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Sunday, May 29, 2011

Another Earth-like planet from Gliese 581

Astronomers are on the lookout for exoplanets that could sustain life. Such a planet would have to have a rocky (as opposed to gaseous) surface, and orbit its star at a distance that allowed for the presence of liquid water. In their search, astronomers led by Robin Wordsworth of the Laboratoire de Météorologie Dynamique have turned once again to Gliese 581.

At just over 20 light years from Earth, the red dwarf Gliese 581 is one of our closest neighbors. The star is known to have at least four planets, several of which have been of great interest. In particular, astronomers once believed that the planet Gliese 581g might be about the same size as Earth with an orbit that placed it well within the habitable zone. Unfortunately, Gliese 581g may not exist, which would be drawback for sustaining life. Since then, astronomers have taken a second look at another Gliese 581 planet: Gliese 581d.

Gliese 581d was originally discounted as an Earth-like planet because it’s too far from its star (receiving less than a third of the starlight that Earth receives from the sun) and may be tidally locked (no longer rotating, but keeping one side of the planet in perpetual darkness). However, computer modeling now indicates that the planet might have a stable and life-sustaining climate. If Gliese 581d has a thick carbon dioxide atmosphere (which is not unlikely), the mostly red light from that star could penetrate the atmosphere and keep the underlying planet warm and cozy. Wind simulations show that heat absorbed on the day-side could be redistributed over the whole planet.

Schematic of the global climate model used to study Gliese 581d. Red / blue shading indicate hot / cold surface temperatures, while the arrows show wind velocities at 2 km height in the atmosphere.

Credit: © LMD/CNRS.

At this stage of the game, finding an Earth-like planet requires a lot of speculation and computer modeling. However, because Gliese 581 is so close, the astronomers are hopeful that we’ll one day have telescopes powerful enough to visually examine its planets. That should make things really interesting.