But as our planetary collection grows, and our telescopes for studying them improve, some astrobiologists say it is time to narrow the search. Taking what we know about the extremes that life can endure, Christopher McKay at NASA's Ames Research Center in California has come up with an expanded checklist for habitability.
"If we go through that checklist and, bang-bang-bang-bang, we've got it all, that is incredibly exciting," says McKay. "Then we have a compelling case for a planet with life." The list may one day pinpoint not only benign, Earth-like environments, but also worlds that may host other forms of life in conditions that would kill humans outright.