Effects of Radius and Gravity on the Inner Edge of the Habitable Zone

Abstract

A rigorous definition of the habitable zone and its dependence on planetary properties is part of the search for habitable exoplanets. In this work, we use the general circulation model ExoCAM to determine how the inner edge of the habitable zone of tidally locked planets orbiting M dwarf stars depends on planetary radius, surface gravity, and surface pressure. We find that the inner edge of the habitable zone for more massive planets occurs at higher stellar irradiation, as found in previous one-dimensional simulations. We also determine the relative effects of varying planetary radius and surface gravity. Increasing the planetary radius leads to a lower planetary albedo and warmer climate, pushing the inner edge of the habitable zone to lower stellar irradiation. This results from a change in circulation regime that leads to the disruption of the thick, reflective cloud deck around the substellar point. Increasing gravity increases the outgoing longwave radiation, which moves the inner edge of the habitable zone to higher stellar irradiation. This is because the column mass of water vapor decreases with increasing gravity, leading to a reduction in the greenhouse effect. The effect of gravity on the outgoing longwave radiation is stronger than the effect of radius on the planetary albedo, so that increasing gravity and radius together causes the inner edge of the habitable zone to move to higher stellar irradiation. Our results show that the inner edge of the habitable zone for more massive terrestrial planets occurs at a larger stellar irradiation.