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SETI and the Search for Life

Christopher F. Chyba, SETI Institute

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Summary: Excerpts from the written testimony submitted by Christopher F. Chyba, SETI Institute, to the "Life in the Universe" hearings held by the House Subcommitee on Space and Aeronautics on July 12, 2001.

Christopher F. Chyba of the SETI Institute discusses the search for life in the Universe. Are we alone?

[[Excerpts from the written testimony submitted by Christopher F. Chyba, SETI Institute, to the "Life in the Universe" hearings held by the House Subcommitee on Space and Aeronautics on July 12, 2001.]]

Over the past decade, there has been a rebirth in the scientific study of life elsewhere in the Universe - and for very good reasons. We've learned that organic molecules - the sort of carbon-based molecules all life on Earth is based upon - are abundant not only in our own solar system, but throughout the space between the stars. They are likely to be present in many other solar systems as well.

We're finally beginning to discover other planets are out there. While we can't yet detect solar systems like our own, at a minimum we now know that planets are not rare. My own suspicion is that just about every kind of solar system that could be out there, will be out there. Our solar system will prove to be neither common nor rare, but instead just one example of a wide variety of possibilities.

Within our solar system we have more and more evidence of other worlds with liquid water, which is an essential ingredient for life as we know it. Water seems to have flowed on the martian surface in the geologically recent past. There is now strong, though still indirect, evidence for a second ocean in our solar system beneath the ice of Jupiter's moon Europa - the evidence from the Voyager and especially Galileo spacecraft missions points towards an ocean whose volume is nearly twice that of all the Earth's oceans combined. If we want to look for life in our solar system, the importance of Europa can hardly be exaggerated. Perhaps even more astonishing, there is now evidence for subsurface oceans under the ice of two of Jupiter's other large moons, Ganymede and Callisto. We've gone from thinking that Earth's ocean is unique to thinking that our ocean may be one of many.

We've also learned that Earth harbors a deep subsurface biosphere, and that the mass of microorganisms beneath our feet, reaching down miles underground, likely equals or exceeds the mass of all the organisms on Earth's surface. This is a dramatically different picture of terrestrial life than the one we experience daily, and makes speculation about subsurface life on Europa or vestigial life on Mars seem much more credible. Our understanding of the Earth helps shape our thinking about other worlds, and vice-versa.

The prospects for finding life elsewhere seem better than ever. But we need to remember that prospects are not proof, and it may be possible that Earth is the only planet where life exists. That would seem extraordinary, and I doubt it's likely in a galaxy with 400 billion stars, but the honest answer is that we don't know yet. But we can use scientific exploration to try and find out.

The SETI (Search for ExtraTerrestrial Intelligence) Institute is a private scientific institute dedicated to research, education, and public outreach. Its mission is to use scientific methods to investigate the origin, nature, and prevalence of life in the Universe. SETI Institute scientists investigate everything from the formation of stars and planets to the development of advanced technical civilizations. Research topics include, for example, interstellar organic chemistry, planet formation, the search for extrasolar planets, the chemistry of life's origins, microbiology and life in extreme environments, planetary climatology and habitability, Mars and Europa, and the role of asteroid and comet impacts in the history of life on Earth.

By understanding the many factors that make a world habitable for complex life, we can put the search for extraterrestrial civilizations into context. For instance, by learning more about the history of life on Earth, we can try to track the events that have led to the development of intelligence. But we don't know whether the evolution of human-style technical intelligence is something that will prove to be incredibly rare or common. Finding evidence for such intelligence elsewhere would have a profound effect on humanity.

One of SETI's best-known projects is the search for artificially produced radio signals in the vicinities of nearby stars. Many natural objects in the Universe produce radio waves (including our own Sun), but no naturally occurring source in the Universe is known that produces bandwidths thinner than 300 Hz (Hertz, a measure of frequency equal to one cycle per second). So the first criterion of an artificial signal is a very narrow spectral bandwidth. In fact, we look for bandwidths below about 1 Hz in width. This wavelength of the signal is extremely precise and highly efficient, because narrow-band signals pack a lot of energy into a small amount of spectral space. Any object producing extremely narrow bandwidth signals is either artificial or represents some entirely unknown astrophysical phenomenon.

To give some sense of how sensitive our radio searches are, it's worth mentioning that we have for many years tested our system by using the signal transmitted by the Pioneer 10 spacecraft, launched from Earth in 1972 and now traveling beyond our Solar System. Pioneer 10 is at a distance of 6 billion miles from Earth and broadcasting with a power of a few watts - much less than a light bulb in your house, but about the power of a small flashlight. It takes more than 10 hours for Pioneer 10's radio signal, traveling at the speed of light, to reach Earth. Because Pioneer 10 really is an extraterrestrial (even extra-Solar System!) artificial source, it provides an excellent test for our system - and it comes in loud and clear.

Since SETI gets so much media attention, it is easy to get the mistaken impression that SETI researchers have searched the galaxy thoroughly for alien radio signals, yet have found nothing. But in fact we've only examined about one-billionth of the galaxy so far. We're looking at the thousand nearest Sun-like stars that lie within about 150 light years of Earth. This is only a tiny fraction of the entire Milky Way galaxy, which contains some 400 billion stars and is 100,000 light years across

Even if alien signals are detected someday, it is unlikely that an interstellar dialogue would occur, except over extremely long timescales. If we detect a signal from a star 100 light years away, that message was sent 100 years ago - so that two-way communication would require 200 years for each reply.

It is quite possible that, while we could detect the signal's carrier wave, we would not have the sensitivity to detect whatever message might be carried by that wave. Even if we could, it is difficult to predict how difficult decipherment might prove to be. A possible analogy could be the decipherment of inscriptions left by the ancient Maya, which proved extremely difficult. Even in this case, we had the advantage of being able to apply linguistic knowledge from extant Maya languages. And of course, we share a genetic and sociological heritage with any other human culture that we will not share with an extraterrestrial civilization. Nevertheless, if we detect an extraterrestrial radio signal, we will at least have in common the physics and mathematics that made that transmission possible, and this could be a starting point.

The scientific Search for Extraterrestrial Intelligence enjoys great public interest. We see this every day at the Institute, where we serve as a resource for the press covering topics across the range of life in the Universe studies. Our web site (http://www.seti.org/) receives about two million hits per month. We view this kind of interest as a tremendous opportunity to teach students and the general public about science and the scientific method -that blend of openness to new ideas coupled with an insistence on hard evidence and skeptical analysis of data.

The goals of the SETI Institute fit naturally with the fundamental questions at the heart of NASA's Astrobiology program: "Does life exist elsewhere in the Universe, or are we alone?" and "What is life's future on Earth and beyond?" Whether any other intelligent civilizations exist elsewhere is a natural extension of these questions. The scientific investigation of these questions is exciting, inspiring, and eventually may help humanity find its place in the Universe.

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