I. The SETI Institute

II. Astrobiology


Observing Projects


Science justification

IV. Education and Public Outreach

I. The SETI Institute

What is the SETI Institute?

The SETI Institute is a non-profit corporation that serves as an institutional home for research and educational projects related to the study of life in the universe. The Institute’s interests include astronomy and planetary sciences, the origins of life, as well as chemical, biological, and cultural evolution.

Historically, Institute projects have been funded by the NASA Ames Research Center, NASA Headquarters, the National Science Foundation, the Department of Energy, the US Geological Survey, the Jet Propulsion Laboratory (JPL), the International Astronomical Union, Argonne National Laboratory, the Alfred P. Sloan Foundation, the David and Lucile Packard Foundation, the Paul G. Allen Foundation, the Moore Family Foundation, the Universities Space Research Association (USRA), the Pacific Science Center, the Foundation for Microbiology, Sun Microsystems, Hewlett Packard Company, the William K. Bowes Jr. Foundation, and other industry corporations or funds. Its many private donors have included William and Rosemary Hewlett, Bernard M. Oliver, Paul Allen, Franklin Antonio, and thousands of other private donors.

The Institute welcomes support from private foundations or other groups or individuals interested in our work.

Each funded effort is supervised by a principal investigator who is responsible to the Board of Trustees for the conduct of the activity. There are currently many active projects, involving approximately a hundred scientists, studying Mars, Pluto, and other bodies in the solar system, as well as exoplanets, exobiology and related topics. In addition, the Institute’s SETI efforts include searches for extraterrestrial intelligence on its own Allen Telescope Array.

Among the Institute’s education and outreach activities are both formal and informal education – including a comprehensive web site, frequent talks by our scientists, a weekly colloquium open to the public, short interviews with our scientists on social media, a vigorous artist-in-residence program, articles printed both by the Institute and in national media, and a weekly, one-hour radio program on science (“Big Picture Science”) now carried by more than 140 broadcast stations.

What sort of research is conducted by the SETI Institute?

The Institute has suites of activities in three are: (1) Astrobiology, the efforts to find and understand the prevalence of life in general (for example, microbial life under the parched landscapes of Mars or the icy crust of the Jovian moon Europa); (2) SETI, experiments designed to detect radio or light signals that would reveal the presence of technically sophisticated beings; and (3) Education and outreach projects that (a) inform the public about our research, (b) encourage young people to become more proficient in science, (c) improve science literacy by explaining the methods of science, and (d) train teachers in STEAM subject areas.

The Institute’s research activities are sometimes referenced to the Drake Equation (see below), which nicely lays out those subject areas that are germane to the question of extraterrestrial life’s prevalence and characteristics.


What is the Drake Equation?

This renowned formulation (sometimes called the second-most famous equation in science, after Einstein’s E=mc2) was devised by astronomer Frank Drake in 1961 as the agenda for a meeting held in Green Bank, West Virginia to discuss the possibility of searching for signals from extraterrestrial intelligence.

The equation defines N, the number of transmitting civilizations in our galaxy, as the product of seven factors, as follows:

N=R* fp ne fl fi fc L


R* = the birth rate of stars in our galaxy, number per year

fp = the fraction of stars with planets

ne = the number of planets per solar system that are suitable for life

fl = the fraction of such planets that actually spawn life

fi = the fraction of planets with life that evolve intelligent life

fc = the fraction of planets with intelligent life that produce technologically capable life

L = the average lifetime (in years) of a technological society

While the first three terms of the equation have been successfully investigated by astronomers and are to some extent known, values for the last four are still speculative. The 1961 Green Bank meeting did not publish any numerical values for the terms of the Drake Equation, although Drake himself estimates that N might be on the order of 10,000. Astronomer Carl Sagan was more optimistic, and said that N could be a million or more. Other people have been less sanguine, and suggest that N might only be 1 – in other words, we might be the only technically sophisticated society in the galaxy.

Until we find evidence for life beyond Earth, many of the terms in the Drake Equation remain unknown.


Who works at the SETI Institute?

The Institute employs scientists, engineers, administrators, technicians, public outreach specialists, educators, and support staff such as IT professionals.


What kind of education do I need to work at the SETI Institute?

Most SETI researchers and engineers have advanced degrees in astronomy, biology, geology, planetary science, etc. We also have employees who have studied electrical engineering, computer science, and education-related disciplines.


How can I contact the SETI Institute?

The best way is to email us at info@seti.org


Does the SETI Institute have public tours?

Not often. There isn’t actually that much to see. The Allen Telescope Array is 350 miles to the north of our headquarters in Mountain View, California. Our office space, some small labs, and our radio studio are located in our headquarters building.

However, tours are available by arrangement. Contact the Institute.

Announcements of weekly science lectures and publications, radio show topics, and appearances by SETI Institute personnel can be found elsewhere on this site.


How could I get a job at the SETI Institute?

Most of the scientists working at the Institute are self-funded. That is to say, they propose research projects to organizations such as NASA and the National Science Foundation and, if awarded monies, elect to become part of the Institute. Projects must fall within the research and education arenas of the Institute – broadly speaking, the question of life’s origins and its distribution.

Other job opportunities, such as for software development or other tasks, are advertised on the Institute’s website, www.seti.org.


II. Astrobiology

Astrobiology research is conducted by the Institute’s Carl Sagan Center for the Study of Life in the Universe.


What is astrobiology and how is it different from exobiology?

Astrobiology is the multidisciplinary study of life in the universe. It addresses some very basic questions: How does life begin? What environments might support life? How common is life in the universe? How can we detect extraterrestrial life? And what is the future of life on Earth and beyond?

Astrobiology research draws upon the talents of scientists from many different fields, including astronomy, astrochemistry, planetary science, geology, biology, biochemistry, and genetics. The word astrobiology was introduced in the middle 1990s, and has largely replaced the older term exobiology. The two terms are often used interchangeably, but astrobiology formally includes the origin and history of life on Earth, while exobiology is more focused on life beyond the Earth. Astrobiologists believe that a study of life on Earth is one of the best ways to be able to identify habitable environments elsewhere.


Have SETI Institute scientists found life, or evidence of life, on any other planets?

No, scientists have found no clear indications of life, past or present, beyond the Earth. There have been several tantalizing suggestions – that the Viking mission might have detected evidence of microbial life on Mars or that there are fossil microbes in some Mars rocks or meteorites – but none of these claims has been verified.

Likewise, our SETI searches have not yet detected any unambiguous narrow-band signals coming from other worlds.

However, one should remember that the technology needed to find evidence of life beyond Earth is improving rapidly, and some astrobiologists have ventured to say that life will be found within two decades or so.


Why do we think that life is out there?

Over the last half-century, scientists have developed a theory of cosmic evolution that predicts that life is a natural phenomenon likely to develop on planets with suitable environmental conditions. Scientific evidence shows that life arose on Earth relatively quickly (only 100 million years after life was even possible), suggesting that life will occur on any planets that have the requisite characteristics, such as liquid oceans (either on the surface or underground). With the recent discovery that the majority of stars have planets – the number of potential habitats for life has been greatly expanded.

In addition, exploration of our own Solar System and analysis of the composition of other systems suggest that the chemical building blocks of life – such as amino acids – are naturally produced and very widespread.

We live in a galaxy with several hundred billion stars, and there are approximately 2 trillion other galaxies in the part of the universe we can see. It would be extraordinary if we were the only thinking beings in all these vast realms.


Why do astrobiologists place so much emphasis on searching for life on Mars? By now we have discovered thousands of other planets.

Mars is accessible, being only about seven months of travel time from the Earth. It also has compelling evidence that – billions of years ago – liquid water pooled and flowed on its surface. We strongly suspect that liquid of some kind is necessary for life.

That makes Mars a compelling place to look for biology, either the remains of microbes that lived long ago, or perhaps are still viable underneath its surface where liquid water might still exist.

None of the other planets in our Solar System seems especially suitable, although the possibility of floating organisms in the atmosphere of Venus has been suggested. Several moons in the outer solar system – Europa, Enceladus, and Titan – are of potential interest to astrobiologists, but they are much farther than Mars and therefore more difficult to reach with spacecraft. And while there are indeed thousands of exoplanets (or exoplanet candidates) that have been discovered, we have no way at present of assessing their habitability. In any event, they are far too distant for spacecraft visits in the foreseeable future.


What is meant by an “Earth-like planet"?

This is a confusing phrase, since not everyone uses it with the same meaning. Usually it denotes a planet with approximately the same size, composition, and temperature as the Earth. Such a planet is thus potentially habitable. By this definition, there are no other Earth-like planets in our Solar System (even though there are other planets and moons that might host life), but there are likely to be tens or even hundreds of millions of exoplanets in our galaxy that meet these criteria.

However, there is an issue of semantics here: Some people think that “Earth-like” should imply the actual presence of life.


How many Earth-like planets are there beyond our Solar System?

Astronomers over the past few years have discovered more than four thousand exoplanets. Most of them were found using the Kepler Space Telescope, a mission that has been partly operated by staff from the SETI Institute.

A few dozen of these exoplanets appear to be of Earth size and, based on the distance from their home star, probably also have temperatures in the range where liquid water is possible. However, we do not yet know anything about their composition or the nature of their atmospheres, so we cannot assess their habitability.


Why don't you take a picture of one of those newly discovered planets to see if they have life?

The Kepler space telescope, which has discovered most of the new exoplanets, detects them only by the fact that they eclipse their home stars, causing the stars to slightly dim. This is like detecting a moth flying in front of a distant streetlamp by the fact that the light intensity dips somewhat. Kepler cannot take photos of these planets. Even with the largest telescopes in the world, taking a picture of any Earth-size exoplanet is currently impossible, although some larger planets have been imaged.


Why don’t we send a space probe to some of the newly discovered planets to see if they have life?

The exoplanets are very distant. The nearest exoplanet is in orbit around the star Alpha Centauri, which is more than 4 light-years from Earth. With our current technology, it would require about 100 thousand years for a probe to travel that far.


Are some of the Kepler planets places that we could move to if our own world becomes too polluted to support human life?

No, it is hard to imagine humans travelling beyond our own Solar System in the foreseeable future. A trip to even the nearest star would require many generations, and the energy needed for such a starship to reach its destination within the lifetime of the crew is far beyond anything we can muster.


What makes Europa special? When will we send a spacecraft to explore the oceans of Europa?

Europa, one of the four largest moons of Jupiter, has more liquid water than all the oceans of the Earth. The presence of liquid water and a source of heat (produced by its gravitational interaction with Jupiter) to keep that water warm makes Europa a very attractive world on which to look for life.

We have data from the Voyager and Galileo spacecraft, but these neither went into orbit around Europa nor did they land. The biggest challenge for exploring its ocean is reaching the liquid water, which lies beneath a crust of ice frozen more solid than granite. The crust is 10 km or more thick. But there are clever schemes to either drill through the ice and send down probes, or look for evidence for biology that’s been “squeezed” onto the surface, all in an effort to find signs of life beneath this ice-skinned moon.


What about other moons, such as Enceladus and Titan? Could they have life?

These two moons of Saturn are also appealing to scientists trying to find life on other worlds. Enceladus, like Europa, has a large body of water in its interior, and the push and pull of gravitational interaction with Saturn squirts some of this water into space. A spacecraft could conceivably grab samples of this cryovolcano, and see if there are any microbes in the water.

Titan is the only world in the Solar System known to have liquid lakes on its surface … but they are lakes of liquid methane and ethane, not water. Still those are organic compounds, and it’s not inconceivable that some sort of life lives in these bodies of liquid, natural gas.


Is the Curiosity Mars Rover searching for life on Mars?

No, Curiosity is investigating the habitability of Mars, and in particular is looking for geological evidence of past water and of organic compounds that might tell us that Mars once had life, perhaps billions of years ago.

The only mission that directly searched for life on Mars was Viking, back in 1976. Two landers carried biology instruments designed to look for evidence of microbes in the soil. The results were interpreted by the majority of the Viking biology team as being negative, and we now know that conditions in the surface soil where the lander collected its samples could not support Earth-like life. We think that if there are living microbes on Mars, they are probably at least several meters below the surface, in special environments where liquid water is present. The Curiosity rover is not able to drill down that deep, nor do we know where such habitable conditions might be found.


Will the Perseverance Rover look for life on Mars?

Perseverance, which is part of the Mars2020 mission to the Red Planet, will look for soil samples that might contain evidence of microbial life that existed on Mars billions of years ago. It is doing this reconnaissance in Jezero Crater, which was once a lake fed by two, now-dry rivers. The samples will be packaged up by Perseverance, and eventually brought back to Earth for detailed analysis by a spacecraft that will be sent to Mars within a few years.

If evidence for ancient life on Mars is found, it will be discovered in a terrestrial lab!


How do we ensure that organisms from Earth, carried to Mars on rovers, do not contaminate the equipment and give false “positive” results showing life on Mars that was actually brought there by us?

Scientists take careful steps to minimize such forward contamination on any Mars lander. However, it’s impossible to completely sterilize the spacecraft, and there will inevitably be a residual of microbial hitchhikers on big devices such as the Curiosity and Perseverance rovers.

We can allow this degree of microbial contamination since we know that the surface of Mars is not able to support living organisms from Earth, being very dry, cold, and stung by lethal ultraviolet light. Therefore, we are confident that microbes from Earth will not grow on the surface and contaminate the planet.


What is the story of arsenic-based life, which was said to be present in samples from Mono Lake? Apparently phosphorus replaces arsenic. Could this be representative of the earliest life on Earth?

Phosphorus is one of a handful of essential elements for life as we know it on Earth. This element is part of the molecular backbone of DNA, and plays a key role in the storage and transfer of chemical energy within cells. Arsenic is an element with a similar atomic structure to phosphorus, but is not important in biochemistry. In large quantities it is a well-known poison.

The experiments referred to here were carried out by a team led by Felisa Wolfe-Simon (NASA and the U.S. Geological Survey). The initial results were misleadingly summarized by Dennis Overbye in the New York Times: “Seeking evidence that life could follow a different biochemical path than what is normally assumed, Dr. Wolfe-Simon grew [microbes from Mono Lake] in an arsenic-rich and phosphorus-free environment, reporting in a NASA news conference on Dec. 2 [2010] that the bacterium had substituted arsenic for phosphorus in many important molecules in its body, including DNA.”

Recent results by other scientists have shown that there was no arsenic in the DNA of this microbe. The ability of this microbe to tolerate arsenic is interesting to astrobiologists, but it is not as dramatic a discovery as the popular article suggested.


There is scientific consensus that global warming is real and caused by human activities. What specific threats will humans face if the carbon dioxide levels continue to increase?

We are already seeing many impacts of global warming. The rapid shrinking of the arctic icecap has opened the Northeast Passage to shipping, and cruise ships already book passengers for voyages through the Northwest Passage. The melting arctic ice and permafrost are exposing oil and mineral deposits for exploitation, but also endangering arctic wildlife. Most important, the melting of arctic snow and ice darkens the surface, leading to more rapid warming during the summer and a shift in weather patterns over North America.

Melting of ice from Greenland and Antarctica is also contributing to sea level rise, making destructive storms like Hurricane Sandy (in 2011) much more likely. Severe droughts in the U.S., Russia, and Australia can also be traced to global warming. Within a few years, the accelerating loss of ice from the Himalayas is expected to lead to the summer drying up of several great Asian rivers, which are the source of water for more than a billion people in China, India, Pakistan, and Bangladesh. By the middle of the century, rising sea levels and stronger storms are likely to lead to the permanent evacuation of much of New Orleans, New York, Miami, Amsterdam, and Venice. We don’t know how fast the warming and sea level rise will take place, but the trend is inexorable unless we stop flooding the atmosphere with carbon dioxide and methane.


If we are able to dramatically increase our planet’s surface temperature, then could we also do the same for Mars? If so, would that cause the ice that lies beneath Mars’ surface to melt, creating massive bodies of water and thus making it more habitable?

The current rapid global warming on Earth is due to the burning of fossil fuels, hydrocarbons like oil, gas, and coal. We are releasing carbon into our atmosphere that was produced by plants and buried millions of years ago. As far as we know, Mars has no such carbon deposits. Without large quantities of oil, gas or coal to burn, and also no atmospheric oxygen to do the burning, we have no easy way of warming Mars as we are now doing on Earth. One suggestion for producing a temporary warming on Mars would be to re-direct a large comet so that it hits the polar areas of Mars, releasing a great deal of water and carbon dioxide. But the fundamental problem with Mars is that its mass is too small to hold on to a substantial atmosphere for very long.


How are SETI Institute scientists studying the effects of global warming?

Institute scientists are investigating the changing conditions and biota in the Earth’s Arctic and Antarctic. It is these fragile environments near the poles (and also in high mountains) that are most sensitive to climate change. Additionally, they also constitute important analogs for conditions on our neighboring world, Mars.

Institute researchers do extensive field work in the Arctic (including a research station on Devon Island), in the Antarctic (including exploratory dives under the ice in the dry valleys), and in the high Andes mountains of South America where the rapid retreat of glaciers is changing the entire ecosystem. We expect to carry out long-term studies of these environments and the changes in their biota under environmental stress. This research may also help us interpret the history of climate change on Mars over hundreds of millions of years.


What are the most extreme conditions on Earth that life can tolerate? Is it theoretically possible that some form of life could adapt to survive in even harsher conditions?

Extremophiles on Earth live at a very wide range of temperatures (from -20 C to +122 C), at high levels of salinity and alkalinity (such as in Mono Lake in California), and even in areas of high radiation such as the cooling systems of nuclear reactors. Life exists deep underground and inside rocks in the Antarctic. There is a comprehensive listing of extremophile record-holders on Wikipedia.

In most cases, there is no reason to think the current extremes couldn’t be exceeded, especially if we consider the evolution of life on other planets with different conditions than those on Earth. But all the life we have studied (”life as we know it”) requires liquid water, organic compounds, and a source of energy. And all of it has DNA.


What would it be like to be at the bottom of the atmosphere of a gas giant planet? Is there a solid surface down there?

Giant planets do not have a solid surface, one you could stand on. If you were dropped onto Jupiter for example, you would sink until crushed by the intense atmospheric pressure (as happened to the Galileo probe that was deliberately plunged into Jupiter’s atmosphere in 1995.) Jupiter and Saturn are composed mainly of hydrogen and helium, and if you could survive the crushing pressure as you went deeper, eventually you’d find that the hydrogen would transition from a molecular state (like a gas) to a metallic state. But there isn’t a specific point that one could call a surface. It’s a gradual change.

The innermost cores of both planets are likely to be “rocky”, containing about 5 to 10 times the mass of Earth, and surrounded by an ice-rich outer core. Uranus and Neptune are more mysterious as scientists have less data than for Jupiter and Saturn. Nonetheless, we also think that those planets do not have a solid surface. In these cases, the gaseous envelopes would transition to an “ocean.” The pressures would not be high enough for metallic hydrogen to form. It’s probable that both Uranus and Neptune also have rocky cores.


Is there any possibility for a planet in the same orbit as Earth, but on the opposite side of the Sun where we can never see it? If so, how could we know about it?

No, there’s not. Such a planet (sometimes called Nemesis), always staying on the opposite side of the Sun from the Earth, would not be in a stable orbit. Perhaps more to the point, if there were anything there, its presence would be easily detected by its gravitational effects on the orbits of other planets, asteroids, and comets. And of course, it would have been seen by many of our planetary space probes. For more information on this history of this idea, look up "counter-earth" in Wikipedia.


Given the gradual warming of the Sun as it ages, how much longer is the Earth expected to be habitable for complex life forms like humans?

Since evolution allows life to adapt to slow changes such as the increasing luminosity of the Sun, it’s not possible to calculate when the solar warming will make all or part of the planet uninhabitable. A less problematic question is to ask when the oceans will evaporate, as the Earth enters a runaway greenhouse state. A recent French study predicts that the oceans will turn to steam approximately one billion years in the future. That time scale is so long that it does not really mean much to most of us.

The current rapid warming from human-caused climate change is an immediate problem that requires action today, unlike the gradual luminosity increase of the Sun due to the compositional changes taking place in its fusing core.


NASA says they have discovered evidence for past water on Mars. Where is the water now?

We have known about H2O on Mars for nearly 50 years, ever since NASA spacecraft showed that the north polar cap was made of water ice. Even 18th century astronomers speculated that the white polar caps of Mars might be composed of water ice (a speculation that is only partially true.)

Scientists also regularly monitor water vapor in the atmosphere. The Viking 2 lander photographed winter frost on the surface in the 1970s, and the Phoenix lander found and photographed ice deposits in 2008.

All of the Mars orbiters since the 1970s have photographed water erosion features, ranging from huge outflow channels to small, recently made gullies in crater walls. Where did the water that made these erosion features go? Some think that water is present now as ice, including large polar deposits. Scientists also think there are very large water deposits below the surface, with ice (permafrost) on top and likely liquid water (aquifers) at depth. Some water escaped along with the rest of the early Martian atmosphere, but most of it is still there. The problem with Mars is not a lack of H 2O, but low temperatures. Those are consequence of the loss of much of its atmosphere and the resultant diminution of any greenhouse effect.


Now that we have found water on the Moon will we look for microbial life there?

Although we often speak of finding water on the Moon, this terminology is confusing. What we have found during the course of several recent missions is ice in permanently shadowed polar craters, presumably deposited by incoming asteroids. There is also new evidence of chemically bound water molecules in the lunar soil. However, we have not found liquid water, which is (as far as we know) required for life. If there were liquid water at the lunar surface, it would instantly evaporate because of the low lunar gravity and the absence of an atmosphere. Ice is stable in permanently shadowed polar craters only because the temperatures are extremely low. It is colder on the floors of some of these craters than on the surface of Pluto. Thus, we have not found anything on the Moon that would encourage us to look for evidence of life there.


What is the shadow biosphere and is it real?

Shadow biosphere is the name given by scientists to a hypothetical microbial biosphere of Earth that would use radically different biochemical and molecular processes than the terrestrial life we know. So far, there is no compelling evidence for a real shadow biosphere on Earth, but by definition it would be difficult to detect with our usual biochemical tools. One reason for skepticism about its existence is the evolutionary fact that stronger life forms tend to out-compete weaker ones, leading to the extinction of the weaker form. Thus, we would have to wonder how two different biospheres could have coexisted on this planet for four billion years. Searching for a shadow biosphere might be useful to help us think about how we could identify an alien biosphere on other worlds.


When astrobiologists say “life as we know it”, what does it really mean in the search for extraterrestrial life? What would life as we don’t know it be like?

There is no clear-cut meaning for “life as we know it.” Usually this phrase refers to life based on DNA or RNA, probably also including viruses (although many biologists do not consider a virus to be alive). Sometimes the meaning is expanded to include any life that is based on the same sort of water-mediated carbon chemistry (with amino acids and proteins) that we have on Earth, but with some other inheritance mechanism that does not use DNA or RNA.

Life as we don’t know it would include life that some speculate could exist on Saturn’s moon Titan, where the temperatures are far below the freezing point of water. However, even in this bitter cold, hydrocarbons like methane and ethane are liquid, and might conceivably form the basis for carbon-based life very different from that on Earth. Astrobiologists are uncertain how we could recognize or detect life as we don’t know it, although presumably any life would use energy to change its chemical environment – in other words, metabolize – thus perhaps providing clues to its existence.


If the presence of life on Earth suggests that life emerges on a planet whenever conditions are favorable, why is there apparently no evidence that life began here more than once?

No one knows whether life emerges on a planet whenever conditions are favorable. The only example of life we have is on our own planet. It is entirely possible that life has begun several times on Earth, but early biology might have been obliterated, perhaps repeatedly, by impacts of some of the large rocks that were flying around the early solar system, remnants of planetary formation. Larger meteors would produce impacts with sufficient energy to boil the oceans, a circumstance guaranteed to destroy any early life.

However, even setting aside the possibility that earlier incarnations of life were wiped out by incoming meteors, there’s also the fact that competition between different life forms would have led to the survival of only one kind. There are no records of what might have happened during the first billion years of Earth’s history, but it seems clear that the life that has survived to the present day all had one common ancestor.


What keeps oxygen on Earth, gravity or our magnetic field? Is it possible for our planet to lose all its oxygen?

Our atmosphere is held in place by Earth’s gravity, which is strong enough that very little gas is lost to space. Mars, with lower gravity, has an atmosphere much less than ours. The Moon, which has lower gravity still, has no atmosphere.

The magnetic field plays very little role. Venus, with no magnetic field, actually has a much thicker atmosphere than Earth. The presence of oxygen as a major component of our atmosphere is a consequence of photosynthetic life, which produces oxygen as a byproduct. If photosynthesis stopped – which is to say, if the plants were to disappear – other oxygen breathing life forms and chemical reactions in the environment (think rust) would deplete most of Earth’s atmospheric oxygen within a few tens of thousands of years.


Is someone hiding aliens?

We don’t think so. One-third of the American public (and a similar fraction of the citizenry in other countries) is convinced that extraterrestrials may be buzzing the countryside in their spacecraft, or occasionally alighting in the back yard to abduct a few humans for breeding experiments.

This would be of enormous interest and importance, and (in our opinion) impossible to hide, particularly if it’s happening internationally. The presence of aliens on our planet is not something you would want to hide: it would be the biggest science story of all time, and tens of thousands of university researchers would be working away on it.

However, despite the popularity of aliens in both movies and TV, and more than 70 years of UFO sightings, the lack of credible physical evidence has made it difficult for serious scientists to believe that UFOs have anything to do with extraterrestrial visitors. Note that witness testimony, which is much ballyhooed in the media, has little horsepower when it comes to swaying scientists.



Observing Projects

What is the premise of SETI?

SETI is an acronym for the Search for Extraterrestrial Intelligence. It’s an effort to detect evidence of technological civilizations that may exist elsewhere in the universe, and most SETI searches are conducted using large antennas (radio telescopes) in an attempt to eavesdrop on signals from other star systems in our Galaxy. Other SETI searches look for flashing laser pulses.

We now know that planets are present in most stellar systems, and on the basis of evidence collected so far can estimate that there are many billions of Earth-size worlds in the Galaxy. This is the primary motivation for SETI experiments: There’s so much planetary real estate that it would be remarkable if Earth were the only place in the Galaxy to host, not just life, but intelligence.


Has the SETI Institute found an extraterrestrial signal? And what about the WOW signal?

No SETI search has yet received a confirmed, extraterrestrial signal. If we had, the world would know about it. YOU would know about it! There is no policy of secrecy and any promising signal would quickly prompt observations at other observatories to confirm that it was real.

In the past, there were several unexplained and intriguing signals detected in SETI experiments. Perhaps the most famous of these was the “Wow” signal picked up at the Ohio State Radio Observatory in 1977. However, none of these signals (including the Wow) was ever detected again, and for scientists that’s not good enough to claim success and travel to Stockholm to collect a Nobel Prize. Who would believe cold fusion unless many researchers could duplicate it in their labs? The same is true of extraterrestrial signals: they are credible only when they can be found more than once.


How would we know that the signal is from ET?

Nearly all radio SETI experiments have looked for what are called “narrow-band signals.” These are radio emissions that span only a small part of the radio spectrum. Imagine tuning your car radio late at night … There’s static everywhere on the dial, but suddenly you hear a squeal – a signal at a particular frequency – and you know you’ve found a station. It’s only at a specific place on the radio spectrum.

Narrow-band signals – perhaps only a few Hertz wide or less – are the mark of a purposely built transmitter. Natural cosmic noisemakers, such as pulsars, quasars, and the turbulent, thin interstellar gas of our own Milky Way, do not make radio signals that are this narrow. The static from these objects is spread all across the spectrum.

In terrestrial radio practice, narrow-band signals are often called “carriers.” They pack a lot of energy into a small amount of spectral space, and consequently are the easiest type of signal to find for any given power level. If E.T. intentionally sends us a signal, those signals may well have at least one narrow-band component to get our attention.


What happens if we find something?

Keep in mind that the receivers used for SETI are designed to detect constant or slowly pulsed carrier signals … something like a flute tone played against the noise of a waterfall. But any rapid variation in the signal – known as modulation, or more colloquially as the “message” – can be smeared out and lost. This is because – to gain sensitivity – SETI receivers average the incoming signals for seconds or minutes.

If E.T.’s electric bills are high (as on Earth) and his received signals are therefore relatively weak, we may have to build far larger instruments to look for the modulation. Fortunately, once a detection is made, we expect the money will become available to do so.

Until we can detect the modulation, we’ll know only a few things about the beings on the other end. We can pinpoint the spot on the sky where the signal is coming from, and slow changes in its frequency will tell us something about the rotation and orbital motion of E.T.’s home planet.

But even though this information is limited, the detection of alien intelligence would be an enormously big story. We’d be aware that we’re neither alone nor the smartest things in the universe. And of course, there will be a clamor to build the big dishes that would allow us to pick up E.T.’s message.


Could we ever understand the message?

No one knows. It’s conceivable that an advanced and altruistic civilization might send us simple pictures and other information. They might do this because they are hundreds (or more) light-years’ distant. That would make real back-and-forth communication tedious at best, so these alien broadcasters might be tempted to send lots of information, and in a format that we could eventually decipher. Then again, we might pick up a signal that was never intended for us, in which case it might be impossible to figure out.


Didn't NASA have a SETI program?

Yes. The NASA effort was called the High-Resolution Microwave Survey (HRMS). In 1993, Nevada Senator Richard Bryan introduced an amendment that eliminated all funding for the NASA SETI program. The cost of the program was less than 0.1% of NASA's annual budget, amounting to about a nickel per taxpayer per year. The Senator cited budget pressures as his reason for ending NASA’s involvement with SETI.


So, who funds the SETI search now?

Current SETI searches are funded by donations, mostly from individuals and a few foundations and corporations. Major donors have included William Hewlett, David Packard, Gordon Moore, Paul Allen, Nathan Myhrvold, Arthur C. Clarke, Barney Oliver, Yuri Milner, and Franklin Antonio.


Why do you think an extraterrestrial civilization will broadcast in the microwave part of the radio spectrum?

There is relatively little background static from galaxies, quasars, high-speed charged particles, and other cosmic noise makers in the microwave part of the spectrum. This makes faint signals easier to pick out. Additionally, the microwave band contains naturally-produced emission lines, including hydrogen at 1420 MHz and methanol at 6667 Mhz, that “broadcast” in narrow frequency ranges. Every radio astronomer (including extraterrestrial ones) will know about these emissions. Such lines may serve as universal “markers” on the radio dial (hailing frequencies, in Star Trek lingo), and indicate good spectral regions to search.

On the other hand, these same frequencies might be avoided by E.T. to prevent “pollution” of these scientifically important bands. At the Allen Telescope Array, we hedge our bets by observing at all frequencies between 1 and 9 GHz (i.e., 1000 – 9000 MHz).


How do you know if you’ve detected an intelligent, extraterrestrial signal?

The main feature distinguishing signals produced by a transmitter from those produced by natural processes is their spectral width, i.e. how much room on the radio dial they take up. Any signal less than about 300 Hz wide must be, as far as we know, artificially produced. Such narrow-band signals are what most radio SETI experiments look for. Other tell-tale characteristics include a signal that is completely polarized or the existence of coded information on the signal.

Unfortunately, SETI searches are burdened with confusion caused by narrow-band, polarized, and coded signals from our own planet. Military radar and telecommunications satellites produce such signals. The Allen Telescope Array sorts out these confusing signals by comparing the cosmic static received from one part of the sky with that from another.


Are SETI researchers looking for the wrong type of signal? (E.g., why not spread spectrum?)

Historically, SETI researchers have looked for narrow-band signals, the type that are confined to a small (usually 1 Hz or less) spot on the dial. But if you have a cellular phone, you may be aware that it, and a lot of other communications on Earth, now use a technique known as “spread spectrum” in which the signal is dispersed over a wide range of frequencies. What if E.T. is also engaged in spread spectrum broadcasting? Would our searches pick up his call?

That depends. If the signal is strong enough, it might be detected with ordinary SETI equipment, although weak broadcasts will be missed. Modern SETI experiments have tried to refine their receiving systems to be sensitive to these other types of communications. Nonetheless, it’s good to keep in mind that any civilization will realize that narrow-band broadcasts are among the most efficient in terms of producing a detectable signal at the receiving end. If they wish to get in touch or, for example, simply have high-powered radars for finding incoming comets, they will generate the type of signals our experiments can find.


Are we also sending any signals?

To date, the SETI Institute has conducted only passive experiments, designed to listen for signals, not to send them. However, humankind has been unintentionally transmitting signals into space – primarily high-frequency radio, television, and radar – for more than seventy years. Our earliest TV broadcasts have reached several thousand nearby stars, although any alien viewers would have to build a very large antenna to detect them.

One reason that SETI researchers have not chosen to broadcast is because of the long time one has to wait for a reply. If the nearest civilization is 100 light-years away, we would have to sit around for 200 years before we could expect a response. Nonetheless, a few intentional messages have been sent. One message, transmitted in 1974 from the Arecibo Observatory, was a simple picture describing our solar system, the compounds important for life, the structure of the DNA molecule, and the form of a human being. The message was transmitted in the direction of the globular star cluster M13, about 25,000 light years away. Since then, both NASA and a small group in Russia have sent several relatively brief, deliberate signals into space. A new organization to pursue METI (Messaging Extraterrestrial Intelligence), called METI International, is researching this issue. It has to date organized one transmitting session using an antenna in Norway.


If an extraterrestrial civilization has a SETI project similar to our own, could they detect signals from Earth?

In general, no. Most earthly transmissions are too weak to be found by equipment similar to ours at the distance of even the nearest star. But there are some important exceptions. High-powered radars and the Arecibo broadcast of 1974 (which lasted for only three minutes) could be detected at distances of tens to hundreds of light-years with a setup similar to our best SETI experiments.


Why can’t we just send a spacecraft into space to look for other planets and life?

The stars are simply too far away. Our best rockets travel at about 10 miles per second. Even to reach the nearest other star system, Proxima Centauri, at about 4.2 light-years’ distance, would take such a rocket approximately 70,000 years. There are several thousand stars within 100 light-years of us. To investigate them all with spacecraft would take millions of years and vast amounts of money. A better scheme is to search for radio waves (which travel at the speed of light) using state-of-the-art technology, and at a relatively modest cost.


Will alien senders have any way of knowing that their signal has been received by us?

No. They wouldn’t be aware that we had received their message any more than a radio disk jockey knows that you’ve tuned in his show. For the extraterrestrials to know, we would have to send a message in reply. Whether or not sending a reply is a good idea is still controversial. It’s worth noting, however, that a complete message exchange might take decades or longer due to the finite speed of light.


Would it be dangerous to reply?

While we can’t pretend to know the behavior or motivations of extraterrestrials, there’s little point in worrying about alerting others to our presence by either deliberately transmitting (METI) or replying to a signal detected by SETI. That’s because we have been unintentionally broadcasting the fact of our existence into space ever since the Second World War. Any society capable of interstellar travel – and therefore a possible threat – would be able to detect these signals. In other words, the evidence for our presence on Earth is already moving into space, and has so far reached several thousand-star systems.


What happens if you detect a signal?

While there is a protocol (available on the internet) for what to do if a signal is found, it is an informal document, without any force of law. But what it lays out is common sense: The first thing to do is to confirm that the signal is truly extraterrestrial. Remember, with hundreds of millions of channels and antennas that are among the world’s largest, SETI picks up thousands of signals. An important test to verify that a signal is truly extraterrestrial would be a confirming observation at another radio telescope.

Once an artificial signal is confirmed as being of extraterrestrial intelligent origin, the discovery would be announced as quickly and as widely as possible. There will be no secrecy, and indeed getting the word out quickly is important as there would be an urgent need to have astronomers world-wide monitor any detected signal, 24 hours a day.

Finally, the protocols state that no reply to any signal should be made without consent of the international community.


What happens if you don’t detect a signal?

We are just scratching the surface of what a modern search can do. Failure to find a signal wouldn’t prove that we’re the only thinking beings in the Galaxy. After all, absence of evidence is not evidence of absence.

The SETI Institute intends to press the search. Needless to say, the march of technology and new scientific discoveries will influence future SETI strategies. But giving up is not in the cards. Christopher Columbus did not turn around simply because he failed to find any new lands during his first few days at sea.


How would you know what the signal means – the message?

The simplest SETI searches look for a “carrier” – a narrow-band signal – that could underpin a transmission. A carrier is just a simple tone, and doesn’t convey any information itself. The message, if there is any, would occupy frequencies near the carrier. This “modulation” would likely be quite weak compared to the carrier, simply because the total energy of the message is spread out over much of the spectrum.

If we do succeed in finding a message, could we understand it? If the signal is intentional, it might be decipherable, simply because the senders want it to be understood. In order to send or receive a signal over interstellar distances, a civilization must understand basic science and mathematics. Hence, a message from another society might use science and math, or even just a lot of simple pictures, to build up a common language with others.

On the other hand, signals produced by a civilization for its own purposes may be impossible to unravel. However, SETI scientists are developing statistics-based algorithms to determine the amount of information sent. This can tell us, almost immediately, something about the aliens’ technical level, even if the message remains mysterious.


How long have astronomers been looking for extraterrestrial signals?

The first scientific paper on using radio waves to transmit information over interstellar distances was published in the journal Nature in 1959 by physicists Phillip Morrison and Giuseppe Cocconi. In the following year, Frank Drake (now emeritus at the SETI Institute) conducted the first radio search for signals using an 85-foot antenna at the National Radio Astronomy Observatory in Green Bank, West Virginia.

Drake called his search Project Ozma, and observed two Sun-like stars, each about 12 light-years away. Since then, approximately 100 searches have been conducted by dozens of astronomers in several countries. However, note that the technology of today’s searches greatly surpasses that of earlier efforts.


Who else is carrying out searches?

Astronomers from the University of California, Berkeley, have a multifaceted SETI program underway called Breakthrough Listen. They have conducted observations at radio telescopes in West Virginia, Puerto Rico and Australia.

The popular SETI screensaver, SETI@home, is a project of the Berkeley group.

Optical SETI programs – which search for very brief flashes of light – are being conducted by the SETI Institute, the University of California, Berkeley and at Harvard University.


Is there an “eerie silence”?

The failure so far to find a signal is hardly evidence that none is to be found. All searches to date have been limited to one degree or another. There are limits on sensitivity, frequency coverage, the types of signals the equipment could detect, and the number of stars or the directions in the sky observed. Note that, while there are hundreds of billions of stars in the Galaxy, only a few thousand have been scrutinized with high sensitivity, and for those, only over a small fraction of the available frequency range.



What is the Allen Telescope Array?

The Allen Telescope Array (ATA) is located at the Hat Creek Observatory in the Cascade Mountains of California, approximately 300 miles to the north of San Francisco and two dozen miles north of Lassen Peak. It comprises an array of 42 antennas, each 6 meters in diameter, which can be simultaneously used for both SETI and cutting-edge radio astronomy research.

The ATA is currently being upgraded with more sensitive receivers. While it is not the world’s largest antenna array, it has the advantage of being able to examine large swaths of the sky, and over a wide range of frequencies from approximately 1 – 14 GHz. Unlike other instruments that are mostly used for radio astronomy projects, the ATA can devote large amounts of time to SETI searches.


What has the Allen Telescope Array observed?

There have been several observing projects on the Allen Telescope Array (ATA). One was a reconnaissance of star systems found to have planets (or planet candidates) by NASA’s Kepler Mission, and especially those planets in the so-called habitable zone. Kepler has uncovered thousands of candidate planets since its launch, and those worlds make routine appearances on the observing lists for the ATA.

In addition, the ATA has observed a small region in the vicinity of the Milky Way’s galactic center. This is the region of highest stellar density in the galaxy, and it’s conceivable that truly advanced societies might place a “beacon” there.

A third effort by the ATA has been to observe nearby, so-called “hab stars”. These are stellar systems less than 1,000 light-years distant that have characteristics that would make them suitable hosts (“habitable”) for planets with life. This last project was an extension of Project Phoenix, a SETI Institute effort that ran from 1995 – 2004, and used radio telescopes in Australia, West Virginia, and Puerto Rico to scrutinize one thousand nearby star systems in a hunt for radio signals.

In 2020, new improved receivers to garner both better sensitivity and a wider frequency range for the ATA have been under construction. Several new SETI observing programs to take advantage of this new hardware are now being formulated.


Why does the Allen Telescope Array have 42 antennas?

In the past, having more antenna collecting area (which increases sensitivity to weak signals) was best attained by building relatively large individual antennas. This was because the electronic amplifiers at the focus of these antennas were expensive, often costing $1 million or more. Fewer antennas meant less money spent on the amplifiers.

However, in the past two decades, the cost of the electronics has plummeted, and it is now less expensive – for any given collecting area – to build lots of smaller antennas. This was the rationale for the ATA’s “large N, small D” design philosophy. That is, a large number of antennas with relatively small diameters.

The original intent was that the ATA would have 350 individual antennas. Unfortunately, the cost of research and development meant that there was only enough funding to construct 42. The hope is that future funding will allow an expansion of the current array – and this would improve both the sensitivity and the imaging capability of the instrument.


Science justification

Why do SETI at all?

There are many reasons for this effort, including such practical considerations as the technological spinoff. But SETI research is first and foremost pursued because it is designed to answer questions that previous generations could only ask. How do we fit into the biological scheme of the cosmos? Is intelligent life rare or common in the universe? Can technological civilizations last for long periods of time, or do they inevitably self-destruct or die out for some other reason?

If we could understand any signal that we detect, there’s always the possibility that it would contain enormously valuable knowledge. It’s probable that any civilization we discover will be far more advanced than ours, and might help us to join a galactic network of intelligent beings. But even if we detect a signal without being able to understand it, that would still tell us that we are not unique in the cosmos. The effect on society might be as profound and long lasting as when Copernicus displaced the Earth from the center of our universe.


What do other scientists think of the search for extraterrestrial civilizations?

Most scientists support the search. Here are some quotations from professional reviews:

From the Report of the Astronomy Survey Committee, National Academy of Sciences, 1972: “More and more scientists feel that contact with other civilizations is no longer something beyond our dreams but a natural event in the history of mankind that will perhaps occur in the lifetime of many of us ... In the long run, this may be one of science’s most important and most profound contributions to mankind and to our civilization.”

From the Report of the Astronomy Survey Committee, National Academy of Sciences, 1982: “It is hard to imagine a more exciting astronomical discovery or one that would have greater impact on human perceptions than the detection of extraterrestrial intelligence.”

From the Report of the Astronomy Survey Committee, National Academy of Sciences, 1991: “The discovery in the last decade of planetary disks (around other stars), and the continuing discovery of highly complex organic molecules in the interstellar medium, lend even greater scientific support to this enterprise.”


IV. Education and Public Outreach

How can I listen to the SETI Institute’s weekly radio program, “Big Picture Science”?

“Big Picture Science” can be most easily found at bigpicturescience.org, where you can immediately play this week’s show or any of hundreds of archived programs. More than 140 radio stations carry the program, and the website will tell you if there’s a station in your area. The show is also available from several podcast outlets, including iTunes, Google Play, Stitcher, and Spotify.


How can I learn about talks and online events from the SETI Institute?

Simple. Just go to the Institute’s web site, SETI.org, and sign up for our free newsletter.