A planet orbiting a star 20 light-years from Earth could have the right conditions for sustaining life. Simulations carried out by a team of scientists in France suggest that the planet, called Gliese 581d, could harbour liquid water, clouds and rainfall, as well as winds that distribute the heat it absorbs from its star. However, the researchers also admit that the simulations might be wrong and the planet could have little or no atmosphere – or even be cloaked in a thick layer of hydrogen and helium.

First observed in 2007, Gliese 581d is thought by some astronomers to be a rocky planet with a mass at least seven times that of Earth, making it a "super Earth". It is one of more than 500 extrasolar planets (exoplanets) that astronomers have spotted orbiting stars other than the Sun. However, none of these exoplanets has been shown to be both Earth-like and to orbit within its star’s "habitable zone", where conditions on the planet would be just right for life to emerge.

Now simulations of the climate on Gliese 581d, which have been carried out by Robin Wordsworth, François Forget and colleagues at the Laboratoire Météorologique Dynamique and the University of Bordeaux, suggest that the exoplanet might be able to harbour life. Indeed, the team describe Gliese 581d as "the first discovered terrestrial-mass exoplanet in the habitable zone".

Gliese 581d is one of six exoplanets thought to orbit the red-dwarf star Gliese 581. It receives about a third of the energy that the Earth receives from the Sun and is also thought to have a hot side that always faces its star and a cold, dark side. The large temperature difference between the two hemispheres was expected to make it difficult for the planet to sustain the thick atmosphere needed for life.

Atmospheric models

Wordsworth and colleagues simulated conditions on Gliese 581d using a 3D model of the atmosphere that is similar to those used to study the Earth's climate. These work on the basis that the planet has a climate dominated by the greenhouse effects of carbon dioxide and water, which the researchers think is a reasonable assumption given that the climates of Venus, Earth and Mars are defined by these gases. The resulting simulations suggest that Gliese 581d could have a thick atmosphere – and that it could be warm enough to have oceans, clouds (both water and carbon dioxide) and rainfall.

One key driver towards habitability, according to the researchers, is the red colour of the exoplanet's parent star. Rayleigh scattering in a planet's atmosphere usually tends to reflect incoming blue light back into space. However, Gliese 581 emits little blue light and therefore the exoplanet absorbs a greater percentage of its star's light compared with the Earth and the Sun. Simulations of circulation within the atmosphere suggest that much of this heat could be transported to the dark side of the exoplanet, perhaps preventing the atmosphere there from condensing completely.

If the simulations are correct, conditions on Gliese 581d would be very different to those here on Earth. The dense atmosphere would let little light get to the surface, which would be in a perpetual murky red twilight – according to the researchers.

Or maybe it is not habitable

The team admits, however, that conditions on Gliese 581d could be very different to that described in the simulations. The exoplanet may have little or no atmosphere, thanks to a fierce stellar wind from Gliese 581 during its early years. Alternatively, Gliese 581d could have a thick layer of hydrogen and helium in its atmosphere, which would lead to a much less-hospitable climate.

To gain a better understanding of the exoplanet's atmosphere, the team has come up with a wish list of spectroscopy measurements of the exoplanet's atmosphere that it hopes will be performed by astronomers in the future. Although the researchers believe that the measurements are beyond the capability of current ground- and space-based telescopes, the exoplanet's close proximity to Earth means that the next generation of instruments could shed more light on Gliese 581d.

The research is described in Astrophys. J. Lett. 733 L48.