  It has been suggested that the recently discovered exoplanet GJ581d might be
able to support liquid water due to its relatively low mass and orbital
distance. However, GJ581d receives 35% less stellar energy than Mars and is
probably locked in tidal resonance, with extremely low insolation at the poles
and possibly a permanent night side. Under such conditions, it is unknown
whether any habitable climate on the planet would be able to withstand global
glaciation and / or atmospheric collapse. Here we present three-dimensional
climate simulations that demonstrate GJ581d will have 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.
We find that atmospheres with over 10 bar CO2 and varying amounts of background
gas (e.g., N2) yield global mean temperatures above 0 degrees Celsius for both
land and ocean-covered surfaces. Based on the emitted IR radiation calculated
by the model, we propose observational tests that will allow these cases to be
distinguished from other possible scenarios in the future.
