The Formation and Habitability of Extra-Solar Planets

Abstract

A quantitative numerical program, developed to model the formation of the terrestrial planets and asteroids of our solar System (Wetherill 1992), has been extended to include a more general range of stellar and preplanetary nebular parameters, as may be expected elsewhere in the Galaxy and Universe. The results of about 500 new simulations of planetary formation are reported. It is found that for circumstellar disk parameters that are not too different from those of the Solar System, the number and radial distribution of final terrestrial planets are insensitive to stellar mass and these planets concentrate in the vicinity of 1 AU. In contrast, the position of the biologically habitable heliocentric distances are strongly dependent on stellar mass (Kastinget al.1993), and the frequency of habitable planets is therefore also dependent on stellar mass. Stars of 1.0 solar mass (M⊙) almost always have at least one planet of mass >1/3 Earth mass (M⊕) in their habitable zones. Larger planets of the smaller stars tend to be too cold, those of the larger stars too hot. Nevertheless, some ∼5 to 15% of the simulated planetary systems associated with stars as small as 0.5M⊙and as large as 1.5M⊙contain a habitable planet. The position and number of simulated terrestrial planets are also insensitive to the initial surface density of solid bodies in the circumstellar disk, but the size of the planets is approximately proportional to the surface density. These results represent planetary systems associated with radial variation of surface densities, disk parameters, and giant planet populations not very different from those of our Solar System. The outer boundary of their “peak” in semimajor axis distribution at 1 AU is determined by the increasing proximity of more distant bodies to strong resonances by Jupiter and Saturn. As a consequence, this boundary will move in or out in accordance with the position, or absence, of such bodies in other systems. In the complete absence of Jupiter, the median planetary mass in the terrestrial planet region is almost 2M⊕, for the same initial surface density used in the models characteristic of our Solar System. The inner boundary is determined by the minimum distance at which planet-forming solids condense in the disk. For some, not necessarily likely, variations in these parameters, abundant populations of habitable planets can be obtained for all the values of stellar mass considered.