Our planet seems to be in just the right spot to sport a mild climate. Not too near the sun's heat, not too far from its warmth, in a narrow habitable zone in which water is liquid and life can thrive. But Earth could still support life even if it were as far from the sun as Saturn, claim two scientists in the US, as long as the air abounded with hydrogen. If they are right, then billions of life-bearing planets may exist much further from their host stars than astronomers had thought possible.
Earth owes much of its warmth to carbon dioxide and water vapour in its atmosphere trapping solar heat, but these greenhouse gases freeze at the low temperatures far from the sun. In contrast, hydrogen stays gaseous, and at high pressure it is also an effective greenhouse gas.
Raymond Pierrehumbert at the University of Chicago and Eric Gaidos at the University of Hawaii in Honolulu calculated the warming effect of a hydrogen blanket on Earth-sized planets, as well as on worlds a few times more massive than our own, known as super-Earths. They found that, swaddled in a hydrogen atmosphere a few dozen times thicker than our nitrogen-oxygen one, such a planet could keep warm at up to 15 times Earth's distance from the sun. And despite the thickness of this alien atmosphere, Pierrehumbert and Gaidos calculate that enough sunlight would reach the planet's surface to foster photosynthesis.
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