Mounting evidence indicates a planet orbiting a star 20 light years away could be the first habitable planet other than Earth, French researchers say.

Planet Gliese 581d is at the colder outer edge of the so-called just-right "Goldilocks zone" around its red dwarf star in which liquid water can be sustained.

A study by French researchers published in Astrophysical Journal Letters suggests its atmosphere may be warm enough for liquid water to exist, the BBC reported Tuesday.

Planet 581d, with a mass about six times that of Earth and twice its size, was initially thought to be too cold to have liquid water.

Now researchers at the Institut Pierre Simon Laplace in Paris have run computer simulations of the planet's atmosphere, arguing it is likely to contain high concentrations of carbon dioxide, and say condition could exist for oceans of liquid water as well as clouds and rainfall.

"This discovery is important because it's the first time climate modellers have proved that the planet is potentially habitable," institute researcher Robin Wordsworth said.

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