Ignored Exoplanet May Be a Watery World

Mark Brown, Wired UK May 18, 2011
http://www.wired.com/wiredscience/2011/05/gliese-581d-exoplanet

Gliese 581g, a habitable Earth-like exoplanet orbiting the red dwarf, thrilled astronomers when it was discovered in September 2010 as it was the most feasibly habitable exoplanet yet observed. But a few months later its entire existence was brought into question — no one has seen any significant signal from 581g since. It could have just been noise in the stellar wobbles of the faraway red dwarf.

The mysterious 581g sat in the so-called Goldilocks zone, where it orbited at just the right distance from its roasting parent star that water, if it existed on the planet, would neither boil nor freeze.

But now a group of French researchers, led by British scientist Robin Wordsworth, have taken another look at the data for 581d — another of the red dwarf’s planets — performing a comprehensive 3-D climate simulation on the planet. The simulation uses fundamental physical principles to look at a wide range of conditions, and account for any atmospheric cocktail of gases, clouds and aerosols.

To the team’s surprise, it believes that 581d would have a dense carbon dioxide atmosphere, which would give the planet a stable and warm climate. In a press release, France’s National Centre for Scientific Research (CNRS) concluded that 581d is likely warm enough to hold liquid water, in “oceans, clouds and rainfall”.

Anyone who stood on the planet would probably see clear blue skies, like on Earth, also. Most planets’ thick atmospheres bounce the majority of sunlight back into space. On Earth, and potentially 581d, the Rayleigh scattering phenomenon lets more sunlight in, leading to blue skies and a warmer climate.

But if humans ever did walk on 581d they’d still find a pretty bizarre planet with very un-Earthlike conditions. The dense air and thick clouds would drape the surface in a perpetually murky red twilight, and its hulking mass (at least 5.6 times that of Earth) means surface gravity would be double that of Earth’s.

“The most important implication of these results,” a spokesperson for the Scientific Research center said in a press release, “may be the idea that life-supporting planets do not in fact need to be particularly like the Earth at all.”