Clouds in the atmospheres of extrasolar planets. V. The impact of CO2 ice clouds on the outer boundary of the habitable zone

TL;DR

This study investigates how CO2 ice clouds affect the climate and habitable zone boundaries of terrestrial exoplanets, revealing that their greenhouse effect varies with stellar type and can slightly extend habitable zones around hotter stars.

Contribution

It provides a detailed radiative-convective model analysis of CO2 ice cloud impacts on exoplanet surface temperatures and habitable zone limits, considering different star types.

Findings

CO2 clouds cause limited greenhouse warming, especially around cool M-stars.

Surface temperature increase can reach up to 30 K around F-type stars.

Outer habitable zone boundaries can extend by about 0.5 au with CO2 clouds around hotter stars.

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.