A novel metric for assessing planetary surface habitability

Planetary surface habitability has so far been, in the main, considered in its entirety. The increasing popularity of 3-D modelling studies of (exo)planetary climate has highlighted the need for a new measure of surface habitability. Combining the observed thermal limits of Earth-based life with surface water fluxes, we introduce such a measure which can be calculated from the climatological outputs from general circulation model simulations. In particular, we pay attention to not only the bounds of macroscopic complex life, but additionally the thermal limits of microbial and extremophilic life which have been vital to the generation of Earth's own biosignatures. This new metric is validated on Earth, using ERA5 reanalysis data to predict the distribution of surface habitability which is then compared to the observed habitability created from satellite-derived data of photosynthetic life. Additionally, the validation against observed habitability is repeated for a selection of popular metrics of surface habitability, allowing for the first time a comparison of metric performance with respect to Earth-based surface life.