| 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 (CO 2), 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 CO 2 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 CO 2 are required to raise its surface temperature ( Ts) 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.
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