theoretical models for the formation of our Solar System that are in accord with many of its observational features.
A quantitative numerical program, developed to model the formation of the terrestrial planets and asteroids of our solar This activity is being carried out in the context of observa-System (Wetherill 1992), has been extended to include a more tional and theoretical advances on the related and more general range of stellar and preplanetary nebular parameters, general problem of star formation. For this reason it also as may be expected elsewhere in the Galaxy and Universe. The has potential relevance to planetary systems surrounding results of about 500 new simulations of planetary formation single stars in which the stellar mass, as well as other are reported. It is found that for circumstellar disk parameters that are not too different from those of the Solar System, the parameters, differs from that of the Solar System. This number and radial distribution of final terrestrial planets are theoretical modeling of the more general case of formation insensitive to stellar mass and these planets concentrate in the of planetary systems is also relevant to the very interesting vicinity of 1 AU. In contrast, the position of the biologically question of the frequency of biologically habitable planets habitable heliocentric distances are strongly dependent on stelaround stars of different mass elsewhere in the Galaxy and lar mass (Kasting et al. 1993), and the frequency of habitable Universe. The nature of other planetary systems has been planets is therefore also dependent on stellar mass. Stars of 1.0 solar mass (M αͺ ) almost always have at least one planet of mass considered by previous authors (Dole 1964, Isaacman and ΟΎ1/3 Earth mass (M ) in their habitable zones. Larger planets Sagan 1977, Hart 1979, Nakano 1988a, 1988b, Fogg 1992, of the smaller stars tend to be too cold, those of the larger Kasting et al. 1993, and Lissauer 1995a, 1995b). The princistars too hot. Nevertheless, some Θ5 to 15% of the simulated pal difference between these earlier studies and the present planetary systems associated with stars as small as 0.5 M αͺ and work is that here the position, size, and the spacing of as large as 1.5 M αͺ contain a habitable planet. The position and number of simulated terrestrial planets are also insensitive the planets are determined by a dynamic model of planet to the initial surface density of solid bodies in the circumstellar formation in which the evolving mutual gravitational perdisk, but the size of the planets is approximately proportional turbations of the growing planetesimals are explicitly conto the surface density. These results represent planetary systems sidered. The relationship of the present study to this previassociated with radial variation of surface densities, disk paous work will be discussed in Section IV.
rameters, and giant planet populations not very different from those of our Solar System. The outer boundary of their ''peak''
A general framework for planet formation (Wetherill in semimajor axis distribution at 1 AU is determined by the 1992, 1994a) is assumed that is compatible with astronomiincreasing proximity of more distant bodies to strong resocal observations and theory of formation of protosolar nances by Jupiter and Saturn. As a consequence, this boundary nebulae and single stars, but is not necessarily a consewill move in or out in accordance with the position, or absence, quence of them (reviewed by Beckwith and Sargent 1993, of such bodies in other systems. In the complete absence of Shu et al. 1993, Tscharnuter and Boss 1993). As discussed Jupiter, the median planetary mass in the terrestrial planet region is almost 2 M , for the same initial surface density used by these authors, single star formation involves three fairly in the models characteristic of our Solar System. The inner distinct stages:
boundary is determined by the minimum distance at which planet-forming solids condense in the disk. For some, not neces-(1) Collapse of a molecular cloud core to form a small sarily likely, variations in these parameters, abundant populacentral object, surrounded by a circumstellar disk extions of habitable planets can be obtained for all the values of tending out to Θ100 AU. The duration of this stage will stellar mass considered.