Habitable planet formation in binary star systems

Assuming current models of terrestrial planet formation in the Solar System, we numerically investigate the conditions under which the secondary star in a binary system will inhibit planet growth in the circumstellar habitable zone. Runaway accretion is assumed to be precluded if the secondary (1) causes the planetesimal orbits to cross within the runaway accretion time scale and (2) if, during crossing, the relative velocities of the planetesimals have been accelerated beyond a certain critical value which results in disruption collisions rather than accretion. For a two solar mass binary with planetesimals in circular orbits about one star at 1 AU, and a typical wide binary eccentricity of 0.5, the minimum binary semimajor axis which would not inhibit planet formation, [a.sub.c], is 32 AU. If the planetesimals orbit the center of mass of the binary system, [a.sub.c] = 0.10 AU, which is inside the tidal circularization radius. We obtain an empirical formula giving the dependencies of [a.sub.c] on the binary eccentricity, secondary mass, planetesimal location, and critical disruption velocity. based on the distributions of orbital elements of a bias-corrected sample of nearby G-dwarfs, we find that [approximately equal to]60% of solar-type binaries cannot be excluded from having a habitable planet solely on the basis of the perturbative effect of the secondary star. This conclusion is independent of when the secondary star formed, nebula dissipative mechanisms, and the time scale for runaway planetesimal accretion, and is relatively insensitive to the mass of the secondary star, the critical disruption velocity, and the location of planetesimals within the circumstellar habitable zone. An earlier study of planet formation in binary star systems came to a different conclusion, namely that planet formation, even at Mercury's distance, is unlikely except in widely separated systems ([greater than or equal to]50 AU), or when the secondary has a very low mass and near circular orbit as in the Sun-Jupiter system. The discrepancy with the present numerical study is due in part to the different runaway accretion time scales assumed and the neglect in the earlier study of an exact criterion for crossing orbits. Key Words: planet formation; binary stars; life.