Confident that life inhabits many other worlds, astrobiologists are now pressing to find evidence of it. Life may take many forms and exist on many scales, from microscopic to planetary. It may dwell on or below the surface of a world, or in an ocean. It may be alive or extinct, within our solar system or on the planet of a star many light-years away. Across this wide spectrum of possibilities, astrobiologists are evolving the techniques needed to catch an uncertain and elusive prey.
What kind of life are they trying to find? By now, the answer should come as no surprise: Astrobiologists are looking for Earth-like life. This isn't because they're an uninspired bunch, unable to contemplate the wildly exotic, but because they increasingly believe that much of the life in the universe probably is Earth-like, at least in its basic biochemistry. If it isn't, they're in trouble, because the only life signs they have a reasonably clear idea of how to look for are those from broadly terrestrial-type organisms. Still, if bio-instruments did turn up anything out of the ordinary, they would at least sound an alarm. Then further research could be focused on finding out whether or not some unfamiliar biological process was at work.
Two episodes have highlighted the problems scientists face in establishing the presence of extraterrestrial life. The life detection package carried aboard Viking in 1976 was the crowning achievement of the first fifteen years of what the geneticist Joshua Lederberg dubbed "exobiology." However, its legacy is troubling. One of the scientists involved, Norman Horowitz, concluded that Viking "found no life and found why there was no life," while another, Gilbert Levin, said "the scientific process forces me to my conclusion that