  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.
