Radiative constraints on the habitability of exoplanets Gliese 581c and Gliese 581d

¹ Laboratory for Climate and Ocean-Atmosphere Studies, Peking University, 100871 Beijing, PR China
e-mail: yyhu@pku.edu.cn
² Dept. of Atmospheric and Oceanic Sciences, School of Physics, Peking University, 100871 Beijing, PR China

Received: 28 April 2010
Accepted: 18 November 2010

Abstract

Aims. The M-type star Gliese 581 is likely to have two super-Earth planets, i.e., Gl 581c and Gl 581d. The present study is to investigate their habitability constrained by radiative properties of their atmospheres and the threshold of carbon-dioxide (CO₂), assuming that the two exoplanets are terrestrial, and that they have similar outgassing processes to those of the terrestrial planets in our own solar system.

Methods. Radiative-convective atmospheric models are used. Different values of CO₂ concentrations and water-vapor mixing ratios are tested.

Results. Our simulation results suggest that Gl 581d is probably a habitable planet. However, at least 6.7 bars of CO₂ are required to raise its surface temperature (T_s) above the freezing point of water. In contrast, Gl 581c might have experienced runaway greenhouse, like Venus, because of its too high surface temperature and the lack of an effective cold trap for water vapor. We compare our results with other independent studies.