Terrestrial Planet Formation in Binary Star Systems

Observations show that more than half of all main sequence stars have a stellar companion tep{duq91}. Perturbations from a companion star can disrupt the formation and long-term stability of planets. We numerically explore the final stages of terrestrial planet formation - the growth from Moon- to Mars-sized planetary embryos into planets - in a wide variety of main sequence binary star systems. We examine planetary accretion around each star in α Centauri AB, the closest binary star system to the Sun, and around individual stars of other `wide' binary systems (with stellar separations from 5 - 40 AU). We also explore planet formation around both members of `close' (0.05 - 0.4 AU) binary star systems. Our results are statistically compared to simulations using the same initial disk around a single Sun-like star (with and without Jupiter and Saturn included).

From 3 - 5 terrestrial planets similar to those in the Solar System formed around each star in α Cen AB in simulations that began with a disk of embryos inclined ⪉ 30° to the binary orbital plane. We found that for wide and close binary star systems, the binary periastron (closest approach) and apastron (maximum stellar separation), respectively, are the most influential stellar parameters on planetary accretion. Terrestrial planets formed in Earth-like (∼1 AU) orbits around individual stars in binary systems with periastron ⪆ 7 AU, and surrounding close binary stars with stellar apastrons ⪉ 0.2 AU. Approximately 50 - 60% of binary star systems - from contact binaries to separations of nearly a parsec - satisfy these constraints. Given that the galaxy contains more than 100 billion star systems, a large number of systems can potentially harbor Earth-like habitable planets.