Zina Deretsky, National Science Foundation

The orbits of planets in the Gliese 581 system are compared to those of our own solar system. The Gliese 581 star has about 30 percent the mass of our sun, and the outermost planet is closer to its star than we are to the sun. Gliese 581d might be able to sustain liquid water on its surface.

The case is building about the habitability of a planet orbiting a red dwarf star about 20-light years away from Earth, according to a new climate modeling study.

The planet, Gliese 581d, is one of a handful of planets orbiting the star Gliese 581. When it was discovered in 2007, astronomers thought it was likely too cold for liquid water, and thus life.

The new study, accepted for publication in the Astrophysical Journal Letters, suggests high concentrations of carbon dioxide in its atmosphere could keep things warm enough for liquid water to be sustained at the surface.

The finding falls on the heels of a similar atmospheric modeling studies published that have reached a similar conclusion.

Atmospheric collapse
However, those studies were based on simple simulations that couldn't determine whether or not the atmosphere would collapse due to the fact the planet likely has a permanent day and night side.

In such a situation, the night side could be cold enough to freeze out the atmosphere, ruining any prospects for a habitable climate.

The new study uses a model that simulates the atmosphere and surface in three-dimensions, much like models used to study climate change on Earth.

To their surprise, the researchers found that with a dense carbon dioxide atmosphere — a likely scenario on such a large planet — the climate of Gliese 581d is stable against collapse and warm enough to have oceans, clouds and rainfall.

The planet "will have a stable atmosphere and surface liquid water for a wide range of plausible cases, making it the first confirmed super-Earth … in the habitable zone," the team, led by Robin Wordsworth at the Institut Pierre Simon Laplace in Paris, concludes in the journal.

Rayleigh scattering 
A key factor of the result is a phenomenon called Rayleigh scattering, which makes the sky blue on Earth. In our solar system, the effect limits the amount of sunlight a thick atmosphere can absorb because a large amount of blue light is scattered back to space.

Since starlight from Gliese 581 is red, however, it is almost unaffected and can penetrate deep into the atmosphere and heat up the planet thanks to the greenhouse effect of carbon dioxide.

The simulations also show that daylight heating is efficiently redistributed across the planet by the atmosphere, preventing atmospheric collapse on the night side or the poles.

If the planet is indeed habitable, the researchers note that it would be a strange place: the dense air and thick clouds would keep the surface in a perpetual murky red twilight.

The Gliese 581 solar system also holds another candidate for habitability, 581g, which was announced last year. However, whether or not that planet actually exists remains up for debate.

The new study on 581d " is important because it's the first time climate modelers have proved that the planet is potentially habitable, and all observers agree that the exoplanet exists," Wordsworth told the British news agency Press Association. 

"The Gliese system is particularly exciting to us as it's very close to Earth, relatively speaking. So with future generations of telescopes, we'll be able to search for alien life on Gliese 581d directly."

Wordswoth added in an email to me that finding a potentially habitable planet that is so unlike Earth bodes well for the search for life in general.

"I think it's becoming clearer with every discovery we make in exoplanet science that the variety of worlds out there in the universe is going to be far greater than the few examples we are used to from our solar system," he said.

More on the Gliese 581 system: 

• Alien planet looks just right for life
• Scientists find most earthlike planet yet
• Scientists find lightest alien planet yet 
• R.I.P possibly habitable planet 581g? Not so fast, co-discoverer says 
• How to find life on an alien planet? 
• Alien planet may be in the habitable zone after all 
• Hopes dashed for life on distant planet