Constraints on the Mass of a Habitable Planet with Water of Nebular Origin

Abstract

From an astrobiological point of view, special attention has been paid to the probability of habitable planets in extrasolar systems. The purpose of this study is to constrain a possible range of the mass of a terrestrial planet that can get water. We focus on the process of water production through oxidation of the atmospheric hydrogen--the nebular gas having been attracted gravitationally--by oxide available at the planetary surface. For the water production to work well on a planet, a sufficient amount of hydrogen and enough high temperature to melt the planetary surface are needed. We have simulated the structure of the atmosphere that connects with the protoplanetary nebula for wide ranges of heat flux, opacity, and density of the nebular gas. We have found both requirements are fulfilled for an Earth-mass planet for wide ranges of the parameters. We have also found the surface temperature of planets of <= 0.3 Earth masses is lower than the melting temperature of silicate (~ 1500K). On the other hand, a planet of more than several Earth masses becomes a gas giant planet through runaway accretion of the nebular gas.