  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.
