Far-out Earths Could Support Alien Life

By Ken Croswell

Published on New Scientist (May 12, 2011)

Even if Earth were as far from the Sun as Saturn is, our planet could support living beings--if it had a thick hydrogen atmosphere. Credit: Voyager 2. NASA/JPL/US Geological Survey.

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 farther from their host stars than astronomers had thought possible.

Earth owes much of its warmth to carbon dioxide and water vapor 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.

"It's a clever idea," says James Kasting of Pennsylvania State University in University Park, "but I'm skeptical as to whether you can form these planets." He doubts that an Earthlike planet or super-Earth would pull in so much hydrogen from the cloud of gas surrounding a young star.

Kasting adds that far-out planets will be fainter and harder to see than close-in planets, so finding these distant worlds will be more difficult, as will studying their atmospheres.

Nevertheless, Pierrehumbert and Gaidos point to one known planet that may fit the bill. Named OGLE-05-390L b, it is about six times as massive as Earth. It orbits a red dwarf--a small, cool, faint star--at 2.6 times Earth's distance from the Sun. A naked planet so far from such a dim star would be a frigid world. But with a thick hydrogen atmosphere it could potentially sustain liquid water at its surface, say the researchers in a study to appear in The Astrophysical Journal Letters.

Yet if a far-out planet did spawn life, that life could sign its own death warrant. Some microbes consume hydrogen and carbon dioxide. By depleting these greenhouse gases, the microbes might turn their warm world into a giant snowball, killing them all.

Ken Croswell is an astronomer and the author of The Lives of Stars.

"A stellar picture of what we know or guess about those distant lights."--Kirkus. See all reviews of The Lives of Stars here.