More Earths Out There than We Thought?

Scouring Space for Life: More Earths Out There than We Thought?

May 24  2011

An artist’s conception shows the inner four planets of the Gliese 581 system and their host star, a red dwarf star 20 light-years away from Earth

Planet hunting is by far the hottest area of astronomy these days, and just about everyone who’s in on the search is looking for the same thing: a distant world where life could exist, at least in theory. That means a world more or less the size of Earth, orbiting its parent star in the habitable zone — the location, just the right distance away from its sun’s heat, where water can exist in liquid form. Size or distance alone aren’t good enough: an Earth-size planet that’s too hot or too cold probably couldn’t support life, and a giant gasbag like Jupiter couldn’t either, even if its temperature were ideal.

It’s a pretty strict set of requirements — but maybe not as strict as scientists have assumed. Two new studies, one purely theoretical and the other focused on a known exoplanet, suggest that planets that would on first blush seem too cold to harbor life may be balmier than expected. That means the habitable zone could be a lot wider, and the prospects for alien life more favorable, than anyone thought. (See pictures of the Hubble telescope’s greatest hits.)

In both cases, these insights come not from the science of astronomy but from atmospherics — and that’s not really so surprising. Without the greenhouse gases carbon dioxide and water vapor to trap heat, for example, Earth would be frozen solid. With an excess of carbon dioxide, Venus is hot enough to melt lead. And Mars, which is technically within our solar system’s habitable zone, has such a thin atmosphere that it’s too frigid for life (although it was much warmer in the distant past, when its atmosphere was thought to have been thicker). Once astronomers know an exoplanet’s size and location with reasonable accuracy, they can turn it over to climate modelers who can create a virtual atmosphere and see how it might behave.

That’s what a French team did for the planet Gliese 581d, which orbits a red dwarf star about 20 light-years from Earth. Given its size — about twice that of Earth — 581d could reasonably have enough gravity to hold onto an atmosphere, even a thick one. But the planet was nonetheless ruled out as a site for life since it probably has one face turned permanently away from its star, the way the far side of the moon never faces Earth. The far side of Gliese 581d would thus be so cold that the atmosphere would freeze out. Beyond that, the planet is too far from the star to get much heat in the first place. (See pictures of Earth from space.)

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