Once the skies had been thronged with gods. Priests and soothsayers-the first astronomers-discerned the biddings of the divine in the stars and planets. Even as man's vision of himself changed from the creation of a supernatural being to a chance byproduct of cosmological theory, from standing at the centre of the celestial spheres to being an infinitesimal speck clinging to a rocky ball spinning round a near-infinite universe, the primordial urge to populate the heavens remained. It is no longer just a spiritual quest. Astronomers now reckon there are billions of planets in the galaxy that are broadly similar to our own. Some of the most sophisticated scientific techniques of our age have been brought to bear in finding and studying them. Yet the question remains the same as it has for millennia: is there anybody out there?
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.
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