Title: Clouds in the atmospheres of extrasolar planets. V. The impact of CO2 ice clouds on the outer boundary of the habitable zone,
Abstract: Clouds have a strong impact on the climate of planetary atmospheres. The
potential scattering greenhouse effect of CO2 ice clouds in the atmospheres of
terrestrial extrasolar planets is of particular interest because it might
influence the position and thus the extension of the outer boundary of the
classic habitable zone around main sequence stars. Here, the impact of CO2 ice
clouds on the surface temperatures of terrestrial planets with CO2 dominated
atmospheres, orbiting different types of stars is studied. Additionally, their
corresponding effect on the position of the outer habitable zone boundary is
evaluated. For this study, a radiative-convective atmospheric model is used the
calculate the surface temperatures influenced by CO2 ice particles. The clouds
are included using a parametrised cloud model. The atmospheric model includes a
general discrete ordinate radiative transfer that can describe the anisotropic
scattering by the cloud particles accurately. A net scattering greenhouse
effect caused by CO2 clouds is only obtained in a rather limited parameter
range which also strongly depends on the stellar effective temperature. For
cool M-stars, CO2 clouds only provide about 6 K of additional greenhouse
heating in the best case scenario. On the other hand, the surface temperature
for a planet around an F-type star can be increased by 30 K if carbon dioxide
clouds are present. Accordingly, the extension of the habitable zone due to
clouds is quite small for late-type stars. Higher stellar effective
temperatures, on the other hand, can lead to outer HZ boundaries about 0.5 au
farther out than the corresponding clear-sky values.