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 fi rst confi rmed super-Earth (exoplanet of 2-10 Earth masses) in the habitable zone. We nd that atmospheres with over 10 bar CO2 and varying amounts of background gas (e.g., N2) yield global mean temperatures above 0 C 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.
Infrared emission spectra for an ocean / rocky GJ581d with a 20-bar atmosphere (blue / red) and a rocky GJ581d with no atmosphere (grey). Thickness of the lines corresponds to the maximum / minimum diff erence in flux over one orbit, for the most probable observation angle of 60. The maximum (minimum) surface temperature in the airless case is 271 K (37 K). For the cases with atmospheres, emission of the ocean planet is higher at most wavelengths due to the increased planetary radius. The major absorption features are labelled by molecule / process (CO2-CO2 corresponds to CO2 collision-induced absorption).
Unlike the majority of the Kepler planetary candidates, Gliese 581d is relatively close to Earth, so in the future it will be possible to establish which scenario applies to it through direct spectroscopic observations. To determine the sensitivity required for this, we used the outgoing longwave radiation (OLR) from our simulations to produce synthetic emission spectra for the habitable climates just discussed, along with those for a planet with no atmosphere (Figure above). We did not calculate emission spectra for the H2-He dominated case, but equilibrium chemistry calculations1 allowed us to establish that for an atmosphere where H2 is the dominant species by mass, the abundance of CO2 should be below the limit of detectability. For a rocky / icy planet where volatiles like CO2 have collapsed on the surface and the atmosphere is thin or non-existent, the flux variations over one orbit will be large (grey region in Figure above). Observations of CO2 and H2O absorption bands with low phase variations (red / blue regions in Figure above) would hence be a strong indicator of the kind of stable, dense habitable atmospheres discussed in this paper.