Greenhouse Effect May Expand Habitable Planet Roster

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Even though Gliese 581 d circles its star every 83.6 days, it still lies on the outer limits of the star's planetary habitable zone.

"In terms of total solar [output], this particular extrasolar planet only receives about a fifth of what Earth receives," Selsis said.

But super Earths, typically several times the mass of our own planet, should have no trouble holding onto their atmospheres.

The data suggest that strong enough greenhouse warming from carbon dioxide clouds and other gases could make such planets habitable even if they lie far from an ideal orbit.

"Carbon dioxide ice clouds are very reflective," said David Crisp, an atmospheric physicist at NASA's Jet Propulsion Laboratory, who was not involved in the study.

"They work like space blankets to trap thermal radiation by reflecting it back to the surface, rather than absorbing it and reemitting it back into space."

More Super-Earths

The results aren't definitive, though.

"Whether or not you can get surface temperatures above freezing at [Gliese 581 d] distance is not clear," said James Kasting, a planetary scientist at Pennsylvania State University and one of the paper's co-authors.

"The greenhouse effect of clouds is very complicated. If you look at the biggest uncertainties of global warming on Earth, they are also the clouds."

The clouds' exact reflectivity would depend on the size of particles within the clouds themselves, Crisp said. Smaller particles would tend to reflect more radiation back into space than back onto the planet's surface.

But even if this one planet doesn't ultimately harbor life, Kasting said he's encouraged by the fact that super Earths are being discovered so readily.

Within the next year or two, someone is going to find rocky planets around red dwarf stars that are "smack inside" the habitable zone, he said.

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