Media playback is unsupported on your device Media caption Kirsty Lang explains the unique qualities of the exoplanet Gliese 581d

A red dwarf star 20 light-years away is again providing hints that it hosts the first definitively habitable planet outside our Solar System.

The planet Gliese 581d is at the colder outer edge of the "Goldilocks zone" in which liquid water can be sustained.

Now a study in Astrophysical Journal Letters suggests its atmosphere may keep things warm enough for water.

The solar system also hosts another contender for habitability, unconfirmed planet Gliese 581g announced in 2010.

However, the existence of that planet has since been called into question.

Gliese 581d is less controversial; it was discovered along with the planet Gliese 581c in 2007, occupying the outer and inner edges of the Goldilocks zone, respectively.

Gliese 581c was soon determined to be too close to its host star to sustain water, with a surface temperature exceeding 1,000C.

With future generations of telescopes, we'll be able to search for life on Gliese 581d directly Robin Wordsworth, Institut Pierre Simon Laplace

Conversely, the outlying planet 581d - with a mass about six times that of the Earth and twice its size - was initially taken to be too cold to have liquid water.

Now, French researchers have run computer simulations of the planet's atmosphere, arguing that it is likely to contain high concentrations of carbon dioxide.

They contend that conditions could be suitable for oceans of liquid water as well as clouds and rainfall.

However, Gliese 581d's denser air and dim red light from its host star would make for a murky environment that would be toxic to humans.

Robin Wordsworth, a member of the team from the Institut Pierre Simon Laplace in Paris, said that the findings were further evidence that the sheer variety of planets and environments far outpaced that which we see in our own Solar System.

Dr Wordsworth said that the simulations are tantamount to a first definitive claim for a habitable exoplanet.

"This discovery is important because it's the first time climate modellers have proved that the planet is potentially habitable, and all observers agree that the exoplanet exists," he told news agency PA.

"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 life on Gliese 581d directly."