Modelling biogeochemical controls on planetary habitability

The length of a planet's `habitable period' is an important controlling factor on the evolution of life and of intelligent observers. This can be \ndefined as the amount of time the surface temperature on the planet remains within defined `habitable' limits. Complex states of habitability derived from complex interactions between multiple factors may arise over the course of the evolution of an individual terrestrial planet with implications for long-term habitability and biosignature detection. The duration of these habitable conditions are controlled by multiple factors, including the orbital distance of the planet, its mass, the evolution of the host star, and the operation of any (bio)geochemical cycles that may serve to regulate planetary climate. A stellar evolution model was developed to investigate the control of increasing main-sequence stellar luminosity on the boundaries of the radiative habitable zone, which was then coupled with a zero-D biogeochemical carbon cycle model to investigate the operation of the carbonate-silicate cycle under conditions of varying incident stellar flux and planet size. \nThe Earth will remain within habitable temperature limits for 6.34 Gyr (1.8 Gyr from present), but photosynthetic primary producers will experience carbon-starvation due to greatly increased terrestrial weathering from 5.38 Gyr (0.84 Gyr from present) onwards, with significant implications for planetary habitability. Planet mass was discovered to have a significant control on the length of the habitable period of Earth-like planets, but more data on the bulk density and atmospheric composition of newly-discovered exoplanets is required before definitive estimates of their long-term habitability can be made. Exoplanet case studies reveal habitable periods significantly longer than that of the Earth, possibly up to 80 Gyr in the case of planets in the orbit of M-dwarfs. Contemporary measures of habitability that rely strongly on surface temperatures are becoming obsolete, and a move towards the inclusion of integrated biogeochemical cycle models and the development of multiparameter habitability indices will strengthen contemporary understanding of the distribution and evolution of potentially habitable terrestrial worlds.