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

From an astrobiological point of view, special attention has been paid to theprobability of habitable planets in extrasolar systems. The purpose of thisstudy is to constrain a possible range of the mass of a terrestrial planet thatcan get water. We focus on the process of water production through oxidation ofthe atmospheric hydrogen--the nebular gas having been attractedgravitationally--by oxide available at the planetary surface. For the waterproduction to work well on a planet, a sufficient amount of hydrogen and enoughhigh temperature to melt the planetary surface are needed. We have simulatedthe structure of the atmosphere that connects with the protoplanetary nebulafor wide ranges of heat flux, opacity, and density of the nebular gas. We havefound both requirements are fulfilled for an Earth-mass planet for wide rangesof 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 gasgiant planet through runaway accretion of the nebular gas.Comment: 25 pages, 8 figures, to appear in the 01 September 2006 issue of Ap