Greenhouse Effect May Expand Habitable Planet Roster

Bruce Dorminey
for National Geographic News
November 7, 2007
In the search for habitable planets around nearby stars, the greenhouse effect blamed for global warming on Earth may not always be bad news.

A French-led group of planetary researchers recently modeled the theoretical effects that a cocktail of "super-greenhouse" gases might have on a recently discovered giant planet.

The team found that the theoretical mixture may warm the planet Gliese 581 d to the right temperature for permanent liquid water, the major criterion of any habitable zone.

"Because this detection came very fast, big terrestrial planets seem to be abundant around red dwarf stars," said Franck Selsis at the University of Lyon, the lead author on the study. (Related: Record-Setting Fifth Planet Found Orbiting Nearby Star" [November 6, 2007].)

Combined with the new research, he said, soon "more planets will be found in the habitable zone of [red dwarfs]."

The new study is slated to appear in an upcoming issue of the journal Astronomy & Astrophysics.

Like Early Mars

The greenhouse effect can have negative consequences—it has made Venus an inhospitable inferno, for example.

But the heat-trapping effects of atmospheric gases helped make life on Earth possible. And if early Mars had been more massive, it might have had enough gravity to hold onto an atmosphere long enough to warm to habitable temperatures. (See a virtual solar system.)

To explore this issue, the team took a look at the three outermost planets circling the cool red dwarf star Gliese 581, which is located in the constellation Libra about 20.5 light-years away.

The group ran simulations of the possible warming effects on the three planets of carbon dioxide ice clouds, water, methane, and even ammonia—all potent greenhouse gases.

The most interesting results had to do with the "super Earth" called Gliese 581 d, which is at least 8.3 times the mass of Earth.

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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