Habitable Climates

The Earth is only partially habitable according to the standard liquid-water definition. We reconsider planetary habitability in the framework of energybalance models, the simplest seasonal models in physical climatology, to assess the spatial and temporal habitability of Earth-like planets. In order to quantify the degree of climatic habitability of our models, we define several metrics of fractional habitability. Previous evaluations of habitable zones may have omitted important climatic conditions by focusing on close Solar System analogies. For example, we find that model pseudo–Earths with different rotation rates or different land–ocean fractions generally have fractional habitabilities that differ significantly from that of the Earth itself. Furthermore, the stability of a planet’s climate against albedo-feedback snowball events strongly impacts its habitability. Therefore, issues of climate dynamics may be central in assessing the habitability of discovered terrestrial exoplanets, especially if astronomical forcing conditions generally differ from the moderate Solar System cases. Subject headings: astrobiology – planetary systems – radiative transfer