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The planetary system around the star Gliese 581, just 20 light years away, finds itself ever more at the centre of attention of astronomers in search of planets that could support life. In 2007 two planets were detected around the red dwarf, Gliese 581 c and Gliese 581 d, located at the inner and outer edges of the habitable zone, the range of orbital radii where a hypothetical rocky planet would be able to retain liquid water on its surface.
Of these two, the first seemed the most promising because it is closer to the star, while "d" was thought to be too cold, so that the atmosphere on the night side would freeze and be precipitated on the ground, making the surface uninhabitable. Later simulations on "c" made life on this planet also seem less likely than originally thought, due to the extremely strong greenhouse effect, like on Venus, that would cause any water to evaporate.
Attention was shifted from both "c" and "d" when, at the end of 2010, another small planet was detected (as ever by the gravitational perturbations that the planets exert on the star), Gliese 581 g, with dimensions similar to the Earth and located in the centre of the habitable zone. Basically the ideal candidate. Unfortunately, follow-up work indicates the "g" may not exist, the original "discovery" being a false detection.
But now a new study has appeared in The Astrophysical Journal Letters by Robin Wordsworth, François Forget (Institute Pierre Simon Laplace de Paris) and their colleagues at the Laboratoire de Météorologie Dynamique (CNRS, UPMC, ENS Paris, Ecole Polytechnique) that re-evaluates the atmospheric conditions on Gliese 581 d. This body is now thought to be the exoplanet most likely to be similar to Earth, although with a mass seven times greater and a surface gravity double the terrestrial value it's not quite a "twin".
The researchers have developed a new 3-dimensional atmospheric model to investigate the surface conditions on the planet, taking into account the many physical factors and atmospheric ingredients that affect the outcome. Amongst the many possible combinations of conditions, they found that if the atmosphere of Gliese 581 d were dominated by carbon dioxide (a likely scenario), then its surface temperature would be sufficient to retain the atmosphere also on the night side of the planet and produce clouds and rain able to sustain surface water.
In addition to this, although the sky would be dim and murky, it may well appear blue, something which would make us feel more at home were we ever to visit!
Given the past surprises involving the Gliese 581 system, it would be prudent not to over-do the optimism and wait until the large telescopes of the future analyse the atmosphere directly, if it has one!
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