hypothesized to exist in the Solar System and throughout the universe. This hypothesis relies on the vast size and consistent physical laws of the observable universe. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, [7] as well as notable personalities such as Winston Churchill, [8] [9] it would be improbable for life not to exist somewhere other than Earth. [10] [11] This argument is embodied in the Copernican principle, which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle, which states that there is nothing special about life on Earth. [12] The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old. [13] [14] Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread—by meteoroids, for example—between habitable planets in a process called panspermia. [15] [16] In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth. [17] According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars. [17]

Since the 1950s, astronomers have proposed that "habitable zones" around stars are the most likely places for life to exist. Numerous discoveries of such zones since 2007 have generated numerical estimates of many billions of planets with Earth-like compositions. [18] As of 2013 [update] , only a few planets had been discovered in these zones. [19] Nonetheless, on 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way, [20] [21] 11 billion of which may be orbiting Sun-like stars. [22] The nearest such planet may be 12 light-years away, according to the scientists. [20] [21] Astrobiologists have also considered a "follow the energy" view of potential habitats. [23] [24]

Evolution Edit

A study published in 2017 suggests that due to how complexity evolved in species on Earth, the level of predictability for alien evolution elsewhere would make them look similar to life on our planet. One of the study authors, Sam Levin, notes "Like humans, we predict that they are made-up of a hierarchy of entities, which all cooperate to produce an alien. At each level of the organism there will be mechanisms in place to eliminate conflict, maintain cooperation, and keep the organism functioning. We can even offer some examples of what these mechanisms will be." [25] There is also research in assessing the capacity of life for developing intelligence. It has been suggested that this capacity arises with the number of potential niches a planet contains, and that the complexity of life itself is reflected in the information density of planetary environments, which in turn can be computed from its niches. [26]

Life on Earth requires water as a solvent in which biochemical reactions take place. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth. [27] [28] Life based on ammonia (rather than water) has been suggested as an alternative, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane. [29]

About 29 chemical elements play active roles in living organisms on Earth. [30] About 95% of living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts. [31] The unique characteristics of carbon make it unlikely that it could be replaced, even on another planet, to generate the biochemistry necessary for life. The carbon atom has the unique ability to make four strong chemical bonds with other atoms, including other carbon atoms. These covalent bonds have a direction in space, so that carbon atoms can form the skeletons of complex 3-dimensional structures with definite architectures such as nucleic acids and proteins. Carbon forms more compounds than all other elements combined. The great versatility of the carbon atom, and its abundance in the visible universe, makes it the element most likely to provide the bases—even exotic ones—for the chemical composition of life on other planets. [32]

Some bodies in the Solar System have the potential for an environment in which extraterrestrial life can exist, particularly those with possible subsurface oceans. [33] Should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophile microorganisms. According to NASA's 2015 Astrobiology Strategy, "Life on other worlds is most likely to include microbes, and any complex living system elsewhere is likely to have arisen from and be founded upon microbial life. Important insights on the limits of microbial life can be gleaned from studies of microbes on modern Earth, as well as their ubiquity and ancestral characteristics." [34] Researchers found a stunning array of subterranean organisms, mostly microbial, deep underground and estimate that approximately 70 percent of the total number of Earth's bacteria and archaea organisms live within the Earth's crust. [35] Rick Colwell, a member of the Deep Carbon Observatory team from Oregon State University, told the BBC: "I think it’s probably reasonable to assume that the subsurface of other planets and their moons are habitable, especially since we’ve seen here on Earth that organisms can function far away from sunlight using the energy provided directly from the rocks deep underground". [36]

Mars may have niche subsurface environments where microbial life might exist. [37] [38] [39] A subsurface marine environment on Jupiter's moon Europa might be the most likely habitat in the Solar System, outside Earth, for extremophile microorganisms. [40] [41] [42]

The panspermia hypothesis proposes that life elsewhere in the Solar System may have a common origin. If extraterrestrial life was found on another body in the Solar System, it could have originated from Earth just as life on Earth could have been seeded from elsewhere (exogenesis). [43] The first known mention of the term 'panspermia' was in the writings of the 5th century BC Greek philosopher Anaxagoras. [44] In the 19th century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834), [45] Kelvin (1871), [46] Hermann von Helmholtz (1879) [47] and, somewhat later, by Svante Arrhenius (1903). [48] Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) are important proponents of the hypothesis who further contended that life forms continue to enter Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution. [49]

Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent from Earth to seed new stellar systems with life. The Nobel prize winner Francis Crick, along with Leslie Orgel, proposed that seeds of life may have been purposely spread by an advanced extraterrestrial civilization, [50] but considering an early "RNA world" Crick noted later that life may have originated on Earth. [51]

Mercury Edit

The spacecraft MESSENGER found evidence of much ice on Mercury. There may be scientific support, based on studies reported in March 2020, for considering that parts of the planet Mercury may have been habitable, and perhaps that life forms, albeit likely primitive microorganisms, may have existed on the planet. [52] [53]

Venus Edit

In the early 20th century, Venus was considered to be similar to Earth for habitability, but observations since the beginning of the Space Age revealed that the Venus surface temperature is around 467 °C (873 °F), making it inhospitable for Earth-like life. [54] Likewise, the atmosphere of Venus is almost completely carbon dioxide, which can be toxic to Earth-like life. Between the altitudes of 50 and 65 kilometers, the pressure and temperature are Earth-like, and it may accommodate thermoacidophilic extremophile microorganisms in the acidic upper layers of the Venusian atmosphere. [55] [56] [57] [58] Furthermore, Venus likely had liquid water on its surface for at least a few million years after its formation. [59] [60] [61] In September 2020, a paper was published announcing the detection of phosphine in Venus' atmosphere in concentrations that could not be explained by known abiotic processes in the Venusian environment, such as lightning strikes or volcanic activity. [62] [63] [64]

The Moon Edit

Humans have been speculating about life on the Moon since antiquity. [65] One of the early scientific inquires into the topic appeared in a 1878 Scientific American article entitled "Is the Moon Inhabited?" [66] Decades later a 1939 essay by Winston Churchill concluded that the Moon is unlikely to harbour life, due to the lack of an atmosphere. [67]

4–3.5 billion years ago, the Moon could have had a magnetic field, sufficient atmosphere, and liquid water to sustain life on its surface. [68] [69] Warm and pressurized regions in the Moon's interior might still contain liquid water. [70]

Several species of terrestrial life were briefly brought to the Moon, including humans, [71] cotton plants, [72] and tardigrades. [73]

As of 2021, no native lunar life has been found, including any signs of life in the samples of Moon rocks and soil. [74]

Mars Edit

Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and now can occasionally be found as low-volume liquid brines in shallow Martian soil. [75] The origin of the potential biosignature of methane observed in Mars' atmosphere is unexplained, although hypotheses not involving life have also been proposed. [76]

There is evidence that Mars had a warmer and wetter past: dried-up riverbeds, polar ice caps, volcanoes, and minerals that form in the presence of water have all been found. Nevertheless, present conditions on Mars' subsurface may support life. [77] [78] Evidence obtained by the Curiosity rover studying Aeolis Palus, Gale Crater in 2013 strongly suggests an ancient freshwater lake that could have been a hospitable environment for microbial life. [79] [80]

Current studies on Mars by the Curiosity and Opportunity rovers are searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. [81] [82] [83] [84] The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective. [81]

Ceres Edit

Ceres, the only dwarf planet in the asteroid belt, has a thin water-vapor atmosphere. [85] [86] The vapor could have been produced by ice volcanoes or by ice near the surface sublimating (transforming from solid to gas). [87] Nevertheless, the presence of water on Ceres had led to speculation that life may be possible there. [88] [89] [90] It is one of the few places in the Solar System where scientists would like to search for possible signs of life. [87] Although the dwarf planet might not have living things today, there could be signs it harbored life in the past. [87]

Jupiter system Edit

Jupiter Edit

Carl Sagan and others in the 1960s and 1970s computed conditions for hypothetical microorganisms living in the atmosphere of Jupiter. [91] The intense radiation and other conditions, however, do not appear to permit encapsulation and molecular biochemistry, so life there is thought unlikely. [92] In contrast, some of Jupiter's moons may have habitats capable of sustaining life. Scientists have indications that heated subsurface oceans of liquid water may exist deep under the crusts of the three outer Galilean moons—Europa, [40] [41] [93] Ganymede, [94] [95] [96] [97] and Callisto. [98] [99] [100] The EJSM/Laplace mission was planned to determine the habitability of these environments, however, due to lack of funding, the program was not continued. Similar missions, like ESA's JUICE and NASA's Europa Clipper are currently in development and are slated for launch in 2022 and 2024, respectively.

Europa Edit

Jupiter's moon Europa has been the subject of speculation about the existence of life, due to the strong possibility of a liquid water ocean beneath its ice surface. [40] [42] Hydrothermal vents on the bottom of the ocean, if they exist, may warm the water and could be capable of supplying nutrients and energy to microorganisms. [102] It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice. [103]

The case for life on Europa was greatly enhanced in 2011 when it was discovered that vast lakes exist within Europa's thick, icy shell. Scientists found that ice shelves surrounding the lakes appear to be collapsing into them, thereby providing a mechanism through which life-forming chemicals created in sunlit areas on Europa's surface could be transferred to its interior. [104] [105]

On 11 December 2013, NASA reported the detection of "clay-like minerals" (specifically, phyllosilicates), often associated with organic materials, on the icy crust of Europa. [106] The presence of the minerals may have been the result of a collision with an asteroid or comet, according to the scientists. [106] The Europa Clipper, which would assess the habitability of Europa, is planned for launch in 2024. [107] [108] Europa's subsurface ocean is considered the best target for the discovery of life. [40] [42]

Saturn system Edit

Like Jupiter, Saturn is not likely to host life. However, Titan and Enceladus have been speculated to have possible habitats supportive of life. [76] [109] [110] [111]

Enceladus Edit

Enceladus, a moon of Saturn, has some of the conditions for life, including geothermal activity and water vapor, as well as possible under-ice oceans heated by tidal effects. [112] [113] The Cassini–Huygens probe detected carbon, hydrogen, nitrogen and oxygen—all key elements for supporting life—during its 2005 flyby through one of Enceladus's geysers spewing ice and gas. The temperature and density of the plumes indicate a warmer, watery source beneath the surface. [76] Of the bodies on which life is possible, living organisms could most easily enter the other bodies of the Solar System from Enceladus. [114]

Titan Edit

Titan, the largest moon of Saturn, is the only known moon in the Solar System with a significant atmosphere. Data from the Cassini–Huygens mission refuted the hypothesis of a global hydrocarbon ocean, but later demonstrated the existence of liquid hydrocarbon lakes in the polar regions—the first stable bodies of surface liquid discovered outside Earth. [109] [110] [111] Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface that are consistent with—but do not prove—the hypothesis that organisms there, if present, could be consuming hydrogen, acetylene and ethane, and producing methane. [115] [116] [117] NASA's Dragonfly mission is slated to land on Titan in the mid 2030s with a VTOL-capable rotorcraft with a launch date set in 2026.

Small Solar System bodies Edit

Small Solar System bodies have also been speculated to host habitats for extremophiles. Fred Hoyle and Chandra Wickramasinghe have proposed that microbial life might exist on comets and asteroids. [118] [119] [120] [121]

Other bodies Edit

Models of heat retention and heating via radioactive decay in smaller icy Solar System bodies suggest that Rhea, Titania, Oberon, Triton, Pluto, Eris, Sedna, and Orcus may have oceans underneath solid icy crusts approximately 100 km thick. [122] Of particular interest in these cases is the fact that the models indicate that the liquid layers are in direct contact with the rocky core, which allows efficient mixing of minerals and salts into the water. This is in contrast with the oceans that may be inside larger icy satellites like Ganymede, Callisto, or Titan, where layers of high-pressure phases of ice are thought to underlie the liquid water layer. [122]

Hydrogen sulfide has been proposed as a hypothetical solvent for life and is quite plentiful on Jupiter's moon Io, and may be in liquid form a short distance below the surface. [123]

The scientific search for extraterrestrial life is being carried out both directly and indirectly. As of September 2017 [update] , 3,667 exoplanets in 2,747 systems have been identified, and other planets and moons in our own solar system hold the potential for hosting primitive life such as microorganisms. As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms on Venus (via of phosphine) and Mars (via methane) was reported. [124]

Direct search Edit

Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites. [13] [14] Some claim to have identified evidence that microbial life has existed on Mars. [127] [128] [129] [130] An experiment on the two Viking Mars landers reported gas emissions from heated Martian soil samples that some scientists argue are consistent with the presence of living microorganisms. [131] Lack of corroborating evidence from other experiments on the same samples suggests that a non-biological reaction is a more likely hypothesis. [131] [132] [133] [134] In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001, formed of rock ejected from Mars. [127] [128]

In February 2005 NASA scientists reported they may have found some evidence of extraterrestrial life on Mars. [135] The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claim on methane signatures found in Mars's atmosphere resembling the methane production of some forms of primitive life on Earth, as well as on their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon distanced NASA from the scientists' claims, and Stoker herself backed off from her initial assertions. [136] Though such methane findings are still debated, support among some scientists for the existence of life on Mars exists. [137]

In November 2011 NASA launched the Mars Science Laboratory that landed the Curiosity rover on Mars. It is designed to assess the past and present habitability on Mars using a variety of scientific instruments. The rover landed on Mars at Gale Crater in August 2012. [138] [139]

The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium, which is relatively easy to determine from a distance by spectroscopy. However, significant advances in the ability to find and resolve light from smaller rocky worlds near their star are necessary before such spectroscopic methods can be used to analyze extrasolar planets. To that effect, the Carl Sagan Institute was founded in 2014 and is dedicated to the atmospheric characterization of exoplanets in circumstellar habitable zones. [140] [141] Planetary spectroscopic data will be obtained from telescopes like WFIRST and ELT. [142]

In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggest DNA and RNA components (adenine, guanine and related organic molecules), building blocks for life as we know it, may be formed extraterrestrially in outer space. [143] [144] [145] In October 2011, scientists reported that cosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. [146] [147] [148] One of the scientists suggested that these compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life." [146]

In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. [149] [150] Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation. [151]

Indirect search Edit

Projects such as SETI are monitoring the galaxy for electromagnetic interstellar communications from civilizations on other worlds. [152] [153] If there is an advanced extraterrestrial civilization, there is no guarantee that it is transmitting radio communications in the direction of Earth or that this information could be interpreted as such by humans. The length of time required for a signal to travel across the vastness of space means that any signal detected would come from the distant past. [154]

The presence of heavy elements in a star's light-spectrum is another potential biosignature such elements would (in theory) be found if the star was being used as an incinerator/repository for nuclear waste products. [155]

Extrasolar planets Edit

Some astronomers search for extrasolar planets that may be conducive to life, narrowing the search to terrestrial planets within the habitable zone of their star. [156] [157] Since 1992 over four thousand exoplanets have been discovered (4,758 planets in 3,517 planetary systems including 783 multiple planetary systems as of 1 June 2021). [158] The extrasolar planets so far discovered range in size from that of terrestrial planets similar to Earth's size to that of gas giants larger than Jupiter. [158] The number of observed exoplanets is expected to increase greatly in the coming years. [159]

The Kepler space telescope has also detected a few thousand [160] [161] candidate planets, [162] [163] of which about 11% may be false positives. [164]

There is at least one planet on average per star. [165] About 1 in 5 Sun-like stars [a] have an "Earth-sized" [b] planet in the habitable zone, [c] with the nearest expected to be within 12 light-years distance from Earth. [166] [167] Assuming 200 billion stars in the Milky Way, [d] that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs are included. [22] The rogue planets in the Milky Way possibly number in the trillions. [168]

The nearest known exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 pc) from Earth in the southern constellation of Centaurus. [169]

As of March 2014 [update] , the least massive exoplanet known is PSR B1257+12 A, which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is DENIS-P J082303.1-491201 b, [170] [171] about 29 times the mass of Jupiter, although according to most definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. Almost all of the planets detected so far are within the Milky Way, but there have also been a few possible detections of extragalactic planets. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life. [4]

One sign that a planet probably already contains life is the presence of an atmosphere with significant amounts of oxygen, since that gas is highly reactive and generally would not last long without constant replenishment. This replenishment occurs on Earth through photosynthetic organisms. One way to analyze the atmosphere of an exoplanet is through spectrography when it transits its star, though this might only be feasible with dim stars like white dwarfs. [172]

Terrestrial analysis Edit

The science of astrobiology considers life on Earth as well, and in the broader astronomical context. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia, when the young Earth was about 400 million years old. [173] [174] According to one of the researchers, "If life arose relatively quickly on Earth, then it could be common in the universe." [173] ¨

Scientists have calculated that there could be at least 36 active, communicating intelligent civilizations in our Milky Way galaxy, according to a study published in The Astrophysical Journal. [175] [176]

In 1961, University of California, Santa Cruz, astronomer and astrophysicist Frank Drake devised the Drake equation as a way to stimulate scientific dialogue at a meeting on the search for extraterrestrial intelligence (SETI). [177] The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. The equation is best understood not as an equation in the strictly mathematical sense, but to summarize all the various concepts which scientists must contemplate when considering the question of life elsewhere. [178] The Drake equation is:

N = the number of Milky Way galaxy civilizations already capable of communicating across interplanetary space

R* = the average rate of star formation in our galaxy fp = the fraction of those stars that have planets ne = the average number of planets that can potentially support life fl = the fraction of planets that actually support life fi = the fraction of planets with life that evolves to become intelligent life (civilizations) fc = the fraction of civilizations that develop a technology to broadcast detectable signs of their existence into space L = the length of time over which such civilizations broadcast detectable signals into space

Drake's proposed estimates are as follows, but numbers on the right side of the equation are agreed as speculative and open to substitution:

The Drake equation has proved controversial since several of its factors are uncertain and based on conjecture, not allowing conclusions to be made. [180] This has led critics to label the equation a guesstimate, or even meaningless.

Based on observations from the Hubble Space Telescope, there are between 125 and 250 billion galaxies in the observable universe. [181] It is estimated that at least ten percent of all Sun-like stars have a system of planets, [182] i.e. there are 6.25 × 10 18 stars with planets orbiting them in the observable universe. Even if it is assumed that only one out of a billion of these stars has planets supporting life, there would be some 6.25 billion life-supporting planetary systems in the observable universe.

A 2013 study based on results from the Kepler spacecraft estimated that the Milky Way contains at least as many planets as it does stars, resulting in 100–400 billion exoplanets. [183] [184] Also based on Kepler data, scientists estimate that at least one in six stars has an Earth-sized planet. [185]

The apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for such civilizations is known as the Fermi paradox. [186]

Cosmic pluralism Edit

Cosmic pluralism, the plurality of worlds, or simply pluralism, describes the philosophical belief in numerous "worlds" in addition to Earth, which might harbor extraterrestrial life. Before the development of the heliocentric theory and a recognition that the Sun is just one of many stars, [187] the notion of pluralism was largely mythological and philosophical. The earliest recorded assertion of extraterrestrial human life is found in ancient scriptures of Jainism. There are multiple "worlds" mentioned in Jain scriptures that support human life. These include Bharat Kshetra, Mahavideh Kshetra, Airavat Kshetra, Hari kshetra, etc. [188] [189] [190] [191] Medieval Muslim writers like Fakhr al-Din al-Razi and Muhammad al-Baqir supported cosmic pluralism on the basis of the Qur'an. [192]

With the scientific and Copernican revolutions, and later, during the Enlightenment, cosmic pluralism became a mainstream notion, supported by the likes of Bernard le Bovier de Fontenelle in his 1686 work Entretiens sur la pluralité des mondes. [193] Pluralism was also championed by philosophers such as John Locke, Giordano Bruno and astronomers such as William Herschel. The astronomer Camille Flammarion promoted the notion of cosmic pluralism in his 1862 book La pluralité des mondes habités. [194] None of these notions of pluralism were based on any specific observation or scientific information.

Early modern period Edit

There was a dramatic shift in thinking initiated by the invention of the telescope and the Copernican assault on geocentric cosmology. Once it became clear that Earth was merely one planet amongst countless bodies in the universe, the theory of extraterrestrial life started to become a topic in the scientific community. The best known early-modern proponent of such ideas was the Italian philosopher Giordano Bruno, who argued in the 16th century for an infinite universe in which every star is surrounded by its own planetary system. Bruno wrote that other worlds "have no less virtue nor a nature different to that of our earth" and, like Earth, "contain animals and inhabitants". [195]

In the early 17th century, the Czech astronomer Anton Maria Schyrleus of Rheita mused that "if Jupiter has (. ) inhabitants (. ) they must be larger and more beautiful than the inhabitants of Earth, in proportion to the [characteristics] of the two spheres". [196]

In Baroque literature such as The Other World: The Societies and Governments of the Moon by Cyrano de Bergerac, extraterrestrial societies are presented as humoristic or ironic parodies of earthly society. The didactic poet Henry More took up the classical theme of the Greek Democritus in "Democritus Platonissans, or an Essay Upon the Infinity of Worlds" (1647). In "The Creation: a Philosophical Poem in Seven Books" (1712), Sir Richard Blackmore observed: "We may pronounce each orb sustains a race / Of living things adapted to the place". With the new relative viewpoint that the Copernican revolution had wrought, he suggested "our world's sunne / Becomes a starre elsewhere". Fontanelle's "Conversations on the Plurality of Worlds" (translated into English in 1686) offered similar excursions on the possibility of extraterrestrial life, expanding, rather than denying, the creative sphere of a Maker.

The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. William Herschel, the discoverer of Uranus, was one of many 18th–19th-century astronomers who believed that the Solar System is populated by alien life. Other scholars of the period who championed "cosmic pluralism" included Immanuel Kant and Benjamin Franklin. At the height of the Enlightenment, even the Sun and Moon were considered candidates for extraterrestrial inhabitants.

19th century Edit

Speculation about life on Mars increased in the late 19th century, following telescopic observation of apparent Martian canals—which soon, however, turned out to be optical illusions. [197] Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilization. [198] The idea of life on Mars led British writer H. G. Wells to write the novel The War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet's desiccation.

Spectroscopic analysis of Mars's atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen was present in the Martian atmosphere. [199] By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis.

The science fiction genre, although not so named during the time, developed during the late 19th century. Jules Verne's Around the Moon (1870) features a discussion of the possibility of life on the Moon, but with the conclusion that it is barren.

20th century Edit

Most unidentified flying objects or UFO sightings [200] can be readily explained as sightings of Earth-based aircraft, known astronomical objects, or as hoaxes. [201] A certain fraction of the public believe that UFOs might actually be of extraterrestrial origin, and the notion has had influence on popular culture.

The possibility of extraterrestrial life on the Moon was ruled out in the 1960s, and during the 1970s it became clear that most of the other bodies of the Solar System do not harbor highly developed life, although the question of primitive life on bodies in the Solar System remains open.

Recent history Edit

The failure so far of the SETI program to detect an intelligent radio signal after decades of effort has at least partially dimmed the prevailing optimism of the beginning of the space age. Belief in extraterrestrial beings continues to be voiced in pseudoscience, conspiracy theories, and in popular folklore, notably "Area 51" and legends. It has become a pop culture trope given less-than-serious treatment in popular entertainment.

In the words of SETI's Frank Drake, "All we know for sure is that the sky is not littered with powerful microwave transmitters". [202] Drake noted that it is entirely possible that advanced technology results in communication being carried out in some way other than conventional radio transmission. At the same time, the data returned by space probes, and giant strides in detection methods, have allowed science to begin delineating habitability criteria on other worlds, and to confirm that at least other planets are plentiful, though aliens remain a question mark. The Wow! signal, detected in 1977 by a SETI project, remains a subject of speculative debate.

In 2000, geologist and paleontologist Peter Ward and astrobiologist Donald Brownlee published a book entitled Rare Earth: Why Complex Life is Uncommon in the Universe. [203] In it, they discussed the Rare Earth hypothesis, in which they claim that Earth-like life is rare in the universe, whereas microbial life is common. Ward and Brownlee are open to the idea of evolution on other planets that is not based on essential Earth-like characteristics (such as DNA and carbon).

Theoretical physicist Stephen Hawking in 2010 warned that humans should not try to contact alien life forms. He warned that aliens might pillage Earth for resources. "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans", he said. [204] Jared Diamond had earlier expressed similar concerns. [205]

In 2013, the exoplanet Kepler-62f was discovered, along with Kepler-62e and Kepler-62c. A related special issue of the journal Science, published earlier, described the discovery of the exoplanets. [206]

On 17 April 2014, the discovery of the Earth-size exoplanet Kepler-186f, 500 light-years from Earth, was publicly announced [207] it is the first Earth-size planet to be discovered in the habitable zone and it has been hypothesized that there may be liquid water on its surface.

On 13 February 2015, scientists (including Geoffrey Marcy, Seth Shostak, Frank Drake and David Brin) at a convention of the American Association for the Advancement of Science, discussed Active SETI and whether transmitting a message to possible intelligent extraterrestrials in the Cosmos was a good idea [208] [209] one result was a statement, signed by many, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent". [210]

On 20 July 2015, British physicist Stephen Hawking and Russian billionaire Yuri Milner, along with the SETI Institute, announced a well-funded effort, called the Breakthrough Initiatives, to expand efforts to search for extraterrestrial life. The group contracted the services of the 100-meter Robert C. Byrd Green Bank Telescope in West Virginia in the United States and the 64-meter Parkes Telescope in New South Wales, Australia. [211]

International organisations and treaties Edit

The 1967 Outer Space Treaty and the 1979 Moon Agreement define rules of planetary protection against potentially hazardous extraterrestrial life. COSPAR also provides guidelines for planetary protection. [212]

A committee of the United Nations Office for Outer Space Affairs had in 1977 discussed for a year strategies in interacting with extraterrestrial life or intelligence. The discussion ended without any conclusions. As of 2010, the UN doesn't have response mechanisms for the case of an extraterrestrial contact. [213]

United States Edit

In November 2011, the White House released an official response to two petitions asking the U.S. government to acknowledge formally that aliens have visited Earth and to disclose any intentional withholding of government interactions with extraterrestrial beings. According to the response, "The U.S. government has no evidence that any life exists outside our planet, or that an extraterrestrial presence has contacted or engaged any member of the human race." [214] [215] Also, according to the response, there is "no credible information to suggest that any evidence is being hidden from the public's eye." [214] [215] The response noted "odds are pretty high" that there may be life on other planets but "the odds of us making contact with any of them—especially any intelligent ones—are extremely small, given the distances involved." [214] [215]

One of the NASA divisions is the Office of Safety and Mission Assurance (OSMA), also known as the Planetary Protection Office. A part of its mission is to “rigorously preclude backward contamination of Earth by extraterrestrial life.” [216]

Russia Edit

In 2020, Dmitry Rogozin, the head of the Russian space agency, said the search for extraterrestrial life is one of the main goals of deep space research. He also acknowledged the possibility of existence of primitive life on other planets of the Solar System. [217]

Japan Edit

In 2020, the Japanese Defense Minister Taro Kono stated that Self-Defense Forces pilots have never encountered a UFO, and that he does not believe in UFOs. He also said he would consider issuing protocols for such encounters. [218] Several months later, the protocols were issued, clarifying what the personnel should do when encountering unidentified flying objects that could potentially pose a threat to national security. [219]

China Edit

In 2016, the Chinese Government released a white paper detailing its space program. According to the document, one of the research objectives of the program is the search for extraterrestrial life. [220] It is also one of the objectives of the Chinese Five-hundred-meter Aperture Spherical Telescope (FAST) program. [221]

EU Edit

The French space agency has an office for the study of “non-identified aero spatial phenomena”. [222] [223] The agency is maintaining a publicly accessible database of such phenomena, with over 1600 detailed entries. According to the head of the office, the vast majority of entries have a mundane explanation but for 25% of entries, their extraterrestrial origin can neither be confirmed nor denied. [222]

In 2018, the German Ministry of Economics stated that the German government has no plans or protocol for the case of a first contact with aliens, as the government perceives such event as "extremely unlikely". It also stated that no cases of a first contact are known. [224]

Israel Edit

In 2020, chairman of the Israel Space Agency Isaac Ben-Israel stated that the probability of detecting life in outer space is "quite large". But he disagrees with his former colleague Haim Eshed who stated that there are contacts between an advanced alien civilization and some of Earth's governments. [225]

  1. ^ For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars wasn't available so this statistic is an extrapolation from data about K-type stars
  2. ^ For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
  3. ^ For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).
  4. ^ About 1/4 of stars are GK Sun-like stars. The number of stars in the galaxy is not accurately known, but assuming 200 billion stars in total, the Milky Way would have about 50 billion Sun-like (GK) stars, of which about 1 in 5 (22%) or 11 billion would be Earth-sized in the habitable zone. Including red dwarfs would increase this to 40 billion.
  1. ^ Frank, Adam (31 December 2020). "A new frontier is opening in the search for extraterrestrial life - The reason we haven't found life elsewhere in the universe is simple: We haven't really looked until now". The Washington Post . Retrieved 1 January 2021 .
  2. ^
  3. Davies, Paul (18 November 2013). "Are We Alone in the Universe?". The New York Times . Retrieved 20 November 2013 .
  4. ^
  5. Pickrell, John (4 September 2006). "Top 10: Controversial pieces of evidence for extraterrestrial life". New Scientist . Retrieved 18 February 2011 .
  6. ^ ab
  7. Overbye, Dennis (6 January 2015). "As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next". The New York Times . Retrieved 6 January 2015 .
  8. ^
  9. Ghosh, Pallab (12 February 2015). "Scientists in US are urged to seek contact with aliens". BBC News.
  10. ^
  11. Baum, Seth Haqq-Misra, Jacob Domagal-Goldman, Shawn (June 2011). "Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis". Acta Astronautica. 68 (11): 2114–2129. arXiv: 1104.4462 . Bibcode:2011AcAau..68.2114B. doi:10.1016/j.actaastro.2010.10.012. S2CID16889489.
  12. ^
  13. Weaver, Rheyanne. "Ruminations on other worlds". State Press. Archived from the original on 24 October 2013 . Retrieved 10 March 2014 .
  14. ^
  15. Livio, Mario (15 February 2017). "Winston Churchill's essay on alien life found". Nature. 542 (7641): 289–291. Bibcode:2017Natur.542..289L. doi: 10.1038/542289a . PMID28202987. S2CID205092694.
  16. ^
  17. De Freytas-Tamura, Kimiko (15 February 2017). "Winston Churchill Wrote of Alien Life in a Lost Essay". The New York Times . Retrieved 18 February 2017 .
  18. ^
  19. Steiger, Brad White, John, eds. (1986). Other Worlds, Other Universes. Health Research Books. p. 3. ISBN978-0-7873-1291-6 .
  20. ^
  21. Filkin, David Hawking, Stephen W. (1998). Stephen Hawking's universe: the cosmos explained . Art of Mentoring Series. Basic Books. p. 194. ISBN978-0-465-08198-1 .
  22. ^
  23. Rauchfuss, Horst (2008). Chemical Evolution and the Origin of Life. trans. Terence N. Mitchell. Springer. ISBN978-3-540-78822-5 .
  24. ^ ab
  25. Loeb, Abraham (October 2014). "The Habitable Epoch of the Early Universe". International Journal of Astrobiology. 13 (4): 337–339. arXiv: 1312.0613 . Bibcode:2014IJAsB..13..337L. CiteSeerX10.1.1.748.4820 . doi:10.1017/S1473550414000196. S2CID2777386.
  26. ^ ab
  27. Dreifus, Claudia (2 December 2014). "Much-Discussed Views That Go Way Back – Avi Loeb Ponders the Early Universe, Nature and Life". The New York Times . Retrieved 3 December 2014 .
  28. ^
  29. Rampelotto, P. H. (April 2010). Panspermia: A Promising Field of Research (PDF) . Astrobiology Science Conference 2010: Evolution and Life: Surviving Catastrophes and Extremes on Earth and Beyond. 20–26 April 2010. League City, Texas. Bibcode:2010LPICo1538.5224R.
  30. ^
  31. Gonzalez, Guillermo Richards, Jay Wesley (2004). The privileged planet: how our place in the cosmos is designed for discovery. Regnery Publishing. pp. 343–345. ISBN978-0-89526-065-9 .
  32. ^ ab
  33. Moskowitz, Clara (29 March 2012). "Life's Building Blocks May Have Formed in Dust Around Young Sun". Space.com . Retrieved 30 March 2012 .
  34. ^
  35. Choi, Charles Q. (21 March 2011). "New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone". Space.com . Retrieved 24 April 2011 .
  36. ^
  37. Torres, Abel Mendez (26 April 2013). "Ten potentially habitable exoplanets now". Habitable Exoplanets Catalog. University of Puerto Rico . Retrieved 29 April 2013 .
  38. ^ ab
  39. Overbye, Dennis (4 November 2013). "Far-Off Planets Like the Earth Dot the Galaxy". The New York Times . Retrieved 5 November 2013 .
  40. ^ ab
  41. Petigura, Eric A. Howard, Andrew W. Marcy, Geoffrey W. (31 October 2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences of the United States of America. 110 (48): 19273–19278. arXiv: 1311.6806 . Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC3845182 . PMID24191033 . Retrieved 5 November 2013 .
  42. ^ ab
  43. Khan, Amina (4 November 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times . Retrieved 5 November 2013 .
  44. ^
  45. Hoehler, Tori M. Amend, Jan P. Shock, Everett L. (2007). "A "Follow the Energy" Approach for Astrobiology". Astrobiology. 7 (6): 819–823. Bibcode:2007AsBio. 7..819H. doi:10.1089/ast.2007.0207. ISSN1531-1074. PMID18069913.
  46. ^
  47. Jones, Eriita G. Lineweaver, Charles H. (2010). "To What Extent Does Terrestrial Life "Follow The Water"?" (PDF) . Astrobiology. 10 (3): 349–361. Bibcode:2010AsBio..10..349J. CiteSeerX10.1.1.309.9959 . doi:10.1089/ast.2009.0428. hdl:1885/8711. ISSN1531-1074. PMID20446874.
  48. ^
  49. "Aliens may be more like us than we think". University of Oxford. 31 October 2017.
  50. ^
  51. Stevenson, David S. Large, Sean (25 October 2017). "Evolutionary exobiology: Towards the qualitative assessment of biological potential on exoplanets". International Journal of Astrobiology. 18 (3): 204–208. doi:10.1017/S1473550417000349. S2CID125275411.
  52. ^
  53. Bond, Jade C. O'Brien, David P. Lauretta, Dante S. (June 2010). "The Compositional Diversity of Extrasolar Terrestrial Planets. I. In Situ Simulations". The Astrophysical Journal. 715 (2): 1050–1070. arXiv: 1004.0971 . Bibcode:2010ApJ. 715.1050B. doi:10.1088/0004-637X/715/2/1050. S2CID118481496.
  54. ^
  55. Pace, Norman R. (20 January 2001). "The universal nature of biochemistry". Proceedings of the National Academy of Sciences of the United States of America. 98 (3): 805–808. Bibcode:2001PNAS. 98..805P. doi:10.1073/pnas.98.3.805. PMC33372 . PMID11158550.
  56. ^
  57. National Research Council (2007). "6.2.2: Nonpolar Solvents". The Limits of Organic Life in Planetary Systems. The National Academies Press. p. 74. doi:10.17226/11919. ISBN978-0-309-10484-5 .
  58. ^
  59. Nielsen, Forrest H. (1999). "Ultratrace Minerals". In Shils, Maurice E. Shike, Moshe (eds.). Modern Nutrition in Health and Disease (9th ed.). Williams & Wilkins. pp. 283–303. ISBN978-0-683-30769-6 .
  60. ^
  61. Mix, Lucas John (2009). Life in space: astrobiology for everyone. Harvard University Press. p. 76. ISBN978-0-674-03321-4 . Retrieved 8 August 2011 .
  62. ^
  63. Horowitz, Norman H. (1986). To Utopia and Back: The Search for Life in the Solar System . W. H. Freeman & Co. ISBN978-0-7167-1765-2 .
  64. ^
  65. Dyches, Preston Chou, Felcia (7 April 2015). "The Solar System and Beyond is Awash in Water". NASA . Retrieved 8 April 2015 .
  66. ^
  67. Hays, Lindsay, ed. (2015). "NASA Astrobiology Strategy 2015" (PDF) . NASA. p. 65. Archived from the original (PDF) on 22 December 2016 . Retrieved 12 October 2017 .
  68. ^
  69. Offord, Catherine (30 September 2018). "Life Thrives Within the Earth's Crust". The Scientist Magazine . Retrieved 2 April 2019 .
  70. ^
  71. Wilke, Carolyn (11 December 2018). "Life Deep Underground Is Twice the Volume of the Oceans: Study". The Scientist Magazine . Retrieved 2 April 2019 .
  72. ^
  73. Summons, Roger E. Amend, Jan P. Bish, David Buick, Roger Cody, George D. Des Marais, David J. Dromart, Gilles Eigenbrode, Jennifer L. et al. (2011). "Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group" (PDF) . Astrobiology. 11 (2): 157–81. Bibcode:2011AsBio..11..157S. doi:10.1089/ast.2010.0506. hdl: 1721.1/66519 . PMID21417945. There is general consensus that extant microbial life on Mars would probably exist (if at all) in the subsurface and at low abundance.
  74. ^
  75. Michalski, Joseph R. Cuadros, Javier Niles, Paul B. Parnell, John Deanne Rogers, A. Wright, Shawn P. (2013). "Groundwater activity on Mars and implications for a deep biosphere". Nature Geoscience. 6 (2): 133–8. Bibcode:2013NatGe. 6..133M. doi:10.1038/ngeo1706.
  76. ^
  77. "Habitability and Biology: What are the Properties of Life?". Phoenix Mars Mission. The University of Arizona . Retrieved 6 June 2013 . If any life exists on Mars today, scientists believe it is most likely to be in pockets of liquid water beneath the Martian surface.
  78. ^ abcd
  79. Tritt, Charles S. (2002). "Possibility of Life on Europa". Milwaukee School of Engineering. Archived from the original on 9 June 2007 . Retrieved 10 August 2007 .
  80. ^ ab
  81. Kargel, Jeffrey S. Kaye, Jonathan Z. Head, James W. Marion, Giles M. Sassen, Roger et al. (November 2000). "Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life". Icarus. 148 (1): 226–265. Bibcode:2000Icar..148..226K. doi:10.1006/icar.2000.6471.
  82. ^ abc
  83. Schulze-Makuch, Dirk Irwin, Louis N. (2001). "Alternative Energy Sources Could Support Life on Europa" (PDF) . Departments of Geological and Biological Sciences, University of Texas at El Paso. Archived from the original (PDF) on 3 July 2006 . Retrieved 21 December 2007 .
  84. ^
  85. Reuell, Peter (8 July 2019). "Harvard study suggests asteroids might play key role in spreading life". Harvard Gazette . Retrieved 29 September 2019 .
  86. ^
  87. O'Leary, Margaret R. (2008). Anaxagoras and the Origin of Panspermia Theory. iUniverse. ISBN978-0-595-49596-2 .
  88. ^
  89. Berzelius, Jöns Jacob (1834). "Analysis of the Alais meteorite and implications about life in other worlds". Annalen der Chemie und Pharmacie. 10: 134–135.
  90. ^
  91. Thomson, William (August 1871). "The British Association Meeting at Edinburgh". Nature. 4 (92): 261–278. Bibcode:1871Natur. 4..261.. doi:10.1038/004261a0. PMC2070380 . We must regard it as probably to the highest degree that there are countless seed-bearing meteoritic stones moving through space.
  92. ^
  93. Demets, René (October 2012). "Darwin's Contribution to the Development of the Panspermia Theory". Astrobiology. 12 (10): 946–950. Bibcode:2012AsBio..12..946D. doi:10.1089/ast.2011.0790. PMID23078643.
  94. ^
  95. Arrhenius, Svante (March 1908). Worlds in the Making: The Evolution of the Universe. trans. H. Borns. Harper & Brothers. OCLC1935295.
  96. ^
  97. Hoyle, Fred Wickramasinghe, Chandra Watson, John (1986). Viruses from Space and Related Matters (PDF) . University College Cardiff Press. Bibcode:1986vfsr.book. H. ISBN978-0-906449-93-6 .
  98. ^
  99. Crick, F. H. Orgel, L. E. (1973). "Directed Panspermia". Icarus. 19 (3): 341–348. Bibcode:1973Icar. 19..341C. doi:10.1016/0019-1035(73)90110-3.
  100. ^
  101. Orgel, L. E. Crick, F. H. (January 1993). "Anticipating an RNA world. Some past speculations on the origin of life: Where are they today?". FASEB Journal. 7 (1): 238–239. doi:10.1096/fasebj.7.1.7678564. PMID7678564. S2CID11314345.
  102. ^
  103. Hall, Shannon (24 March 2020). "Life on the Planet Mercury? 'It's Not Completely Nuts' - A new explanation for the rocky world's jumbled landscape opens a possibility that it could have had ingredients for habitability". The New York Times . Retrieved 26 March 2020 .
  104. ^
  105. Roddriquez, J. Alexis P. et al. (16 March 2020). "The Chaotic Terrains of Mercury Reveal a History of Planetary Volatile Retention and Loss in the Innermost Solar System". Scientific Reports. 10 (4737): 4737. Bibcode:2020NatSR..10.4737R. doi: 10.1038/s41598-020-59885-5 . PMC7075900 . PMID32179758.
  106. ^
  107. Redd, Nola Taylor (17 November 2012). "How Hot is Venus?". Space.com . Retrieved 28 January 2020 .
  108. ^
  109. Clark, Stuart (26 September 2003). "Acidic clouds of Venus could harbour life". New Scientist . Retrieved 30 December 2015 .
  110. ^ Redfern, Martin (25 May 2004). "Venus clouds 'might harbour life'". BBC News. Retrieved 30 December 2015.
  111. ^
  112. Dartnell, Lewis R. Nordheim, Tom Andre Patel, Manish R. Mason, Jonathon P. et al. (September 2015). "Constraints on a potential aerial biosphere on Venus: I. Cosmic rays". Icarus. 257: 396–405. Bibcode:2015Icar..257..396D. doi:10.1016/j.icarus.2015.05.006.
  113. ^
  114. "Did the Early Venus Harbor Life? (Weekend Feature)". The Daily Galaxy. 2 June 2012. Archived from the original on 28 October 2017 . Retrieved 22 May 2016 .
  115. ^
  116. "Was Venus once a habitable planet?". European Space Agency. 24 June 2010 . Retrieved 22 May 2016 .
  117. ^
  118. Atkinson, Nancy (24 June 2010). "Was Venus once a waterworld?". Universe Today . Retrieved 22 May 2016 .
  119. ^
  120. Bortman, Henry (26 August 2004). "Was Venus Alive? 'The Signs are Probably There ' ". Space.com . Retrieved 22 May 2016 .
  121. ^
  122. Greaves, Jane S. et al. (14 September 2020). "Phosphine gas in the cloud decks of Venus". Nature Astronomy. arXiv: 2009.06593 . Bibcode:2020NatAs.tmp..178G. doi:10.1038/s41550-020-1174-4. S2CID221655755 . Retrieved 14 September 2020 .
  123. ^
  124. Stirone, Shannon Chang, Kenneth Overbye, Dennis (14 September 2020). "Life on Venus? Astronomers See a Signal in Its Clouds - The detection of a gas in the planet's atmosphere could turn scientists' gaze to a planet long overlooked in the search for extraterrestrial life". The New York Times . Retrieved 14 September 2020 .
  125. ^
  126. Johnson, J. C. Johnson, P. A. Mardon, A. A. (November 2020). "Prospecting Microbial Biosignatures from Venusian Clouds". LPI Contributions. 2356: 8024. Bibcode:2020LPICo2356.8024J. ISSN0161-5297.
  127. ^ see Moon in fiction for many examples
  128. ^
  129. Scientific American, "Is The Moon Inhabited?". Munn & Company. 20 July 1878. p. 36.
  130. ^
  131. Livio, Mario (15 February 2017). "Winston Churchill's essay on alien life found". Nature . Retrieved 26 March 2021 .
  132. ^
  133. "Mysteries from the moon's past". Washington State University. 23 July 2018 . Retrieved 22 August 2020 .
  134. ^
  135. Schulze-Makuch, Dirk Crawford, Ian A. (2018). "Was There an Early Habitability Window for Earth's Moon?". Astrobiology. 18 (8): 985–988. Bibcode:2018AsBio..18..985S. doi:10.1089/ast.2018.1844. PMC6225594 . PMID30035616.
  136. ^
  137. "Could Life Exist Deep Underground on Mars?". Center for Astrophysics (Harvard & Smithsonian). Archived from the original on 28 January 2021 . Retrieved 28 January 2021 .
  138. ^
  139. Loff, Sarah (1 February 2019). "The Apollo Missions". nasa.gov. NASA . Retrieved 26 March 2021 .
  140. ^
  141. Wong, Sam (15 January 2019). "First moon plants sprout in China's Chang'e 4 biosphere experiment". New Scientist . Retrieved 26 March 2021 .
  142. ^
  143. Virk, Kameron (7 August 2019). "Tardigrades: 'Water bears' stuck on the moon after crash". BBC . Retrieved 26 March 2021 .
  144. ^
  145. Smith, Kimberly Anderson, James (15 July 2019). "NASA Searches for Life from the Moon in Recently Rediscovered Historic Footage". nasa.gov. NASA . Retrieved 26 March 2021 .
  146. ^
  147. Ojha, L. Wilhelm, M. B. Murchie, S. L. McEwen, A. S. Wray, J. J. Hanley, J. Massé, M. Chojnacki, M. (2015). "Spectral evidence for hydrated salts in recurring slope lineae on Mars". Nature Geoscience. 8 (11): 829–832. Bibcode:2015NatGe. 8..829O. doi:10.1038/ngeo2546.
  148. ^ abc
  149. "Top 10 Places To Find Alien Life : Discovery News". News.discovery.com. 8 June 2010 . Retrieved 13 June 2012 .
  150. ^
  151. Baldwin, Emily (26 April 2012). "Lichen survives harsh Mars environment". Skymania News . Retrieved 27 April 2012 .
  152. ^
  153. de Vera, J.-P. Kohler, Ulrich (26 April 2012). "The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars" (PDF) . European Geosciences Union. Archived from the original (PDF) on 4 May 2012 . Retrieved 27 April 2012 .
  154. ^
  155. Chang, Kenneth (9 December 2013). "On Mars, an Ancient Lake and Perhaps Life". The New York Times . Retrieved 9 December 2013 .
  156. ^
  157. "Science – Special Collection – Curiosity Rover on Mars". Science. 9 December 2013 . Retrieved 9 December 2013 .
  158. ^ ab
  159. Grotzinger, John P. (24 January 2014). "Introduction to Special Issue – Habitability, Taphonomy, and the Search for Organic Carbon on Mars". Science. 343 (6169): 386–387. Bibcode:2014Sci. 343..386G. doi: 10.1126/science.1249944 . PMID24458635.
  160. ^
  161. "Special Issue – Table of Contents – Exploring Martian Habitability". Science. 343 (6169): 345–452. 24 January 2014 . Retrieved 24 January 2014 .
  162. ^
  163. "Special Collection – Curiosity – Exploring Martian Habitability". Science. 24 January 2014 . Retrieved 24 January 2014 .
  164. ^
  165. Grotzinger, J. P. et al. (24 January 2014). "A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars". Science. 343 (6169): 1242777. Bibcode:2014Sci. 343A.386G. CiteSeerX10.1.1.455.3973 . doi:10.1126/science.1242777. PMID24324272. S2CID52836398.
  166. ^
  167. Küppers, M. O'Rourke, L. Bockelée-Morvan, D. Zakharov, V. Lee, S. Von Allmen, P. Carry, B. Teyssier, D. Marston, A. Müller, T. Crovisier, J. Barucci, M. A. Moreno, R. (23 January 2014). "Localized sources of water vapour on the dwarf planet (1) Ceres". Nature. 505 (7484): 525–527. Bibcode:2014Natur.505..525K. doi:10.1038/nature12918. ISSN0028-0836. PMID24451541. S2CID4448395.
  168. ^
  169. Campins, H. Comfort, C. M. (23 January 2014). "Solar system: Evaporating asteroid". Nature. 505 (7484): 487–488. Bibcode:2014Natur.505..487C. doi: 10.1038/505487a . PMID24451536. S2CID4396841.
  170. ^ abc
  171. "In Depth | Ceres". NASA Solar System Exploration . Retrieved 29 January 2020 .
  172. ^
  173. O'Neill, Ian (5 March 2009). "Life on Ceres: Could the Dwarf Planet be the Root of Panspermia". Universe Today . Retrieved 30 January 2012 .
  174. ^
  175. Catling, David C. (2013). Astrobiology: A Very Short Introduction. Oxford: Oxford University Press. p. 99. ISBN978-0-19-958645-5 .
  176. ^
  177. Boyle, Alan (22 January 2014). "Is There Life on Ceres? Dwarf Planet Spews Water Vapor". NBC . Retrieved 10 February 2015 .
  178. ^
  179. Ponnamperuma, Cyril Molton, Peter (January 1973). "The prospect of life on Jupiter". Space Life Sciences. 4 (1): 32–44. Bibcode:1973SLSci. 4. 32P. doi:10.1007/BF02626340. PMID4197410. S2CID12491394.
  180. ^
  181. Irwin, Louis Neal Schulze-Makuch, Dirk (June 2001). "Assessing the Plausibility of Life on Other Worlds". Astrobiology. 1 (2): 143–160. Bibcode:2001AsBio. 1..143I. doi:10.1089/153110701753198918. PMID12467118.
  182. ^
  183. Dyches, Preston Brown, Dwayne (12 May 2015). "NASA Research Reveals Europa's Mystery Dark Material Could Be Sea Salt". NASA . Retrieved 12 May 2015 .
  184. ^
  185. "NASA's Hubble Observations Suggest Underground Ocean on Jupiter's Largest Moon". NASA News. 12 March 2015 . Retrieved 15 March 2015 .
  186. ^
  187. Clavin, Whitney (1 May 2014). "Ganymede May Harbor 'Club Sandwich' of Oceans and Ice". NASA. Jet Propulsion Laboratory . Retrieved 1 May 2014 .
  188. ^
  189. Vance, Steve Bouffard, Mathieu Choukroun, Mathieu Sotina, Christophe (12 April 2014). "Ganymede's internal structure including thermodynamics of magnesium sulfate oceans in contact with ice". Planetary and Space Science. 96: 62–70. Bibcode:2014P&SS. 96. 62V. doi:10.1016/j.pss.2014.03.011.
  190. ^
  191. "Video (00:51) – Jupiter's 'Club Sandwich' Moon". NASA. 1 May 2014 . Retrieved 2 May 2014 .
  192. ^
  193. Chang, Kenneth (12 March 2015). "Suddenly, It Seems, Water Is Everywhere in Solar System". The New York Times . Retrieved 12 March 2015 .
  194. ^
  195. Kuskov, O. L. Kronrod, V. A. (2005). "Internal structure of Europa and Callisto". Icarus. 177 (2): 550–569. Bibcode:2005Icar..177..550K. doi:10.1016/j.icarus.2005.04.014.
  196. ^
  197. Showman, Adam P. Malhotra, Renu (1999). "The Galilean Satellites" (PDF) . Science. 286 (5437): 77–84. doi:10.1126/science.286.5437.77. PMID10506564.
  198. ^
  199. Hsiao, Eric (2004). "Possibility of Life on Europa" (PDF) . University of Victoria.
  200. ^Europa may be home to alien life. Melissa Hogenboom, BBC News. 26 March 2015.
  201. ^
  202. Atkinson, Nancy (2009). "Europa Capable of Supporting Life, Scientist Says". Universe Today . Retrieved 18 August 2011 .
  203. ^
  204. Plait, Phil (17 November 2011). "Huge lakes of water may exist under Europa's ice". Discover. Bad Astronomy Blog.
  205. ^
  206. "Scientists Find Evidence for "Great Lake" on Europa and Potential New Habitat for Life". The University of Texas at Austin. 16 November 2011.
  207. ^ ab
  208. Cook, Jia-Rui C. (11 December 2013). "Clay-Like Minerals Found on Icy Crust of Europa". NASA . Retrieved 11 December 2013 .
  209. ^
  210. Wall, Mike (5 March 2014). "NASA hopes to launch ambitious mission to icy Jupiter moon". Space.com . Retrieved 15 April 2014 .
  211. ^
  212. Clark, Stephen (14 March 2014). "Economics, water plumes to drive Europa mission study". Spaceflight Now . Retrieved 15 April 2014 .
  213. ^ ab
  214. Than, Ker (13 September 2005). "Scientists Reconsider Habitability of Saturn's Moon". Space.com.
  215. ^ ab
  216. Britt, Robert Roy (28 July 2006). "Lakes Found on Saturn's Moon Titan". Space.com.
  217. ^ ab
  218. "Lakes on Titan, Full-Res: PIA08630". 24 July 2006. Archived from the original on 29 September 2006.
  219. ^
  220. Coustenis, A. et al. (March 2009). "TandEM: Titan and Enceladus mission". Experimental Astronomy. 23 (3): 893–946. Bibcode:2009ExA. 23..893C. doi: 10.1007/s10686-008-9103-z .
  221. ^
  222. Lovett, Richard A. (31 May 2011). "Enceladus named sweetest spot for alien life". Nature. doi:10.1038/news.2011.337 . Retrieved 3 June 2011 .
  223. ^
  224. Czechowski, L (2018). "Enceladus as a place of origin of life in the Solar System". Geological Quarterly. 61 (1). doi: 10.7306/gq.1401 .
  225. ^
  226. "What is Consuming Hydrogen and Acetylene on Titan?". NASA/JPL. 2010. Archived from the original on 29 June 2011 . Retrieved 6 June 2010 .
  227. ^
  228. Strobel, Darrell F. (2010). "Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions". Icarus. 208 (2): 878–886. Bibcode:2010Icar..208..878S. doi:10.1016/j.icarus.2010.03.003.
  229. ^
  230. McKay, C. P. Smith, H. D. (2005). "Possibilities for methanogenic life in liquid methane on the surface of Titan". Icarus. 178 (1): 274–276. Bibcode:2005Icar..178..274M. doi:10.1016/j.icarus.2005.05.018.
  231. ^
  232. Hoyle, Fred (1982). Evolution from Space (The Omni Lecture) and Other Papers on the Origin of Life. Enslow. pp. 27–28. ISBN978-0-89490-083-9 .
  233. Hoyle, Fred Wickramasinghe, Chandra (1984). Evolution from Space: A Theory of Cosmic Creationism. Simon & Schuster. ISBN978-0-671-49263-2 .
  234. ^
  235. Hoyle, Fred (1985). Living Comets. Cardiff: University College, Cardiff Press.
  236. ^
  237. Wickramasinghe, Chandra (June 2011). "Viva Panspermia". The Observatory. Bibcode:2011Obs. 131..130W.
  238. ^
  239. Wesson, P (2010). "Panspermia, Past and Present: Astrophysical and Biophysical Conditions for the Dissemination of Life in Space". Sp. Sci.Rev. 1–4. 156 (1–4): 239–252. arXiv: 1011.0101 . Bibcode:2010SSRv..156..239W. doi:10.1007/s11214-010-9671-x. S2CID119236576.
  240. ^ ab
  241. Hussmann, Hauke Sohl, Frank Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects". Icarus. 185 (1): 258–273. Bibcode:2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005.
  242. ^
  243. Choi, Charles Q. (10 June 2010). "The Chance for Life on Io". Astrobiology Magazine . Retrieved 25 May 2013 .
  244. ^
  245. Chang, Kenneth Stirone, Shannon (8 February 2021). "Life on Venus? The Picture Gets Cloudier - Despite doubts from many scientists, a team of researchers who said they had detected an unusual gas in the planet's atmosphere were still confident of their findings". The New York Times . Retrieved 8 February 2021 .
  246. ^
  247. Cofield, Calla Chou, Felicia (25 June 2018). "NASA Asks: Will We Know Life When We See It?". NASA . Retrieved 26 June 2018 .
  248. ^
  249. Nightingale, Sarah (25 June 2018). "UCR Team Among Scientists Developing Guidebook for Finding Life Beyond Earth". UCR Today. University of California, Riverside . Retrieved 26 June 2018 .
  250. ^ ab
  251. Crenson, Matt (6 August 2006). "Experts: Little Evidence of Life on Mars". Associated Press. Archived from the original on 16 April 2011 . Retrieved 8 March 2011 .
  252. ^ ab
  253. McKay, David S. Gibson, Everett K., Jr. Thomas-Keprta, Kathie L. Vali, Hojatollah Romanek, Christopher S. et al. (August 1996). "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001". Science. 273 (5277): 924–930. Bibcode:1996Sci. 273..924M. doi:10.1126/science.273.5277.924. PMID8688069. S2CID40690489.
  254. ^
  255. Webster, Guy (27 February 2014). "NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars". NASA . Retrieved 27 February 2014 .
  256. ^
  257. Gannon, Megan (28 February 2014). "Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life". Space.com . Retrieved 28 February 2014 .
  258. ^ ab
  259. Chambers, Paul (1999). Life on Mars The Complete Story. London: Blandford. ISBN978-0-7137-2747-0 .
  260. ^
  261. Klein, Harold P. Levin, Gilbert V. Levin, Gilbert V. Oyama, Vance I. Lederberg, Joshua Rich, Alexander Hubbard, Jerry S. Hobby, George L. Straat, Patricia A. Berdahl, Bonnie J. Carle, Glenn C. Brown, Frederick S. Johnson, Richard D. (1 October 1976). "The Viking Biological Investigation: Preliminary Results". Science. 194 (4260): 99–105. Bibcode:1976Sci. 194. 99K. doi:10.1126/science.194.4260.99. PMID17793090. S2CID24957458.
  262. ^
  263. Beegle, Luther W. Wilson, Michael G. Abilleira, Fernando Jordan, James F. Wilson, Gregory R. (August 2007). "A Concept for NASA's Mars 2016 Astrobiology Field Laboratory". Astrobiology. 7 (4): 545–577. Bibcode:2007AsBio. 7..545B. doi:10.1089/ast.2007.0153. PMID17723090.
  264. ^
  265. "ExoMars rover". ESA . Retrieved 14 April 2014 .
  266. ^
  267. Berger, Brian (16 February 2005). "Exclusive: NASA Researchers Claim Evidence of Present Life on Mars". Space.com.
  268. ^
  269. "NASA denies Mars life reports". spacetoday.net. 19 February 2005.
  270. ^
  271. Spotts, Peter N. (28 February 2005). "Sea boosts hope of finding signs of life on Mars". The Christian Science Monitor . Retrieved 18 December 2006 .
  272. ^
  273. Chow, Dennis (22 July 2011). "NASA's Next Mars Rover to Land at Huge Gale Crater". Space.com . Retrieved 22 July 2011 .
  274. ^
  275. Amos, Jonathan (22 July 2011). "Mars rover aims for deep crater". BBC News . Retrieved 22 July 2011 .
  276. ^
  277. Glaser, Linda (27 January 2015). "Introducing: The Carl Sagan Institute". Archived from the original on 27 February 2015 . Retrieved 11 May 2015 .
  278. ^
  279. "Carl Sagan Institute – Research". May 2015 . Retrieved 11 May 2015 .
  280. ^
  281. Cofield, Calla (30 March 2015). "Catalog of Earth Microbes Could Help Find Alien Life". Space.com . Retrieved 11 May 2015 .
  282. ^
  283. Callahan, M.P. Smith, K.E. Cleaves, H.J. Ruzica, J. Stern, J.C. Glavin, D.P. House, C.H. Dworkin, J.P. (11 August 2011). "Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases". Proceedings of the National Academy of Sciences. 108 (34): 13995–13998. Bibcode:2011PNAS..10813995C. doi:10.1073/pnas.1106493108. PMC3161613 . PMID21836052.
  284. ^
  285. Steigerwald, John (8 August 2011). "NASA Researchers: DNA Building Blocks Can Be Made in Space". NASA . Retrieved 10 August 2011 .
  286. ^
  287. "DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests". ScienceDaily. 9 August 2011 . Retrieved 9 August 2011 .
  288. ^ ab
  289. Chow, Denise (26 October 2011). "Discovery: Cosmic Dust Contains Organic Matter from Stars". Space.com . Retrieved 26 October 2011 .
  290. ^
  291. "Astronomers Discover Complex Organic Matter Exists Throughout the Universe". ScienceDaily. 26 October 2011 . Retrieved 27 October 2011 .
  292. ^
  293. Kwok, Sun Zhang, Yong (26 October 2011). "Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features". Nature. 479 (7371): 80–3. Bibcode:2011Natur.479. 80K. doi:10.1038/nature10542. PMID22031328. S2CID4419859.
  294. ^
  295. Than, Ker (29 August 2012). "Sugar Found in Space". National Geographic . Retrieved 31 August 2012 .
  296. ^
  297. "Sweet! Astronomers spot sugar molecule near star". Associated Press. 29 August 2012 . Retrieved 31 August 2012 .
  298. ^
  299. Jørgensen, Jes K. Favre, Cécile Bisschop, Suzanne E. Bourke, Tyler L. van Dishoeck, Ewine F. Schmalzl, Markus (September 2012). "Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA" (PDF) . The Astrophysical Journal Letters. 757 (1). L4. arXiv: 1208.5498 . Bibcode:2012ApJ. 757L. 4J. doi:10.1088/2041-8205/757/1/L4. S2CID14205612.
  300. ^
  301. Schenkel, Peter (May–June 2006). "SETI Requires a Skeptical Reappraisal". Skeptical Inquirer . Retrieved 28 June 2009 .
  302. ^
  303. Moldwin, Mark (November 2004). "Why SETI is science and UFOlogy is not". Skeptical Inquirer. Archived from the original on 13 March 2009.
  304. ^
  305. "The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum". The Columbus Optical SETI Observatory.
  306. ^
  307. Whitmire, Daniel P. Wright, David P. (April 1980). "Nuclear waste spectrum as evidence of technological extraterrestrial civilizations". Icarus. 42 (1): 149–156. Bibcode:1980Icar. 42..149W. doi:10.1016/0019-1035(80)90253-5.
  308. ^
  309. "Discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy exoplanet". IAP.fr. 25 January 2006.
  310. ^
  311. Than, Ker (24 April 2007). "Major Discovery: New Planet Could Harbor Water and Life". Space.com.
  312. ^ ab
  313. Schneider, Jean (10 September 2011). "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopaedia . Retrieved 30 January 2012 .
  314. ^
  315. Wall, Mike (4 April 2012). "NASA Extends Planet-Hunting Kepler Mission Through 2016". Space.com.
  316. ^
  317. "NASA – Kepler". Archived from the original on 5 November 2013 . Retrieved 4 November 2013 .
  318. ^
  319. Harrington, J. D. Johnson, M. (4 November 2013). "NASA Kepler Results Usher in a New Era of Astronomy".
  320. ^
  321. Tenenbaum, P. Jenkins, J. M. Seader, S. Burke, C. J. Christiansen, J. L. Rowe, J. F. Caldwell, D. A. Clarke, B. D. Li, J. Quintana, E. V. Smith, J. C. Thompson, S. E. Twicken, J. D. Borucki, W. J. Batalha, N. M. Cote, M. T. Haas, M. R. Hunter, R. C. Sanderfer, D. T. Girouard, F. R. Hall, J. R. Ibrahim, K. Klaus, T. C. McCauliff, S. D. Middour, C. K. Sabale, A. Uddin, A. K. Wohler, B. Barclay, T. Still, M. (2013). "Detection of Potential Transit Signals in the First 12 Quarters of Kepler Mission Data". The Astrophysical Journal Supplement Series. 206 (1): 5. arXiv: 1212.2915 . Bibcode:2013ApJS..206. 5T. doi:10.1088/0067-0049/206/1/5.
  322. ^
  323. "My God, it's full of planets! They should have sent a poet" (Press release). Planetary Habitability Laboratory, University of Puerto Rico at Arecibo. 3 January 2012.
  324. ^
  325. Santerne, A. Díaz, R. F. Almenara, J.-M. Lethuillier, A. Deleuil, M. Moutou, C. (2013). "Astrophysical false positives in exoplanet transit surveys: Why do we need bright stars?". Sf2A-2013: Proceedings of the Annual Meeting of the French Society of Astronomy and Astrophysics: 555. arXiv: 1310.2133 . Bibcode:2013sf2a.conf..555S.
  326. ^
  327. Cassan, A. et al. (11 January 2012). "One or more bound planets per Milky Way star from microlensing observations". Nature. 481 (7380): 167–169. arXiv: 1202.0903 . Bibcode:2012Natur.481..167C. doi:10.1038/nature10684. PMID22237108. S2CID2614136.
  328. ^
  329. Sanders, R. (4 November 2013). "Astronomers answer key question: How common are habitable planets?". newscenter.berkeley.edu.
  330. ^
  331. Petigura, E. A. Howard, A. W. Marcy, G. W. (2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences. 110 (48): 19273–19278. arXiv: 1311.6806 . Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC3845182 . PMID24191033.
  332. ^
  333. Strigari, L. E. Barnabè, M. Marshall, P. J. Blandford, R. D. (2012). "Nomads of the Galaxy". Monthly Notices of the Royal Astronomical Society. 423 (2): 1856–1865. arXiv: 1201.2687 . Bibcode:2012MNRAS.423.1856S. doi:10.1111/j.1365-2966.2012.21009.x. S2CID119185094. estimates 700 objects >10 −6 solar masses (roughly the mass of Mars) per main-sequence star between 0.08 and 1 Solar mass, of which there are billions in the Milky Way.
  334. ^
  335. Chang, Kenneth (24 August 2016). "One Star Over, a Planet That Might Be Another Earth". The New York Times . Retrieved 4 September 2016 .
  336. ^
  337. "DENIS-P J082303.1-491201 b". Caltech . Retrieved 8 March 2014 .
  338. ^
  339. Sahlmann, J. Lazorenko, P. F. Ségransan, D. Martín, E. L. Queloz, D. Mayor, M. Udry, S. (August 2013). "Astrometric orbit of a low-mass companion to an ultracool dwarf". Astronomy & Astrophysics. 556: 133. arXiv: 1306.3225 . Bibcode:2013A&A. 556A.133S. doi:10.1051/0004-6361/201321871. S2CID119193690.
  340. ^
  341. Aguilar, David A. Pulliam, Christine (25 February 2013). "Future Evidence for Extraterrestrial Life Might Come from Dying Stars". Harvard-Smithsonian Center for Astrophysics. Release 2013-06 . Retrieved 9 June 2017 .
  342. ^ ab
  343. Borenstein, Seth (19 October 2015). "Hints of life on what was thought to be desolate early Earth". Excite. Yonkers, NY: Mindspark Interactive Network. Associated Press. Archived from the original on 23 October 2015 . Retrieved 8 October 2018 .
  344. ^
  345. Bell, Elizabeth A. Boehnike, Patrick Harrison, T. Mark et al. (19 October 2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon" (PDF) . Proc. Natl. Acad. Sci. U.S.A. 112 (47): 14518–21. Bibcode:2015PNAS..11214518B. doi:10.1073/pnas.1517557112. ISSN1091-6490. PMC4664351 . PMID26483481 . Retrieved 20 October 2015 . Early edition, published online before print.
  346. ^
  347. "Are there really 36 alien civilizations out there? Well, maybe". Live Science. 16 June 2020.
  348. ^
  349. "There could be 36 communicating intelligent civilizations in our galaxy, study says". CNN. 16 June 2020.
  350. ^
  351. "Chapter 3 – Philosophy: "Solving the Drake Equation". SETI League. December 2002 . Retrieved 24 July 2015 .
  352. ^
  353. Burchell, M. J. (2006). "W(h)ither the Drake equation?". International Journal of Astrobiology. 5 (3): 243–250. Bibcode:2006IJAsB. 5..243B. doi:10.1017/S1473550406003107. S2CID121060763.
  354. ^
  355. Aguirre, L. (1 July 2008). "The Drake Equation". Nova ScienceNow. PBS . Retrieved 7 March 2010 .
  356. ^
  357. Cohen, Jack Stewart, Ian (2002). "Chapter 6: What does a Martian look like?". Evolving the Alien: The Science of Extraterrestrial Life. Hoboken, NJ: John Wiley and Sons. ISBN978-0-09-187927-3 .
  358. ^
  359. Temming, M. (18 July 2014). "How many galaxies are there in the universe?". Sky & Telescope . Retrieved 17 December 2015 .
  360. ^
  361. Marcy, G. Butler, R. Fischer, D. et al. (2005). "Observed Properties of Exoplanets: Masses, Orbits and Metallicities". Progress of Theoretical Physics Supplement. 158: 24–42. arXiv: astro-ph/0505003 . Bibcode:2005PThPS.158. 24M. doi:10.1143/PTPS.158.24. S2CID16349463. Archived from the original on 2 October 2008.
  362. ^
  363. Swift, Jonathan J. Johnson, John Asher Morton, Timothy D. Crepp, Justin R. Montet, Benjamin T. et al. (January 2013). "Characterizing the Cool KOIs. IV. Kepler-32 as a Prototype for the Formation of Compact Planetary Systems throughout the Galaxy". The Astrophysical Journal. 764 (1). 105. arXiv: 1301.0023 . Bibcode:2013ApJ. 764..105S. doi:10.1088/0004-637X/764/1/105. S2CID43750666.
  364. ^
  365. "100 Billion Alien Planets Fill Our Milky Way Galaxy: Study". Space.com. 2 January 2013. Archived from the original on 3 January 2013 . Retrieved 10 March 2016 .
  366. ^
  367. "Alien Planets Revealed". Nova. Season 41. Episode 10. 8 January 2014. Event occurs at 50:56.
  368. ^
  369. Overbye, Dennis (3 August 2015). "The Flip Side of Optimism About Life on Other Planets". The New York Times . Retrieved 29 October 2015 .
  370. ^
  371. "Who discovered that the Sun was a Star?". Stanford Solar Center.
  372. ^
  373. Mukundchandra G. Raval (2016). Meru: The Center of our Earth. Notion Press. ISBN978-1-945400-10-0 .
  374. ^
  375. Crowe, Michael J. (1999). The Extraterrestrial Life Debate, 1750–1900. Courier Dover Publications. ISBN978-0-486-40675-6 .
  376. ^
  377. Wiker, Benjamin D. (4 November 2002). "Alien Ideas: Christianity and the Search for Extraterrestrial Life". Crisis Magazine. Archived from the original on 10 February 2003.
  378. ^
  379. Irwin, Robert (2003). The Arabian Nights: A Companion. Tauris Parke Paperbacks. p. 204 & 209. ISBN978-1-86064-983-7 .
  380. ^ David A. Weintraub (2014). "Islam," Religions and Extraterrestrial Life (pp 161–168). Springer International Publishing.
  381. ^
  382. de Fontenelle, Bernard le Bovier (1990). Conversations on the Plurality of Worlds. trans. H. A. Hargreaves. University of California Press. ISBN978-0-520-91058-4 .
  383. ^
  384. "Flammarion, (Nicolas) Camille (1842–1925)". The Internet Encyclopedia of Science.
  385. ^
  386. "Giordano Bruno: On the Infinite Universe and Worlds (De l'Infinito Universo et Mondi) Introductory Epistle: Argument of the Third Dialogue". Archived from the original on 13 October 2014 . Retrieved 4 October 2014 .
  387. ^
  388. "Rheita.htm". cosmovisions.com.
  389. ^
  390. Evans, J. E. Maunder, E. W. (June 1903). "Experiments as to the actuality of the "Canals" observed on Mars". Monthly Notices of the Royal Astronomical Society. 63 (8): 488–499. Bibcode:1903MNRAS..63..488E. doi: 10.1093/mnras/63.8.488 .
  391. ^
  392. Wallace, Alfred Russel (1907). Is Mars Habitable? A Critical Examination of Professor Lowell's Book "Mars and Its Canals," With an Alternative Explanation. London: Macmillan. OCLC8257449.
  393. ^
  394. Chambers, Paul (1999). Life on Mars The Complete Story. London: Blandford. ISBN978-0-7137-2747-0 .
  395. ^
  396. Cross, Anne (2004). "The Flexibility of Scientific Rhetoric: A Case Study of UFO Researchers". Qualitative Sociology. 27 (1): 3–34. doi:10.1023/B:QUAS.0000015542.28438.41. S2CID144197172.
  397. ^
  398. Ailleris, Philippe (January–February 2011). "The lure of local SETI: Fifty years of field experiments". Acta Astronautica. 68 (1–2): 2–15. Bibcode:2011AcAau..68. 2A. doi:10.1016/j.actaastro.2009.12.011.
  399. ^
  400. "LECTURE 4: MODERN THOUGHTS ON EXTRATERRESTRIAL LIFE". The University of Antarctica . Retrieved 25 July 2015 .
  401. ^
  402. Ward, Peter Brownlee, Donald (2000). Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus. Bibcode:2000rewc.book. W. ISBN978-0-387-98701-9 .
  403. ^
  404. "Hawking warns over alien beings". BBC News. 25 April 2010 . Retrieved 2 May 2010 .
  405. ^
  406. Diamond, Jared M. (2006). "Chapter 12". The Third Chimpanzee: The Evolution and Future of the Human Animal. Harper Perennial. ISBN978-0-06-084550-6 .
  407. ^
  408. "Special Issue: Exoplanets". Science. 3 May 2013 . Retrieved 18 May 2013 .
  409. ^
  410. Chang, Kenneth (17 April 2014). "Scientists Find an 'Earth Twin', or Maybe a Cousin". The New York Times.
  411. ^
  412. Borenstein, Seth (13 February 2015). "Should We Call the Cosmos Seeking ET? Or Is That Risky?". The New York Times. Associated Press. Archived from the original on 14 February 2015.
  413. ^
  414. Ghosh, Pallab (12 February 2015). "Scientist: 'Try to contact aliens ' ". BBC News . Retrieved 12 February 2015 .
  415. ^
  416. "Regarding Messaging To Extraterrestrial Intelligence (METI) / Active Searches For Extraterrestrial Intelligence (Active SETI)". University of California, Berkeley. 13 February 2015 . Retrieved 14 February 2015 .
  417. ^
  418. Katz, Gregory (20 July 2015). "Searching for ET: Hawking to look for extraterrestrial life". Excite!. Associated Press . Retrieved 20 July 2015 .
  419. ^
  420. Matignon, Louis (29 May 2019). "THE FRENCH ANTI-UFO MUNICIPAL LAW OF 1954". Spacelegalissues.com . Retrieved 26 March 2021 .
  421. ^https://www.un.org/press/en/2010/101014_Othman.doc.htm
  422. ^ abc
  423. Larson, Phil (5 November 2011). "Searching for ET, But No Evidence Yet". White House. Archived from the original on 24 November 2011 . Retrieved 6 November 2011 .
  424. ^ abc
  425. Atkinson, Nancy (5 November 2011). "No Alien Visits or UFO Coverups, White House Says". UniverseToday . Retrieved 6 November 2011 .
  426. ^https://time.com/5793520/coronavirus-alien-life/
  427. ^https://tass.ru/kosmos/9160789
  428. ^https://www.japantimes.co.jp/news/2020/04/28/world/science-health-world/pentagon-officially-releases-military-videos-ufos/
  429. ^https://www.dw.com/en/japan-orders-military-pilots-to-report-ufo-sightings/a-55081061
  430. ^https://particle.scitech.org.au/space/china-next-space-superpower/
  431. ^http://www.xinhuanet.com/english/2019-07/11/c_138218290.htm
  432. ^ abhttps://www.newscientist.com/article/dn11443-france-opens-up-its-ufo-files/
  433. ^https://www.bbc.com/news/magazine-29755919
  434. ^https://www.dw.com/en/germany-lacks-plan-in-case-of-alien-contact/a-45126643
  435. ^https://www.timesofisrael.com/israeli-space-chief-says-aliens-may-well-exist-but-they-havent-met-humans/
Wikimedia Commons has media related to Extraterrestrial life .
Wikiquote has quotations related to: Alien life
Wikisource has original works on the topic: Extraterrestrial life
  • Baird, John C. (1987). The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings. Hanover: University Press of New England. ISBN978-0-87451-406-3 .
  • Cohen, Jack Stewart, Ian (2002). Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. ISBN978-0-09-187927-3 .
  • Crowe, Michael J. (1986). The Extraterrestrial Life Debate, 1750–1900. Cambridge. ISBN978-0-521-26305-4 .
  • Crowe, Michael J. (2008). The extraterrestrial life debate Antiquity to 1915: A Source Book. University of Notre Dame Press. ISBN978-0-268-02368-3 .
  • Dick, Steven J. (1984). Plurality of Worlds: The Extraterrestrial Life Debate from Democratis to Kant. Cambridge.
  • Dick, Steven J. (1996). The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science. Cambridge. ISBN978-0-521-34326-8 .
  • Dick, Steven J. (2001). Life on Other Worlds: The 20th Century Extraterrestrial Life Debate. Cambridge. ISBN978-0-521-79912-6 .
  • Dick, Steven J. Strick, James E. (2004). The Living Universe: NASA And the Development of Astrobiology . Rutgers. ISBN978-0-8135-3447-3 .
  • Fasan, Ernst (1970). Relations with alien intelligences – the scientific basis of metalaw. Berlin: Berlin Verlag.
  • Goldsmith, Donald (1997). The Hunt for Life on Mars. New York: A Dutton Book. ISBN978-0-525-94336-5 . , "Alone in the Milky Way: Why we are probably the only intelligent life in the galaxy", Scientific American, vol. 319, no. 3 (September 2018), pp. 94–99.
  • Grinspoon, David (2003). Lonely Planets: The Natural Philosophy of Alien Life. HarperCollins. ISBN978-0-06-018540-4 .
  • Lemnick, Michael T. (1998). Other Worlds: The Search for Life in the Universe. New York: A Touchstone Book. Bibcode:1998owsl.book. L.
  • Michaud, Michael (2006). Contact with Alien Civilizations – Our Hopes and Fears about Encountering Extraterrestrials . Berlin: Springer. ISBN978-0-387-28598-6 .
  • Pickover, Cliff (2003). The Science of Aliens. New York: Basic Books. ISBN978-0-465-07315-3 .
  • Roth, Christopher F. (2005). Debbora Battaglia (ed.). Ufology as Anthropology: Race, Extraterrestrials, and the Occult. E.T. Culture: Anthropology in Outerspaces. Durham, NC: Duke University Press.
  • Sagan, Carl Shklovskii, I. S. (1966). Intelligent Life in the Universe. Random House.
  • Sagan, Carl (1973). Communication with Extraterrestrial Intelligence. MIT Press. ISBN978-0-262-19106-7 .
  • Ward, Peter D. (2005). Life as we do not know it-the NASA search for (and synthesis of) alien life. New York: Viking. ISBN978-0-670-03458-1 .
  • Tumminia, Diana G. (2007). Alien Worlds – Social and Religious Dimensions of Extraterrestrial Contact . Syracuse: Syracuse University Press. ISBN978-0-8156-0858-5 .

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Is There Anybody out There?

About 2,000 years ago, just before the start of the Common Era, the Romans conquered Spain. The Roman Empire was powered by money, and the currency of the time was silver. Fortunately for the Romans, there were an ample number of silver mines in their new Spanish territory.

It takes a lot of energy to smelt silver into coins, so the Romans cut down vast swaths of Spain's forests to burn the wood for fuel. A byproduct of the smelting process is lead, which the Romans used for plumbing. For the first time, our species was engaged in large-scale industrial manufacturing—and also large-scale pollution. Signs of all this can be found in Greenland ice cores.

Pete Worden is the executive director of Breakthrough Initiatives, which funds efforts to search for life beyond Earth. He recently told me Roman silver mining is arguably the first time humans' impact on the planet was noticeable from outer space.

"If you were sitting at a nearby star and had the ability to take a spectrum of the atmosphere, with technology that we can imagine in the next few decades, you would detect these things that are at least, from our understanding, clearly industrial pollutants," he said.

A popular science fiction notion, as portrayed in the novel and movie Contact, by Planetary Society co-founder Carl Sagan, is that intelligent life might pick up our stray TV transmissions. But that's not possible with Earthling-level technology. If aliens in orbit around Proxima Centauri, our nearest stellar neighbor, broadcasted us episodes of I Love Lucy, we wouldn't hear them, unless they put a lot more power in their transmitters than ours.

We are, however, on the verge of being able to pick up missile detection radar-level signals. And if something as noisy as the Arecibo Observatory planetary radar in Puerto Rico, which is used to zap near-Earth asteroids, was aimed in our direction, we'd definitely hear—assuming we were listening and pointing in the right direction.

But in the end, it might not be our radio traffic that gives us away. Intelligent beings might already know we're here, thanks to the way we've tinkered with our planet's ecosystem.

Crescent Earth from Rosetta Rosetta viewed Earth in a thin crescent phase as it approached for its November 13, 2009 flyby. This image is one in a series taken to make an animation of the rotating Earth over a 24-hour period. Image: ESA / OSIRIS Team MPS / UPD / LAM / IAA / RSSD / INTA / UPM / DASP / IDA / Gordan Ugarković

The question of whether or not we are alone in the universe lies at the heart of many reasons we explore space. But for more than half a century, one branch of science has tried to answer the question more directly. SETI, the search for extraterrestrial intelligence, started as a fringe science, surged to a taxpayer-funded endeavor and receded into a privately funded effort.

The field's history involves semi-secret meetings, blustering congressional representatives and unexplained signal detections. Now, a surge of cash has given SETI new life. New arrays of powerful radio telescopes rising in South Africa and Australia could help revolutionize the field. Meanwhile, other upcoming projects promise to realize the dream of watching the entire sky for signals, all of the time.

Is there anybody out there? We may be closer to the answer today than ever before.

The story of modern SETI begins in 1959, when Cornell University scientists Giuseppe Cocconi and Philip Morrison published a paper in Nature titled "Searching for Interstellar Communications." The paper opens with a simple assumption: If intelligent beings know we're here, they might try to get in touch. This premise remains one of the bedrocks of the field.

What sort of beacon would extraterrestrials use? The electromagnetic spectrum is large, ranging from low-frequency radio waves to high-frequency gamma rays. Fortunately, there's a practical limitation that narrows the possibilities: Earth's atmosphere blocks large portions of the spectrum. Cocconi and Morrison reasoned an advanced civilization would recognize our limitations and transmit something we could detect on the ground.

Furthermore, since high-frequency wavelengths require more transmitting power, Cocconi and Morrison believed a good place to search would be in the radio and microwave spectrum, between 1 and 10,000 megahertz. From FM radio to X-band spacecraft communications, this is indeed where we humans do most of our over-the-air communications.

The atmosphere's effect on electromagnetic radiation Earth's atmosphere prevents large chunks of the electromagnetic spectrum from reaching the ground, providing a natural limit on where ground-based observatories can search for SETI signals. Image: The Planetary Society

Modern, digitally equipped radio telescopes can listen to large swaths of the spectrum simultaneously. But early on, analog receivers were limited to small slices at a time. To narrow the search further, Cocconi and Morrison turned to the universal language of science. Hydrogen, the lightest and most abundant chemical element in the cosmos, emits photons at frequencies of 1,420 megahertz. Later, a cluster of hydroxyl photon emission frequencies around 1,660 megahertz were proposed. When hydrogen and hydroxyl combine, they form H2O—water. Since life as we know it requires water, the region between these two frequencies became known as the water hole, a proverbial place for galactic citizens to meet in the desert of space. It was a frequent target of early SETI scans, and most modern searches still include it.

The same year Cocconi and Morrison published their landmark paper, Frank Drake, a staff astronomer at the National Radio Astronomy Observatory in Green Bank, West Virginia, independently prepared to conduct the first SETI search using the institution's new 26-meter Tatel radio telescope. The search was dubbed Project Ozma, after a fictional princess from the Land of Oz, and started scanning the water hole for signals from nearby stars Tau Ceti and Epsilon Eridani in 1960.

One year later, the National Academy of Sciences hosted an invitation-only meeting at Green Bank to discuss how to go about conducting further SETI research. The eclectic, interdisciplinary group included Drake, Cocconi, Morrison, the biochemist Melvin Calvin (who won the Nobel Prize in Chemistry during the meeting), Bernard Oliver, who was the vice president of research and development at Hewlett-Packard, the young Carl Sagan, and the scientist John Lilly, who had recently published a controversial book arguing dolphins were an intelligent species.

With a nod to Lilly's book, the participants dubbed themselves "The Order of the Dolphin." One product of the meeting was the Drake equation, which attempts to predict the number of advanced civilizations in the Milky Way able to contact Earth. The equation includes variables such as average star formation rate, the number of habitable planets per star, and the number of planets where intelligent life could evolve.

After 200 hours of observing, Project Ozma came up empty. But the field of SETI was officially born.

SETI close calls: LGM-1 Image: The Planetary Society

For the rest of the 1960s, SETI research remained mostly dormant, aside from a few searches in the Soviet Union. Starting in 1971, two Project Ozma follow-ups named Ozpa and Ozma II used bigger dishes and listened to more stars.

In 1973, another SETI search began, using a radio telescope called Big Ear at Ohio State University. Big Ear was a flat, aluminum dish three football fields wide, with reflectors at both ends.

On the night of August 15, 1977, Big Ear picked up a signal from the constellation Sagittarius that was 30 times stronger than the cosmic background noise, right at the 1,420 megahertz hydrogen line frequency. No one noticed it for a few days, until a volunteer sifting through the previous week's data circled the signal and wrote "Wow!" in the margin.

The Wow! signal On the night of August 15, 1977, Ohio State's Big Ear radio telescope picked up a signal from the constellation Sagittarius that was 30 times stronger than the cosmic background noise. Image: Courtesy of the Ohio History Connection

Jill Tarter is a legendary SETI scientist. She's the Bernard Oliver Chair for SETI at the SETI Institute in Mountain View, California, and is the inspiration for protagonist Ellie Arroway in Contact, played by Jodie Foster in the film adaptation.

Tarter told me Big Ear's automated search program had no built-in logic to stop and focus on the Wow! signal. Furthermore, there was no confirmation system such as a second telescope located elsewhere, which could help determine whether the signal was local to Earth or truly from the stars.

Despite decades of follow-up searches, the Wow! signal was never heard again. To this day, Tarter favors SETI programs that can process data in real time, rapidly follow-up on detections, and rule out local interference.

I asked her if the Wow! signal still haunts the SETI field. "Well, it haunts me," she said.

NASA gets involved

By the end of the 1970s, NASA got involved, giving the field a huge boost in credibility. The agency planned to conduct SETI research on two fronts. A systematic, all-sky search would be led by NASA's Jet Propulsion Laboratory, primarily using the agency's Deep Space Network facilities. JPL director Bruce Murray, who co-founded The Planetary Society in 1980, was a strong proponent of this approach.

At the same time, NASA's Ames Research Center would examine nearby stars similar to our Sun, using Arecibo, Green Bank, Parkes Observatory in Australia, and Nancay Observatory in France. Both JPL and Ames would look at frequencies between 1,000 and 3,000 megahertz, covering a large swath of the spectrum near the water hole.

But before the search began, it was met with political resistance. In 1979, Senator William Proxmire bestowed his ignominious "Golden Fleece Award" on the program, calling it a waste of taxpayer dollars. Three years later, Proxmire got SETI funding on the chopping block. Carl Sagan personally met with the senator to convince him SETI was a worthwhile endeavor, arguing it would spur development of advanced technologies and potentially offer evidence Earthlings can survive what Sagan dubbed our "technological adolescence," should we make contact with other beings. A petition signed by leading world scientists and Nobel laureates bolstered Sagan's argument.

At the time, the program had strong support from the scientific community, said Andrew Siemion, the director of the SETI Research Center at the University of California, Berkeley.

"Looking back at some of the study conferences that were held, before the first time Congress tried to kill the funding, it's just absolutely incredible," he said. "I mean, it's virtually a who's-who of not just astronomy, but of science and technology."

Proxmire backed down, and following another decade of development, NASA's SETI program officially went online in 1992, innocuously re-branded as the High Resolution Microwave Survey.

The SETI Institute

NASA contracted much of its SETI activities to the SETI Institute, a nonprofit formed in 1984. A group of SETI researchers, including John Billingham, who spearheaded some of NASA's initial SETI efforts at Ames, grew irked that the academic researchers NASA hired came with high overhead costs, in the form of extra percentages paid to researchers' universities. In the case of Stanford University, Tarter said, the overhead rate was 100 percent—meaning every dollar paid for a researcher's time required another dollar paid to Stanford.

Billingham had an idea: What if an independent institution could hire the researchers directly, charge less, and then apply for the same grants?

"Our whole motive was to save NASA money," said Tarter, who became the SETI Institute's first employee. "We could actually set our own overhead rate at the true cost of doing business. As opposed to 100 percent, it was more like 20 percent." The cost savings were funneled into building hardware for SETI searches.

But the NASA-led search was barely underway when it came under fire again. This time, Nevada Senator Richard Bryan attacked the program. Once more, Sagan and others prepared to lobby on NASA's behalf, but Bryan outright refused to meet with anyone involved with SETI. In 1993, after a total investment of $60 million and just one year of operations, the High Resolution Microwave Survey was canceled.

"This hopefully will be the end of Martian hunting season at the taxpayer's expense," said a press release from Bryan.

Jill Tarter at Arecibo Observatory Image: Louis Psihoyos / Courtesy of Jill Tarter

The SETI Institute scrambled to pick up the pieces. Fortunately, said Tarter, the proposals for observing time at Arecibo and Parkes had already been peer-reviewed and accepted.

"I then wrote to all of the (observatories) on which we had been granted time and said, 'If we can come up with the money to keep this project going, can we still have the time?' They all responded yes," she said.

The Ames portion of the SETI program rose from the ashes as Project Phoenix, a suite of equipment that could be connected to large radio dishes like Arecibo, Parkes, and Green Bank. From 1995 through 2004, Project Phoenix scanned Sun-like stars at a frequency between 1,000 and 3,000 megahertz.

The JPL all-sky search using NASA's Deep Space Network, however, could not be salvaged. A final attempt to install SETI equipment at NASA's Deep Space Network facility in Goldstone, California, which would passively collect data from wherever the telescope aimed, was swiftly rejected.

"NASA came back and said, 'Oh no you won't.' They really shut it down hard," said Tarter. "Bryan had done this with such vengeance that we became the four-letter-S-word you couldn't say at NASA headquarters."

The Planetary Society

In 1978, during one of his many appearances on The Tonight Show, Planetary Society co-founder Carl Sagan discussed SETI at length with host Johnny Carson. In a pre-Internet era where most Americans only had a few channels, Sagan and Carson spent 15 minutes on prime-time television discussing everything from Star Wars ("I felt very bad that, at the end, the Wookiee didn't get a medal," Sagan said) to how aliens might send us a signal using prime numbers.

"The remarkable thing is, for all the history of mankind, people have wondered about intelligence elsewhere—I think it's in religion and philosophy, legends—but this is the first time that we have the competence and ability to actually do such a search, and we are just beginning," Sagan said.

The Planetary Society's involvement in SETI practically began when the organization was founded in 1980. Just one year later, NASA and the Society funded Suitcase SETI, a portable spectrum analyzer that could be installed on large radio telescopes like Arecibo. Suitcase SETI eventually grew into Sentinel, an all-sky search using a 26-meter radio telescope at Harvard University. Next came META, the Megachannel ExtraTerrestrial Assay, funded with a significant donation from Steven Spielberg, who was then a Planetary Society board member.

These projects were led by Paul Horowitz, a Harvard physicist and electrical engineer. Horowitz said META was able to pick through 8 million slices of radio frequency at a time, making it the most advanced SETI search ever when it came online in 1985. Yet when compared to modern processing capabilities, its performance was paltry.

"I had that thing (the META computer) in a double rack, and at the top, it said, 'META supercomputer—75 million instructions per second,'" Horowitz said. "Now, your cell phone is better than that."

During a decade of operations, META found 37 "candidate events"—strong signals of unknown origin. None have ever repeated. The Society launched a southern hemisphere clone of the project named META II, and META eventually evolved into BETA, which increased the processing capacity of the Harvard telescope to a quarter-billion channels at once, scanning the water hole between 1,400 and 1,700 megahertz. BETA operated until 1999, when a storm damaged the antenna's drive gear.

Flipping the switch Paul Horowitz, Carl Sagan and Steven Spielberg (holding his son, Max) activate The Planetary Society’s META project in 1985. Image: The Planetary Society

Around the same time, Horowitz's group, motivated by Charles Townes, who invented the laser, started tinkering with optical SETI searches. Visible light has a higher frequency than radio waves, allowing more data to be encoded over any given period of time. Like radio waves, visible light also filters through our atmosphere, making it a logical portion of the spectrum for SETI searches.

In 2006, Horowitz and The Planetary Society constructed a 1.8-meter telescope at Harvard which began the first dedicated, all-sky optical SETI survey. The search is still in operation, completing a full survey of the sky visible from Massachusetts every 200 nights.

Meanwhile, in the late 1980s, The Planetary Society, NASA and the National Science Foundation helped fund a west coast SETI effort called SERENDIP at the University of California, Berkeley. SERENDIP, as its name implies, looks for serendipitous SETI detections by piggybacking on traditional astronomical observations made by large radio telescopes. The program has undergone many upgrades and relocations over the years, and was still running at Arecibo when the telescope was damaged by Hurricane Maria in September 2017.

SERENDIP originally processed data in real-time, but Berkeley soon began archiving the data and sifting through them using computer algorithms. There were more data available than could be processed using supercomputers, said Dan Werthimer, who is now chief scientist of the Berkeley SETI Research Center. Werthimer and three other engineers and scientists designed a program to allow home computers to help with the data crunching.

"We had this wild and crazy idea to use volunteers to analyze our data, but we took it around to various people, and nobody seemed to think it would ever work," Werthimer told me. "The Planetary Society said, 'Hey this wild, crazy idea? We want to get behind it.' And they gave us the money to launch the project."

In 1999, Berkeley released the result, [email protected] , and since then more than 8 million people have downloaded the program and donated spare computing power to help search for intelligent life. The open-source software, BOINC, on which [email protected] is based, is now used for other projects. This led to what Werthimer calls "the democratization of supercomputing," where users can choose individual research programs to assist.

Andrew Siemion, the Berkeley SETI Research Center director, credits The Planetary Society's [email protected] involvement for helping keep the field alive prior to his arrival at Berkeley as a student in 2004.

"Science is about standing on the shoulders of giants," he said. "Frankly, we would not be here today were it not for the support specifically of the Planetary Society."

The Allen Telescope Array Image: Seth Shostak / The SETI Institute

The Allen Telescope Array

Before the SETI Institute's resurrected NASA program, Project Phoenix, came to a close in 2004, the group planned for what would come next.

A series of workshops involving scientists, engineers and Silicon Valley computing experts concluded the next step in SETI radio research should be a large array of small telescopes. Signals from small dishes can be combined, ultimately covering larger swaths of the sky than single, large dishes, and smaller dishes are cheaper, often using off-the-shelf hardware.

In 2007, the Allen Telescope Array, named after its benefactor, Microsoft co-founder Paul Allen, went online at Hat Creek Observatory in northern California. Using digital technology to process incoming signals, the array was built with long-term scalability in mind. Early on, it collected more data than could be processed. Now, there is a near-match between data volume and processing power, and by the end of the project, SETI scientists should be data-starved.

The original design called for 350 dishes. But the antennas cost more than predicted, and Berkeley, originally a project partner, could not secure hoped-for operating funds from the National Science Foundation. The project was downsized, and taking a page from the Hitchhiker's Guide to the Galaxy, the SETI Institute stopped construction at 42.

"What number are you going to pick if you can't get to 350, right?" said Tarter.

SETI close calls: The Wow! signal Image: The Planetary Society

Part II: SETI now

Shortly before 2000, Shelley Wright was working on her physics bachelor's degree at the University of California, Santa Cruz. One day, she saw a flyer for an astrobiology conference at NASA's Ames Research Center, and decided to attend.

During lunch at the conference, she unknowingly sat next to SETI experts Frank Drake and Dan Werthimer. She struck up a conversation, grew interested in the field, and ended up with a gig at Lick Observatory near San Jose, California, where she built an optical SETI detector as part of her undergraduate thesis. Drake became one of her advisors.

Many SETI scientists start in subjects like astronomy, astrophysics or engineering, and either work in related areas or find homes at places like the SETI Institute. Wright is now an assistant professor of physics at the University of California, San Diego, where she designs and builds telescope instruments used to study galaxies and black holes.

She also uses her talents for SETI research. Earlier in her career, she did so quietly, "because it was semi-taboo," she told me.

"The truth is, I think people want to work on SETI," Wright said. "It's just that we all have to pay rent. People need a career path, and if there is no government funding for this, it's really challenging."

Among those who do work on SETI, there are generation gaps corresponding to the field's ups and downs. First came the pioneers, like Drake. A second generation is represented by Tarter, Horowitz and Werthimer. The latest group includes Wright and Siemion, but both are quickly becoming mid-career scientists. A fourth generation needs training.

"There aren't a lot of graduate students that work on it," said Siemion. "This is a huge problem, obviously, for the field to kind of keep it going."

Generational gaps occur in other scientific fields, too, Werthimer said. "But SETI is a little more fragile."

In 2009, NASA launched the Kepler space telescope to hunt for planets around other stars, known as exoplanets. Before Kepler, only a handful of exoplanets were confirmed to exist that number has now jumped to more than 3,500, with more than 2,000 confirmed by Kepler alone. Another 4,500 await independent confirmation. So far, scientists have found around 30 Earth-sized exoplanets in their stars' habitable zones, where liquid water could exist.

Kepler made the notion of intelligent beings on other worlds far less abstract. A SETI revolution was ready to happen the field just needed a financial breakthrough.

The breakthrough

In 2012, a Russian billionaire named Yuri Milner announced a new annual award called the Breakthrough Prize, designed to hand out cash to scientists making major contributions to fundamental physics.

Milner, who is named after Yuri Gagarin, the first human to fly in space, was born in Moscow and initially studied physics before becoming a successful technology entrepreneur. In its 2017 iteration, the Breakthrough Prize awarded more than $25 million. It is the largest individual monetary science prize, and has expanded to include life sciences and mathematics.

By 2015, Pete Worden, then director of NASA Ames, heard Milner was interested in expanding his philanthropy to bolster the search for life. Under an umbrella group named Breakthrough Initiatives, Milner's benefaction would be split between SETI observations, characterizing nearby exoplanets, and developing a fleet of miniature, laser-powered sail spacecraft to visit the neighboring Alpha Centauri star system.

As a NASA center director, a lot of Worden's day-to-day work focused on "budget and personnel, and, you know, that the toilets don't work in building 18," he told me. He was happiest when he had time to arm himself with snacks, escape his office and visit scientists and engineers working on missions like Kepler.

When Worden found out about Milner's desire to seek answers to fundamental questions about life in the cosmos, he was intrigued. "Probably since I was a teenager—maybe even younger—I've had these questions on my mind," he said. "And they were clearly on Yuri Milner's mind, and he felt that now was the time to begin a set of initiatives to address those questions."

Worden retired from NASA that same year, and became the executive director of Breakthrough Initiatives. "I'm spending most of my time now on these big questions, which is really cool. I can't believe I'm so lucky," he said.

Yuri Milner Yuri Milner, flanked by Stephen Hawking, announces the Breakthrough Listen initiative in 2015. Image: Stuart C. Wilson / Getty Images / Breakthrough Initiatives

Money dilemma

A large cash influx could drastically alter the SETI landscape. The field's two biggest players were the SETI Institute and the SETI Research Center at Berkeley. At the time, representatives from both groups were working together on an initiative called FIRSST, which sought to create a long-term endowment for SETI research.

"If you don't have institutional funding—and apparently we're not going to be able to get that for SETI—then you need something like an endowment, so that people can take risks and do new things that don't necessarily provide publication next month," said Tarter, who was a FIRSST institutional liaison.

Milner ultimately funneled the funding through Berkeley, under a new program called Breakthrough Listen. Breakthrough would use the giant telescopes at Green Bank and Parkes to scan the nearest 1 million stars—a sphere about 1,000 light years across, covering the round-trip distance where other beings may have noticed our 2,000-year-old pollution habits and tried to get in touch. The program would also scan the Milky Way's galactic plane and the nearest 100 galaxies, while another telescope, the Automated Planet Finder at Lick Observatory, would conduct optical SETI searches.

Berkeley engineers set to work upgrading the digital processing capabilities at Green Bank and Parkes. The university's SETI Research Center would process the data, eventually making some of it available for [email protected] users.

Milner allocated $100 million to Breakthrough Listen, to be spent over a 10-year period. Worden said the annual spending rate varies most recently it was between 6 and 7 million. The FIRSST endowment fizzled, and three SETI Institute representatives, including Tarter, joined the Breakthrough Listen advisory committee. But thus far, no Breakthrough funds have gone to the SETI Institute, or the Allen Array.

"We may just now be finding a way to work gracefully in concert with them," Tarter said. "It's been very disappointing for us, that we were essentially excluded."

A reciprocal arrangement

For the radio observatories participating in the search, Breakthrough Listen was a welcome new funding source.

In 2013, the National Science Foundation announced it would divest the Green Bank Telescope, forcing the observatory to start funding itself. The process was completed in 2016. In Australia, Parkes faces a similar, uncertain future. Parkes declined to say how much money the observatory has received from Breakthrough Listen thus far, while Green Bank did not respond to interview requests. But the funding was clearly welcome Breakthrough is now paying to use a quarter of both telescopes' time. In 2015, Karen O'Neil, who is now the director at Green Bank, told The Planetary Society the new money would "go a long way toward helping secure the long-term future of the facility."

Securing observing arrangements at both locations required contracts with the U.S. and Australian governments, but Worden said the process went remarkably smoothly.

"I worked for the government most of my career, and I never saw the government move so quickly," he said. "We signed an agreement within a few weeks for millions of dollars."

Dedicating a quarter of observing time at Green Bank and Parkes to SETI impacts other telescope users. But somewhat making up for that is a reciprocal arrangement between SETI and traditional radio astronomy. The equipment Breakthrough installs can be used by other observers. At both locations, Berkeley engineers spent about a year installing high-end graphical processing units—the same found in video game cards—to process incoming signals. Those processing units are now "the most powerful digital instruments the telescopes have, full stop," said Siemion.

The arrangement also works in reverse. Through a technology called multibeam receivers, Breakthrough plans to use surplus observing bandwidth during other observations to run SETI searches on nearby swaths of sky. And because Breakthrough openly publishes all its data, SETI observations can be used to make other scientific discoveries.

Jimi Green is a senior systems scientist at CSIRO, Australia's national science agency, which runs Parkes Observatory. When I spoke to him via Skype, he sat in an operations center in Sydney, next to an array of screens used to remotely control the telescope. Green said the Breakthrough search is contributing to the quest to understand a mysterious astronomical phenomenon called Fast Radio Bursts, or FRBs.

FRBs are short, high-energy bursts of radio waves believed to come from outside our galaxy. The first was discovered in 2007, by a student searching archival Parkes data for a different reason. Astronomers aren't sure what causes FRBs—they could be anything from colliding black holes to laser-driven sail spacecraft.

"There's a joke in the community that there are more theories for what they are than there are detections," Green said.

Because SETI searches encompass seemingly random locations across the sky, they uniquely contribute to the hunt for FRBs.

"We don't know enough about (FRBs) yet, like where they come from, what mechanisms generate them, and so on," said Green. "So just doing this sort-of blind searching—wherever (Breakthrough) is looking, we're running at the same time—is a great way to do it." In August, the Breakthrough team, working with a collaboration of international researchers, announced they had detected 15 new pulses from a known FRB at a higher frequency and wider bandwidth than ever before.

The broadest search thus far

In April, Breakthrough Listen published the data from 692 stars observed during initial Green Bank telescope observations, and described the results in an upcoming paper for The Astrophysical Journal. Berkeley scientists also published a corresponding list of 11 "significant events" where radio signals spiked above the cosmic background noise. None were believed to be signals from intelligent beings, but 692 is only a fraction of the million stars the project will survey.

Back at Hat Creek Observatory in northern California, the Allen Array continues its own SETI search. Until recently, the array was looking at Kepler exoplanet candidates, but since it is now clear most stars have planets, the array is scanning the nearest 20,000 stars, most of which are red dwarfs. To pay for operating costs, the array allocates half of its observing time to SRI International, which subcontracts telescope usage for various research and communications projects.

Together, Breakthrough and the SETI Institute are conducting the broadest-ever targeted SETI radio search. The telescopes roughly cover frequencies between 1,000 and 15,000 megahertz NASA's original HRMS program, for comparison, only searched between 1,000 and 3,000 megahertz.

Bill Diamond, the CEO and president of the SETI Institute, said Breakthrough Listen has hardly made the Allen Array redundant. He sees the two efforts as complementary, and notes the Allen Array has its own unique capabilities, including the ability to process data in real time.

"That immediate analysis gives us the opportunity to verify and eliminate false positives," he said.

Tarter said she would still like to find a way to get the array up to 350 dishes, which would increase its sensitivity and allow it to see more of the sky at once.

"I'm not giving up on that one," she said. "That would be one hell of an instrument for both SETI and radio astronomy."

SETI close calls: Tabby's star Image: The Planetary Society

Part III: SETI next

In a 1991 issue of The Planetary Report, Harvard's Paul Horowitz described a discussion he had with SETI pioneer Philip Morrison, and Michael Davis, then director of Arecibo Observatory. At Morrison's house, the trio talked about the future of SETI, theorizing about how to revolutionize the field. Horowitz put forth what he called "a favorite idea," that future radio searches would scrap large, traditional telescopes for massive arrays of small receivers listening to the entire sky all at once.

"There it is," he wrote. "A dry lake bed tiled with glistening purple checkerboards of silicon, quietly receiving radio signals from a multiplicity of directions."

I asked Horowitz if this 26-year-old idea was still feasible. He said it was, but that I shouldn't hold my breath for it to happen anytime soon.

"If someone wanted to put a billion dollars in it, we could probably build such a thing," he said. "It's not harder than getting to the Moon. It's just different."

At Parkes in Australia, Breakthrough Listen is currently surveying the Milky Way's galactic plane. During a single observation, the observatory's 64-meter telescope can listen to an area of sky about three Moons wide at a time. Surveying the galactic plane will take about 1,500 hours, and since Breakthrough only gets about one-fourth of the telescope's observing time, it will take a year to complete the survey. As a result, the telescope can only listen to each slice of sky for five minutes before moving on.

The Planetary Society's 1.8-meter optical SETI telescope at Harvard covers an even smaller area, just slightly larger than the Moon. Though the telescope is allocated entirely to SETI, it still has to fight cloudy Massachusetts nights and takes a year to complete one full sky survey. The total time spent staring at each patch of stars? Forty-eight seconds.

Herein lies the challenge of SETI: If intelligent beings are currently sending us a beacon, it would have to be near-continuous for us to receive it using our current methods.

"If we wanted to do the best possible experiment we could do, we would want to look at every wavelength, every frequency, all the time, in as many ways as we could possibly conceive of," said Andrew Siemion. "For now, we have to have to make choices."

PANO-SETI

Two new optical SETI projects are hoping to make giant leaps forward in our ability to watch the entire sky, all of the time. At the University of California, San Diego, Shelley Wright leads a team developing a system called PANO-SETI. Wright and other SETI experts, including Horowitz, recently spent a year brainstorming a way to continuously monitor the entire sky at once, which Wright calls "the ultimate optical solution."

A single telescope dish focuses and amplifies the light from one patch of sky at a time, creating sharp, high-resolution images of celestial objects. But using large telescopes for SETI is akin to searching the heavens through a soda straw. To watch the entire sky at once, you could build an array of giant telescopes, but that would be unthinkably expensive, and for optical SETI, you don't really need high-resolution images. Instead, you only need to count the number of photons coming through the telescope and see if that changes.

PANO-SETI relies on Fresnel lenses, named after their French inventor, who originally built them for lighthouses. A Fresnel lens takes a patch of sky and converts it into a single point, and a detector behind the lens called a solid state photomultiplier samples the light every nanosecond, watching for changes that could indicate the presence of a laser signal. The sample rate is faster than any known natural phenomenon, such as pulsars or the twinkling effect created by Earth's atmosphere.

Each hexagonal Fresnel lens is a half-meter across, and Wright's team envisions 126 of them mounted on the outside of an observatory dome just taller than a person. Like a fly's eye, the array would continuously sample the sky as it rotates overhead. Just a few of these 126-lens domes scattered around the world could continually monitor the entire celestial sphere.

Two views of a completed PANO-SETI dome Image: Wright et. al (2017)

PANO-SETI covers the entire visible spectrum, and as a bonus, a little bit of infrared. This is another area of Wright's expertise in 2015, she commissioned NIROSETI, the first dedicated near-infrared SETI search. It successfully surveyed 1,000 stars, and Wright's team is preparing to publish the results. The idea of infrared SETI has been around for a long time, but it's expensive because the telescope instruments must be cooled to stop radiant heat from interfering with observations, and the detectors have traditionally been difficult to work with. Nevertheless, infrared is a logical place to search for signals higher-energy visible wavelengths, like blues, scatter easily (this is why our sky is blue), making infrared a compelling choice for would-be extraterrestrial phone calls. (PANO-SETI will not require cooling because it does not dip deep enough into the infrared.)

Right now, Wright's team is testing a single Fresnel lens. They're almost ready to take it out for trial runs, and she also sees potential to use the technology in other fields.

"We're learning a ton of things, which I think are applicable to other applications in astronomy, and other science," she said.

PANO-SETI could detect the most powerful lasers we have on Earth from a distance out to a few thousand light-years. The team has enough funding for design and prototyping, but to make the concept a reality, they'll need additional investment. They pitched the idea to Breakthrough Listen, but have yet to get a response.

Laser SETI

Another advantage of all-sky surveys over targeted searches is that they also capture the void of space between stars and galaxies. If there's something unseen saying hello from the blackness, an all-sky search could pick it up.

Eliot Gillum is the director of the SETI Institute's optical SETI program, which recently introduced an all-sky solution called Laser SETI. Goal-wise, Laser SETI is similar to PANO-SETI the former is less sensitive, but cheaper.

Laser SETI uses off-the-shelf, wide-field camera lenses to monitor the sky. A single observatory has eight cameras, housed in small, horn-shaped enclosures small enough to carry. The concept is relatively inexpensive thanks to advances in computing, 3D printing and electronics, Gillum told me. "You couldn't have done this ten years ago," he said.

Through a device called diffraction grating, Laser SETI splits each point of starlight into threes. As the Earth rotates, the three dots smear across electronic detectors under each lens. The cameras work in pairs to track the points of light in both the horizontal and vertical directions, and the results are fed into a computer that analyzes the trails for brightness changes.

Gillum envisions installing 12 Laser SETI observatories around the world to continually monitor the entire celestial sphere. Each is self-contained, requiring little intervention besides "power and connectivity, and physical security, to make sure somebody doesn't walk away with your very expensive cameras," he said.

In August, the SETI Institute raised more than $100,000 for Laser SETI on the crowdfunding website Indiegogo. Gillum has a working prototype, and said the next step is getting multiple cameras working together. He is in the process of formalizing a partnership with a yet-to-be-disclosed California observatory for testing.

The Square Kilometer Array

To this day, most SETI searches envision intelligent civilizations intentionally contacting us with radio or laser signals. The I Love Lucy scenario—where we accidentally pick up aliens' weak, wayward transmissions—has never been possible.

That will soon change. A giant network of new radio telescopes coming online will be sensitive enough to pick up "leakage" signals equivalent to those we inadvertently beam away from Earth. The telescopes are being built for traditional astronomical research, but SETI scientists are salivating at the chance to use them to search for intelligent life.

The new network is called the Square Kilometer Array, or SKA, and as its name suggests, it will have a combined collecting area of one square kilometer. Like the Allen Array, the SKA combines the signals from multiple dishes one proposal for the final design involves 2,000, 15-meter-wide dishes and a million smaller antennas. If it were a single, circular dish, the SKA would be 1.13 kilometers wide, dwarfing any other telescope on Earth. "We really don't want to make single dishes that big," one astronomer told me. "You lose some efficiency (using smaller dishes), but it's worth it."

The SKA is headquartered at Jodrell Bank Observatory in the U.K., with dishes and antennas located in South Africa and Australia. The project, which won't be completed until at least the end of the 2020s, is so large it has precursor projects—and those precursors have their own precursors.

The first phase, SKA1, is currently underway. In Australia, the precursor Murchison Widefield Array (MWA) consists of 2,048 small antennas, laying the groundwork for a system of 130,000 by the time SKA1 is complete. In South Africa, another precursor called MeerKAT will have 64, 13.5-meter-wide dishes, 16 of which are already online. By itself, MeerKAT will be the largest radio telescope in the Southern Hemisphere when finished, and the dishes will be integrated into a total pool of 200 for SKA1.

Square Kilometre Array An artist’s concept of a portion of the Square Kilometre Array, in South Africa. Image: SKA Organisation

MeerKAT picks up mid-frequency radio waves, including the part of the spectrum traditionally targeted by SETI searches. As part of its commissioning phase, MeerKAT will conduct sky surveys, mapping the structure of the galaxy while hunting for pulsars and FRBs. Pete Worden said Breakthrough Listen is "deep in discussions" with the MeerKAT team, hoping to run piggyback SETI searches at the same time.

"These are next-generation facilities in terms of their sensitivities, and their field of view, and also in terms of the way that we can access them digitally," said Andrew Siemion. "So it's incredibly easy for us to plug instrumentation into them, and that makes it possible for us to use them for SETI very easily."

MWA is a low-frequency array. Low frequencies are not typically targeted by SETI searches because longer wavelengths constrain how much information can be transmitted over any given period of time.

Low-frequency arrays can, however, cover the entire sky all at once. For this reason, scientists like Dan Werthimer are increasingly interested in conducting SETI searches with arrays like MWA, "not necessarily because we think E.T. might be broadcasting there," but because the experience could be used to build higher-wavelength all-sky arrays like Horowitz envisioned.

"The microwave part of the spectrum that most people think about for SETI experiments—that will be a little harder," said Werthimer.

The second phase of the Square Kilometer Array, SKA-2, includes a mid-frequency array called MFAA. One of its proposed precursors, MANTIS, would use 250,000 antennas to cover 200 square degrees of sky at once—the equivalent of 1,000 Moons. Its frequency range is 450 to 1,450 megahertz—a large chunk of the water hole, making it a good tool for SETI research.

Though the U.S. did not help fund SKA, the National Radio Astronomy Observatory is considering its own ambitious, SKA-like facility called the next-generation Very Large Array, or ngVLA. Should it be built, the ngVLA would consist of 214, 28-meter high-frequency radio dishes in the southwestern U.S. capable of receiving signals from 1,200 megahertz to 116 gigahertz. Jill Tarter said SETI researchers are making the case that the search for life should be part of the ngVLA's science justification, should it be built.

Big data

In an era where the term "big data" dominates the technology landscape, many SETI scientists are eyeing improvements to the way telescope signals are stored and processed.

At Parkes, Breakthrough stores up to 100 times more data than a typical telescope user—down to individual voltage levels bouncing off the dish itself. The team currently stores a petabyte, or 1,000 terabytes, of those data. "But they quickly get through that," said Jimi Green, the Parkes scientist.

New algorithms and machine learning could help rule out spurious signals while finding hidden ones scientists don't know to look for. Pete Worden hopes to bring in help, perhaps on a volunteer basis, from unique places like the intelligence community.

"We'd really kind of like to get somebody that may work in the daytime for a three-letter agency, who goes home at night, downloads the data says 'Hey, I found something interesting,'" Worden said.

About a year ago, the SETI Institute teamed up with IBM to start using machine learning to sift through the 54 terabytes of data per day the Allen Array captures. Bill Diamond said IBM was interested in placing the data in "a gymnasium for software" to test new computing algorithms. In return, the SETI Institute gets access to cloud computing resources and tools.

"It's almost like building a new instrument, or building a new telescope," Diamond said.

The big picture

During his 1978 Tonight Show appearance, Carl Sagan theorized about what it might mean to establish two-way communication with another civilization.

"We are at a very dangerous moment in human history," he said. "We have weapons of mass destruction, we are in the process of inadvertently altering our climate—exhaustion of fossil fuels and minerals—all kinds of problems that come with technology. We are not certain that we will be able to survive this period of what I like to call technological adolescence. Were we to receive a message from somewhere else, it would show that it's possible to survive this kind of period. And that's a useful bit of information to have."

Humans have only had radio technology for a century—just a blip on the galactic timescale. If we do find intelligent beings, it would be "awfully unusual to find them where they just discovered radio a hundred years ago," Werthimer said. Odds are they'll be much more advanced than us, and potentially able to offer the kind of guidance Sagan envisioned.

Hearing nothing won't necessarily mean no one is out there perhaps we aren't looking the right way. What if intelligent beings communicate using a form of energy stronger than gamma rays? Or chat via subspace, like on Star Trek?

"We have to reserve the right to get smarter," Tarter said. "We may be doing a fantastic, excellent job at just the wrong thing."

Or, we could truly be alone in the cosmos. Paraphrasing Arthur C. Clarke, Siemion said the non-detection of intelligent life would be just as profound as detecting it.

"The only thing weirder than there being intelligent life out there is that there is not intelligent life out there," he said. "These ideas are kind of equally compelling—equally strange and amazing."

At a Breakthrough Initiatives meeting just before the Listen project was announced, each team member predicted the odds the effort would find intelligent life, Worden said.

"The numbers ranged from a percent or two, to ten to the minus fifth," he said. Milner was among the low end of the estimates.

"I was just thinking at the time, if I had gone to the U.S. government and said I want a hundred million bucks to do something, and they asked me what I thought the probability of success was, even if I had said two percent, they would have said, 'you must be nuts,'" he said.

Even negative results help define the boundaries of where and how to look. And the only way to know for sure whether there's anybody out there is to keep searching.

"I think most people in the public think we're always looking, which is totally not true," said Shelley Wright. "We've barely looked, contrary to all of our sci-fi movies."

Acknowledgements

Parkes Observatory story header timelapse and imagery by Angelo Papakostas and John Cassimatis.

Many thanks to Amir Alexander, who previously published a Planetary Society SETI overview, which was enormously helpful while researching this story.

To support The Planetary Society's feature reporting, or our science and technology projects that bolster the search for life, consider becoming a member today.


Have We Already Been Visited by Aliens?

An eminent astrophysicist argues that signs of intelligent extraterrestrial life have appeared in our skies. What’s the evidence for his extraordinary claim?

On October 19, 2017, a Canadian astronomer named Robert Weryk was reviewing images captured by a telescope known as Pan-STARRS1 when he noticed something strange. The telescope is situated atop Haleakalā, a ten-thousand-foot volcanic peak on the island of Maui, and it scans the sky each night, recording the results with the world’s highest-definition camera. It’s designed to hunt for “near-Earth objects,” which are mostly asteroids whose paths bring them into our planet’s astronomical neighborhood and which travel at an average velocity of some forty thousand miles an hour. The dot of light that caught Weryk’s attention was moving more than four times that speed, at almost two hundred thousand miles per hour.

Weryk alerted colleagues, who began tracking the dot from other observatories. The more they looked, the more puzzling its behavior seemed. The object was small, with an area roughly that of a city block. As it tumbled through space, its brightness varied so much—by a factor of ten—that it had to have a very odd shape. Either it was long and skinny, like a cosmic cigar, or flat and round, like a celestial pizza. Instead of swinging around the sun on an elliptical path, it was zipping away more or less in a straight line. The bright dot, astronomers concluded, was something never before seen. It was an “interstellar object”—a visitor from far beyond the solar system that was just passing through. In the dry nomenclature of the International Astronomical Union, it became known as 1I/2017 U1. More evocatively, it was dubbed ‘Oumuamua (pronounced “oh-mooah-mooah”), from the Hawaiian, meaning, roughly, “scout.”

Even interstellar objects have to obey the law of gravity, but ‘Oumuamua raced along as if propelled by an extra force. Comets get an added kick thanks to the gases they throw off, which form their signature tails. ‘Oumuamua, though, didn’t have a tail. Nor did the telescopes trained on it find evidence of any of the by-products normally associated with outgassing, like water vapor or dust.

“This is definitely an unusual object,” a video produced by NASA observed. “And, unfortunately, no more new observations of ‘Oumuamua are possible because it’s already too dim and far away.”

As astronomers pored over the data, they excluded one theory after another. ‘Oumuamua’s weird motion couldn’t be accounted for by a collision with another object, or by interactions with the solar wind, or by a phenomenon that’s known, after a nineteenth-century Polish engineer, as the Yarkovsky effect. One group of researchers decided that the best explanation was that 1I/2017 U1 was a “miniature comet” whose tail had gone undetected because of its “unusual chemical composition.” Another group argued that ‘Oumuamua was composed mostly of frozen hydrogen. This hypothesis—a variation on the mini-comet idea—had the advantage of explaining the object’s peculiar shape. By the time it reached our solar system, it had mostly melted away, like an ice cube on the sidewalk.

By far the most spectacular account of 1I/2017 U1 came from Avi Loeb, a Harvard astrophysicist. ‘Oumuamua didn’t behave as an interstellar object would be expected to, Loeb argued, because it wasn’t one. It was the handiwork of an alien civilization.

In an equation-dense paper that appeared in The Astrophysical Journal Letters a year after Weryk’s discovery, Loeb and a Harvard postdoc named Shmuel Bialy proposed that ‘Oumuamua’s “non-gravitational acceleration” was most economically explained by assuming that the object was manufactured. It might be the alien equivalent of an abandoned car, “floating in interstellar space” as “debris.” Or it might be “a fully operational probe” that had been dispatched to our solar system to reconnoitre. The second possibility, Loeb and Bialy suggested, was the more likely, since if the object was just a piece of alien junk, drifting through the galaxy, the odds of our having come across it would be absurdly low. “In contemplating the possibility of an artificial origin, we should keep in mind what Sherlock Holmes said: ‘when you have excluded the impossible, whatever remains, however improbable, must be the truth,’ ” Loeb wrote in a blog post for Scientific American.

Not surprisingly, Loeb and Bialy’s theory received a lot of attention. The story raced around the world almost at the speed of ‘Oumuamua. TV crews crowded into Loeb’s office, at the Harvard-Smithsonian Center for Astrophysics, and showed up at his house. Film companies vied to make a movie of his life. Also not surprisingly, much of the attention was unflattering.

“No, ‘Oumuamua is not an alien spaceship, and the authors of the paper insult honest scientific inquiry to even suggest it,” Paul M. Sutter, an astrophysicist at Ohio State University, wrote.

“Can we talk about how annoying it is that Avi Loeb promotes speculative theories about alien origins of ‘Oumuamua, forcing [the] rest of us to do the scientific gruntwork of walking back these rumors?” Benjamin Weiner, an astronomer at the University of Arizona, tweeted.

Far from being deterred, Loeb doubled down. Together with Thiem Hoang, a researcher at the Korea Astronomy and Space Science Institute, he blasted the frozen-hydrogen theory. In another equation-packed paper, the pair argued that it was fantastical to imagine solid hydrogen floating around outer space. And, if a frozen chunk did manage to take shape, there was no way for a block the size of ‘Oumuamua to survive an interstellar journey. “Assuming that H2 objects could somehow form,” Hoang and Loeb wrote, “sublimation by collisional heating” would vaporize them before they had the chance to, in a manner of speaking, take off.

Loeb has now dispensed with the scientific notation and written “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” (Houghton Mifflin Harcourt). In it, he recounts the oft-told story of how Galileo was charged with heresy for asserting that Earth circled the sun. At his trial in Rome, in 1633, Galileo recanted and then, legend has it, muttered, sotto voce, “Eppur si muove” (“And yet it moves”). Loeb acknowledges that the quote is probably apocryphal still, he maintains, it’s relevant. The astronomical establishment may wish to silence him, but it can’t explain why ‘Oumuamua strayed from the expected path. “And yet it deviated,” he observes.

In “Extraterrestrial,” Loeb lays out his reasoning as follows. The only way to make sense of ‘Oumuamua’s strange acceleration, without resorting to some sort of undetectable outgassing, is to assume that the object was propelled by solar radiation—essentially, photons bouncing off its surface. And the only way the object could be propelled by solar radiation is if it were extremely thin—no thicker than a millimetre—with a very low density and a comparatively large surface area. Such an object would function as a sail—one powered by light, rather than by wind. The natural world doesn’t produce sails people do. Thus, Loeb writes, “ ‘Oumuamua must have been designed, built, and launched by an extraterrestrial intelligence.”

The first planet to be found circling a sunlike star was spotted in 1995 by a pair of Swiss astronomers, Michel Mayor and Didier Queloz. Its host star, 51 Pegasi, was in the constellation Pegasus, and so the planet was formally dubbed 51 Pegasi b. By a different naming convention, it became known as Dimidium.

Dimidium was the ‘Oumuamua of its day—a fantastic discovery that made headlines around the world. (For their work, Mayor and Queloz were eventually awarded a Nobel Prize.) The planet turned out to be very large, with a mass about a hundred and fifty times that of Earth. It was whipping around its star once every four days, which meant that it had to be relatively close to it and was probably very hot, with a surface temperature of as much as eighteen hundred degrees. Astronomers hadn’t thought such a large body could be found so close to its parent star and had to invent a whole new category to contain it it became known as a “hot Jupiter.”

Mayor and Queloz had detected Dimidium by measuring its gravitational tug on 51 Pegasi. In 2009, NASA launched the Kepler space telescope, which was designed to search for exoplanets using a different method. When a planet passes in front of its star, it reduces the star’s brightness very slightly. (During the last transit of Venus, in 2012, viewers on Earth could watch a small black dot creep across the sun.) Kepler measured variations in the brightness of more than a hundred and fifty thousand stars in the vicinity of the constellations Cygnus and Lyra. By 2015, it had revealed the existence of a thousand exoplanets. By the time it stopped operating, in 2018, it had revealed sixteen hundred more.

NASA’s ultimate goal for the telescope was to work out a figure known as eta-Earth, or η⊕. This is the average number of rocky, roughly Earth-size planets that can be found orbiting an average sunlike star at a distance that might, conceivably, render them habitable. After spending two years analyzing the data from Kepler, researchers recently concluded that η⊕ has a value somewhere between .37 and .6. Since there are at least four billion sunlike stars in the Milky Way, this means that somewhere between 1.5 billion and 2.4 billion planets in our galaxy could, in theory, harbor life. No one knows what fraction of potentially habitable planets are, in fact, inhabited, but, even if the proportion is trivial, we’re still talking about millions—perhaps tens of millions—of planets in the galaxy that might be teeming with living things. At a public event a few years ago, Ellen Stofan, who at the time was NASA’s chief scientist and is now the director of the National Air and Space Museum, said that she believed “definitive evidence” of “life beyond earth” would be found sometime in the next two decades.

“It’s definitely not an ‘if,’ it’s a ‘when,’ ” Jeffrey Newmark, a NASA astrophysicist, said at the same gathering.

What will life on other planets look like, when—not if—it’s found? Arik Kershenbaum, a researcher at the University of Cambridge, takes up this question in “The Zoologist’s Guide to the Galaxy: What Animals on Earth Reveal About Aliens—and Ourselves” (Penguin Press). “It’s a popular belief that alien life is too alien to imagine,” he writes. “I don’t agree.”

Kershenbaum argues that the key to understanding cosmic zoology is natural selection. This, he maintains, is the “inevitable mechanism” by which life develops, and therefore it’s “not just restricted to the planet Earth” or even to carbon-based organisms. However alien biochemistry functions, “natural selection will be behind it.”

From this premise, Kershenbaum says, it follows that life on other planets will have evolved, if not along the same lines as life on this planet, then at least along lines that are generally recognizable. On Earth, for instance, where the atmosphere is mostly made of nitrogen and oxygen, feathers are a useful feature. On a planet where clouds are made of ammonia, feathers probably wouldn’t emerge, “but we should not be surprised to find the same functions (i.e. flight) that we observe here.” Similarly, Kershenbaum writes, alien organisms are apt to evolve some form of land-based locomotion—“Life on alien planets is very likely to have legs”—as well as some form of reproduction analogous to sex and some way of exchanging information: “Aliens in the dark will click like bats and dolphins, and aliens in the clear skies will flash their colours at each other.”

Assuming that there is, in fact, alien life out there, most of it seems likely to be microscopic. “We are not talking about little green men” is how Stofan put it when she said we were soon going to find it. “We are talking about little microbes.” But Kershenbaum, who studies animal communication, jumps straight to complex organisms, which propels him pretty quickly into Loebian territory.

On Earth, many animals possess what we would broadly refer to as “intelligence.” Kershenbaum argues that, given the advantages that this quality confers, natural selection all across the galaxy will favor its emergence, in which case there should be loads of life-forms out there that are as smart as we are, and some that are a whole lot smarter. This, in his view, opens up quite a can of interstellar worms. Are we going to accord aliens “human rights”? Will they accord us whatever rights, if any, they grant their little green (or silver or blue) brethren? Such questions, Kershenbaum acknowledges, are difficult to answer in advance, “without any evidence of what kind of legal system or system of ethics the aliens themselves might have.”

As disconcerting as encountering intelligent aliens would be, the fact that we haven’t yet heard from any is, arguably, even more so. Why this is the case is a question that’s become known as the Fermi paradox.

One day in 1950, while lunching at Los Alamos National Laboratory, the physicist Enrico Fermi turned to some colleagues and asked, “Where are they?” (At least, this is how one version of the story goes according to another version, he asked, “But where is everybody?”) This was decades before Pan-STARRS1 and the Kepler mission. Still, Fermi reckoned that Earth was a fairly typical planet revolving around a fairly typical star. There ought, he reasoned, to be civilizations out there far older and more advanced than our own, some of which should have already mastered interstellar travel. Yet, strangely enough, no one had shown up.

Much human intelligence has since been devoted to grappling with Fermi’s question. In the nineteen-sixties, an astronomer named Frank Drake came up with the eponymous Drake equation, which offers a way to estimate—or, if you prefer, guesstimate—how many alien cultures exist with which we might hope to communicate. Key terms in the equation include: how many potentially habitable planets are out there, what fraction of life-hosting planets will develop sophisticated technology, and how long technologically sophisticated civilizations endure. As the list of potentially habitable planets has grown, the “Where are they?” mystery has only deepened. At a workshop on the subject held in Paris in 2019, a French researcher named Jean-Pierre Rospars proposed that aliens haven’t reached out to us because they’re keeping Earth under a “galactic quarantine.” They realize, he said, that “it would be culturally disruptive for us to learn about them.”

Loeb proposes that Fermi may be the answer to his own paradox. Humanity has been capable of communicating with other planets, via radio wave, for only the past hundred years or so. Seventy-five years ago, Fermi and his colleagues on the Manhattan Project invented the atomic bomb, and a few years after that Edward Teller, one of Fermi’s companions at the lunch table at Los Alamos, came up with the design for a hydrogen bomb. Thus, not long after humanity became capable of signalling to other planets, it also became capable of wiping itself out. Since the invention of nuclear weapons, we’ve continued to come up with new ways to do ourselves in these include unchecked climate change and manufactured microbes.