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Implications of Astrobiology for SETI
by Albert A. Harrison, Ph.D. (email aaharrison @

Astrobiologists use telescopes, interferometers, spectrometers and other devices to discover suitable locations for life, find life's physical or chemical precursors, and identify signs of past or current biological activity. As currently practiced, astrobiology strengthens preliminary terms in the Drake Equation (stars, planets, habitability, initiation of life), for now avoiding concluding terms such as the evolution of intelligence and longevity. SETI searches bypass specific components of the Drake Equation and seek unequivocal signs of technologically advanced civilizations. By developing evidence in support of specific parts of the Drake Equation, astrobiologists bolster the rationale for SETI. This brief editorial identifies other possible implications of astrobiology for SETI.

Astrobiologists are discovering large (multiples of Jupiter) planets almost faster than they can be catalogued. Within a decade, new Earth-based and space-based telescopes will go on line. NASA's planned Terrestrial Planet Finder (TPF) and Next Generation Space Telescope (NGST) are but two examples. These instruments will make it possible to image Earth-like planets in other solar systems. Subsequent generation (circa 2020) devices will enable us to identify more distant habitable planets and monitor chemical activities suggestive of simple forms of such as bacteria. Within the 21st Century, the range of these imaging devices could increase from 30 to 100 LY.

Despite various rules of thumb or "heuristics" such as exploring frequencies in the vicinity of the "cosmic water hole," a continuing difficulty confronting SETI astronomers is where they should look. Advances in astrobiology will lead to new heuristics that should help astronomers aim their telescopes in promising directions. Of course, astrobiology cannot help astronomers locate machine intelligence broadcasting from interstellar space, and many promising planets will host life that does not progress to technologically advanced civilizations. Still, astrobiology may help astronomers steer clear of really unpromising locations.

Many SETI organizations including the Planetary Society, the SETI Institute, and the SETI League are supplementing microwave observation (MSETI) with optical observation (OSETI) intended to find continuous or pulsed lasers. One rationale for adding OSETI is that Earth may be "pinged" by a civilization within 50 LY that has detected terrestrial radio activity. Perhaps a much older and more technologically advanced civilization could use advanced astrobiological procedures (including remote imaging from multiple locations) to discover life on Earth from hundreds or even thousands of LY away. If so, we may not be invisible beyond the 50 LY boundary of our radio signature. Astrobiology increases the plausibility of theories suggesting that Earth has been detected by civilizations that fall beyond the range of our radio emissions.

We discount interstellar probes (such as Pioneer and Voyager) as practical means for encountering distant cultures. One of the many reasons for this is that there are countless possible destinations, and another is the tremendous time requirement for traversing interstellar distances. Still, an ancient civilization's discovery of life on Earth would give their scientists a promising destination and eliminate the need to dispatch fleets of probes in every imaginable direction. Because extraterrestrial civilizations are estimated to be billions of years older than our own, and because imaging extrasolar planets (like radio, lasers, and spacefaring) is a relatively early technology, extraterrestrial astrobiologists could have found our life-bearing planet well before the evolution of humans. There could have been ample time for a probe traveling at a small fraction of the speed of light to reach our solar system.

Thus, astrobiology is more than an esteemed partner conducting routine grunt work to increase our confidence that SETI will lead to a confirmed detection. Advances in astrobiology force us to rethink our search strategies and our plans for managing contact and its aftermath. Could it be that the step-by-step approach of astrobiology, rather than the direct approach of SETI, would first confirm the existence of extraterrestrial intelligence? Astrobiology's advantage over SETI is that it does not have to depend on civilizations billions of years older than our own sharing contemporary terrestrial technology. Breathe easy, SETI aficionados, NASA specifically prohibits its astrobiologists from searching for extraterrestrial intelligence!


Disclaimer: The opinions expressed in editorials are those of the individual authors, and do not necessarily reflect the position of The SETI League, Inc., its Trustees, officers, Advisory Board, members, donors, or commercial sponsors.

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