The extrasolar planet GL 581 d: A potentially habitable planet?

The planetary system around the M star Gliese 581 contains at least three close-in potentially low-mass planets, GL 581 c, d, and e. In order to address the question of the habitability of GL 581 d, we performed detailed atmospheric modeling studies for several planetary scenarios. A 1D radiative-convective model was used to calculate temperature and pressure profiles of model atmospheres, assumed to be composed of molecular nitrogen, water, and carbon dioxide. The model allows for changing surface pressures caused by evaporation/condensation of water and carbon dioxide. Furthermore, the treatment of the energy transport has been improved in the model to account in particular for high CO2, high-pressure Super-Earth conditions. For four high-pressure scenarios of our study, the resulting surface temperatures were above 273 K, indicating a potential habitability of the planet. These scenarios include three CO2-dominated atmospheres (95% CO2 concentration with 5, 10, and 20 bar surface pressure) and a high-pressure CO2-enriched atmosphere (5% CO2 concentration with 20 bar surface pressure). For all other considered scenarios, the calculated GL 581 d surface temperatures were below the freezing point of water, suggesting that GL 581 d would not be habitable then. The results for our CO2-dominated scenarios confirm very recent model results by Wordsworth et al. (2010). However, our model calculations imply that also atmospheres that are not CO2-dominated (i.e., 5% vmr instead of 95% vmr) could result in habitable conditions for GL 581 d.