FAQ (Frequently Asked Questions)

I. The SETI Institute

II. Astrobiology


Observing Projects


Science justification

IV. SETI, 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 relating to the study of life in the universe. The Institute conducts research in a number of fields including astronomy and planetary sciences, chemical evolution, the origin of life, biological and cultural evolution.

Institute projects have been sponsored 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, other private industry, William and Rosemary Hewlett, Bernard M. Oliver and many other private donations. 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 over one hundred active projects, involving more than 50 scientists at the Institute investigating Mars, planetary science, exobiology and related topics. In addition, the Institute’s SETI group is conducting several searches for extraterrestrial intelligence.

Among the Institute’s educational activities are both formal and informal education – including outreach connected with NASA’s Kepler Mission – frequent talks by our scientists, a weekly colloquium open to the public and a weekly, one-hour radio program on science (“Big Picture Science”) now carried by more than 80 broadcast stations.

What sorts of research are conducted at the SETI Institute?

The Institute has suites of activities in three arenas: (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 inform the public about our research, encourage young people to become more proficient in science, and train teachers in so-called STEM 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 nature.

What is the Drake Equation?

This famous equation (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 ft L


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

fp = the fraction of stars with planet

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

ft = 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 as high as 10,000. 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.

Who works at the SETI Institute?

The Institute employs scientists, engineers, administrators, technicians, public outreach specialists, educators and other support staff.

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. This facility houses office space, some small labs, and our radio studio.

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 funders 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 web site, www.seti.org.

Can I become a member of the SETI Institute?

Yes! Learn more about the SETI Institute’s membership program, TeamSETI at www.seti.org

You can keep up with the daily SETI search at http://setiquest.info/ and beta test our online human-powered search at http://www.setilive.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? 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 it 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 study of life on Earth is one of the best ways to learn 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.

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.

There are several hundred billion other stars in our Galaxy, and more than 100 billion 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, just a few months of travel time from the Earth. It also has compelling evidence for a history of liquid water on its surface, which is the main requirement we know of for life. None of the other planets in our solar system seems suitable. Two moons – Europa and Titan – are of potential interest to astrobiologists, but they are much farther than Mars and 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 millions of exoplanets in our galaxy that meet these criteria. However, some people go further and 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 thousands of exoplanets, most of them using the Kepler Space Telescope, a mission that is partly operated by staff from the SETI Institute. A small number of these exoplanets appear to be of Earth size and, based on the distance from their 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. If we assume that many of these are habitable and extrapolate from the small sample studied so far, it seems likely that there are at least millions of Earth-like planets in our Galaxy.

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 shadow they cast. It cannot take photos of them. Even with the largest telescopes in the world, taking a picture of any Earth-size exoplanet is currently impossible. And even we had a picture, it would not show any surface detail and so would be unlikely to answer questions about the presence of life.

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 this 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 is far beyond anything we can muster. We are stuck with our own planet Earth, and we need to take care of it. There is nowhere else to go.

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

Europa, one of the moons of Jupiter, has more liquid water than the oceans of the Earth. The presence of liquid water and of an internal heat source to keep that water warm makes Europa a very attractive target for space exploration. We already have data from the Voyager and Galileo spacecraft, but these spacecraft did not go into orbit or land on Europa. The biggest challenge for exploring its ocean is reaching the liquid water, which lies beneath a crust of frozen ice that is 10 km or more thick. But someday we do hope to send hi-tech robotic drilling equipment that can overcome these challenges, and search for life in Europa’s vast, dark and ice-covered ocean.

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 be indications that Mars once supported 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 microbial life 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 microbes. We think that if there are living microbes on Mars, they are probably at least several meters below the surface, in special environments that include liquid water. The Curiosity rover is not able to drill down that deep, nor do we know where such habitable conditions might be found on Mars.

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

Scientists take careful steps to minimize such forward contamination on any Mars lander. However, it is impossible to completely sterilize the spacecraft, and there will inevitably be a residual of microbial hitchhikers on a big spacecraft like Curiosity. We are able to tolerate this degree of microbial contamination since we know the surface of Mars is not able to support living organisms from Earth. The surface is very dry, cold, and bathed in lethal ultraviolet light. Therefore we are confident that microbes from Earth will not grow on the surface and contaminate the planet.

The most important consideration is making sure that microbial contamination from Earth doesn’t generate a false positive in any life detection experiment. It is partly for this reason that Curiosity doesn’t carry any life detection experiments. Most of the instruments on Curiosity are studying the geology, chemistry and potential habitability of Mars, past and present. These investigations will not be compromised by a very small amount of contamination from Earth.

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 it is not important in biochemistry, and in large quantities it is a poison. The experiments you refer to were carried out by a team led by Felisa Wolfe-Simon (NASA and the U.S. Geological Survey). The initial results were 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 was originally reported.

There is an emerging scientific consensus that global warming is real and caused by human activities. What specific threats humans will face if the carbon dioxide levels continue to increase, in the worst-case scenario.

We are already seeing many impacts of global warming. The rapid shrinking of the arctic icecap has opened the Northeast Passage to shipping, and within a decade it will also open 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. The recent 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 inescapable 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 to Mars? If so, would that cause the ice 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 or gas or coal to burn, and also no atmospheric oxygen to do the burning, we have no way of warming Mars as we are now doing on Earth. One suggestion to produce 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 the 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 neighbor world Mars. We do extensive field work in the Arctic (including a research station on Devon Island), in the Antarctic (including in the dry valleys and exploratory dives under the ice), 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 survive? Is it theoretically possible that some form of life could adapt to survive even harsher conditions?

Extremophiles on Earth live at a very wide range of temperatures (from -20C to +122C), 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 record-holders in Wikipediahttp://en.wikipedia.org/wiki/Extremophile.

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.

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 that you could stand on. If you were dropped on Jupiter for example, you would sink until crushed by the intense atmospheric pressure (as happened to the Galileo probe that plunged into the atmosphere of Jupiter in 1995). Jupiter and Saturn are composed mainly of hydrogen and helium, and as you go deeper, the hydrogen would transition from a molecular state (like a gas) down to a metallic state. But there isn’t a limit that one would call a surface. It’s a gradual change.

The 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 a lot less data compared to Jupiter and Saturn. Nonetheless, we can still infer that those planets do not have a solid surface you could stand on. The gaseous envelopes would transition to an “ocean”. The pressures are not high enough for metallic hydrogen to form, however. Both Uranus and Neptune also probably 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, 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 and asteroids and comets, and of course 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 http://en.wikipedia.org/wiki/Counter-Earth.

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 tends to allow life to adapt to slow changes such as those in the luminosity of the Sun, it is not possible to calculate when the solar warming will make all or part of the planet uninhabitable. A less uncertain question is to ask when the oceans will evaporate. This has been predicted for 3-4 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 the human-amplified greenhouse effect is an immediate problem that needs our action today, unlike the gradual luminosity increase of the Sun as it transforms hydrogen to helium in its core.

Recently NASA announced that 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 H2O ice. 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 that H2O is present now as ice, including large polar deposits. Scientists also think there are very large deposits of water below the surface, with ice (permafrost) on top and likely liquid water (aquifers) at depth. Some water also escaped along with the rest of the early martian atmosphere, but most of it is still there. The problem with Mars is not lack of H 2O, but low temperatures, resulting from the loss of much of its atmosphere and hence its greenhouse effect.

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

Although we often speak of finding water on the Moon, this terminology is confusing. What we have found from several recent missions is ice in permanently shadowed polar craters. 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 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, 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 know is on our planet. It is entirely possible that life has begun several times on Earth. However, if this happened the 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, our gravity or magnetic field? Is it possible for our planet to lose all its oxygen?

Our atmosphere is held in place by gravity. The Earth’s gravity is strong enough that very little gas is lost to space. Mars, with lower gravity, has an atmosphere much smaller than ours. The Moon, which has lower gravity yet, has no atmosphere. The magnetic field plays very little role. Venus, with no magnetic field, actually has a much larger atmosphere than the Earth. The presence of oxygen as a major component of our atmosphere depends on photosynthetic life, which produces oxygen as a byproduct of photosynthesis. If photosynthesis stopped, our atmosphere would lose almost all of its oxygen within a few tens of thousands of years. But the oxygen would be lost due to chemical reactions, not escape to space.

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 a half-century 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 moving scientists.


The Institute’s SETI projects are conducted by the Center for SETI Research

Observing Projects

What is the premise of SETI?

SETI is an acronym for the Search for Extraterrestrial Intelligence. It is an effort to detect evidence of technological civilizations that may exist elsewhere in the universe, particularly in our galaxy. There are potentially billions of locations outside our solar system that may host life. With our current technology, we have some ability to discover evidence of cosmic habitation, and in the specific case of our SETI experiments to find beings that are at a technological level at least as advanced as our own.

Has the SETI Institute found an extraterrestrial signal?

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

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 was ever detected again, and for scientists that’s not good enough to claim success and boogie off 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?

Virtually all radio SETI experiments have looked for what are called “narrow-band signals.” These are radio emissions that extend over 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.

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 dial.

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 find 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 will be an enormously big story. We’ll 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 will 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 it 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 among the public 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, 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, 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, and indicate good frequencies to search. Alternatively, these same frequencies might be avoided by E.T. to prevent “pollution” of these scientifically important wavelength bands. At the ATA, 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 do 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 all 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. Since 2011, the SETI Institute has been expanding its search to discover 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 sixty 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.

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, Alpha Centauri, at about 4.2 light-years’ distance, would take such a rocket close to 100,000 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) with 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 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 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 be 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?

The first thing to do is to confirm that it’s truly extraterrestrial. Remember, with tens of millions of channels and antennas that are among the world’s largest, SETI picks up thousands of signals daily. 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 will 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.

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 search 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, might be much weaker.

If we do succeed in finding a message, could we understand it? If the signal is intentional, it might be decipherable. In order to send or receive a signal over interstellar distances, a civilization must understand basic science and mathematics. Hence, a message from another civilization might use science and math (or simply pictures) to build up a common language with other societies.

Although, signals sent by a civilization for its own purposes may be impossible to unravel, SETI scientists are developing statistics-based algorithms to determine the amount of information sent. This can tell us, almost immediately, something about their level of intelligence.

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 at the SETI Institute) conducted the first radio search for evidence of technology in other solar systems 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 several SETI programs underway, including SERENDIP which collects data at the Arecibo Observatory in Puerto Rico. About 3% of these data are made available for processing by the popular SETI@home screen saver software.

Another radio SETI search is being conducted at the Medecina Radio Observatory of the University of Bologna, in Italy.

Optical SETI programs – which search for very brief (nanosecond) flashes of light – are being conducted at the University of California Berkeley’s Leuschner Observatory (Project SEVENDIP) 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 in one respect or another. These include limits on sensitivity, frequency coverage, types of signals the equipment could detect, and the number of stars or the directions in the sky observed. For example, while there are hundreds of billions of stars in our 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. It is currently managed by SRI International, in Menlo Park, California.

Because it has the ability to study many areas on the sky at once, and is continually being upgraded with improved receivers and spectral analyzers, the ATA is able to sift through targeted stellar systems far more quickly than previous SETI experiments. To date, several thousand star systems have been observed with high sensitivity and over a wide range of radio frequencies. The ATA will permit an expansion of this reconnaissance to 100 thousand or even 1 million nearby stars in the next two decades.

What is the Allen Telescope Array observing now?

There are several observing projects currently running on the Allen Telescope Array (ATA). One is a reconnaissance of star systems found to have planets (or planet candidates) by NASA’s Kepler Mission, and especially those planets in their own stellar habitable zone. Kepler has uncovered thousands of candidate planets since its launch, and these are on the observing list for the ATA. They will be examined over a spectral range from 1 to 9 GHz (1000 – 9000 MHz).

In addition, the ATA is observing 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, and on-going effort by the ATA is 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 is 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.

Why does the Allen Telescope Array (ATA) have 42 antennas?

In the past, large 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 GOAL 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, 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 a rare event or a common one 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 likely 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. SETI, 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 eighty radio stations carry the program, and the web site will tell you if there’s a station in your area. The show is also available from several podcast outlets, including iTunes.