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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% the mass of our sun, and the outermost planet is closer to its star than we are to the sun. The fifth planet, 581d, could sustain life. (National Science Foundation/Zina Deretsky)
French astrophysicists have determined that a rocky planet 20 light years from Earth is the first planet outside our solar system—an exoplanet—that is in the habitable zone.
Gliese 581d is one of several exoplanets in a system orbiting a red dwarf star, Gliese 581, that has already received considerable attention since its discovery in 2007. Last September, Gliese 581g was controversially proposed as a Goldilocks planet, capable of supporting life. Since then doubt has been cast on its existence.
Now, scientists from the Institute Pierre Simon Laplace in Paris have used new computer modeling techniques that can simulate exoplanet climates and surfaces in 3D to predict that Gliese 581d, initially thought to be too cold to support life, may be warm and wet enough for Earth-like life to exist there.
Gliese 581d is approximately double Earth’s size, with a mass at least seven times that of our planet. With a permanent day and night side, and less than one-third of the stellar energy that shines on Earth, the exoplanet seems unlikely to be habitable as an atmosphere thick enough for warming would probably freeze out on the night side.
However, the team’s climate simulations demonstrate that Gliese 581d has “a stable atmosphere and surface liquid water for a wide range of plausible cases, making it the first confirmed super-Earth (exoplanet of 2-10 Earth masses) in the habitable zone,” as stated in the abstract of the study, which is published in The Astrophysical Journal Letters.
According to a press release from France's National Center for Scientific Research (CNRS), if Gliese 581d is modelled to have a dense carbon dioxide atmosphere, which is a likely scenario, its climate is “not only stable against collapse, but warm enough to have oceans, clouds, and rainfall.”
As the starlight from Gliese 581 is red, light can travel much further into the planet’s carbon dioxide atmosphere, creating heat via the greenhouse effect. In our Solar System, this would not be possible due to Rayleigh scattering whereby a thick atmosphere reflects the blue component of sunlight back into space, causing Earth’s sky to be blue.
The modeling also shows that the atmosphere efficiently redistributes daylight heating around the planet through the atmosphere, preventing atmospheric collapse on the poles and the night side.
The institute's press release states that in the future telescopes will be able to detect the exoplanet's atmosphere directly because it is relatively close to Earth. The team has devised several simple tests that will allow future observers to glean other information, such as whether Gliese 581d has retained some atmospheric hydrogen, like Uranus or Neptune.
As well as being bathed in red light, the planet’s large mass means its surface gravity would be approximately twice that of Earth’s, suggesting that life-supporting planets may not need to be particularly Earth-like at all, according to the release.
