HABITABLE PLANET FORMATION IN BINARY STAR SYSTEMS

Assuming current models of terrestrial planet formation in the Solar S ystem, we numerically investigate the conditions under which the secon dary star in a binary system will inhibit planet growth in the circums tellar 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 certai n critical value which results in disruption collisions rather than ac cretion. For a two solar mass binary with planetesimals in circular or bits 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(c), is 32 AU. If the planetesimals orbit the center of m ass of the binary system, a(c) = 0.10 AU, which is inside the tidal ci rcularization radius. We obtain an empirical formula giving the depend encies of a(c) on the binary eccentricity, secondary mass, planetesima l location, and critical disruption velocity. Based on the distributio ns of orbital elements of a bias-corrected sample of nearby G-dwarfs, we find that approximate to 60% of solar-type binaries cannot be exclu ded from having a habitable planet solely on the basis of the perturba tive effect of the secondary star. This conclusion is independent of w hen the secondary star formed, nebula dissipative mechanisms, and the time scale for runaway planetesimal accretion, and is relatively insen sitive to the mass of the secondary star, the critical disruption velo city, and the location of planetesimals within the circumstellar habit able 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 (g reater than or equal to 50 AU), or when the secondary has a very low m ass and near circular orbit as in the Sun-Jupiter system. The discrepa ncy with the present numerical study is due in part to the different r unaway accretion time scales assumed and the neglect in the earlier st udy of an exact criterion for crossing orbits. (C) 1998 Academic Press .