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When deciding which planets beyond Earth could host life, astronomers usually follow the water. Exoplanets with rocky surfaces are declared habitable if they orbit far enough from their star to be warm, while potentially supporting oceans and seas.

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.

Some items on the checklist can be inferred just from knowing a planet's size, mass and distance from its host star. Others will require directly photographing the planets and probing the contents of their atmospheres. Spacecraft with those capabilities, like the proposed StarshadeMovie Camera mission and the upcoming James Webb Space Telescope, are already being developed.

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