On the Formation and Long Term Stability of Earth-like Planets in Binary Star Systems

The majority of solar-type stars live in binary systems, and binary companions can disrupt both the formation of terrestrial planets and their long term prospects for stability. We present the results from a large survey (~ 120 numerical simulations) of the effects of a stellar companion on the final stages of terrestrial planet formation in S-type orbits around one component of a main-sequence binary star system. We examine stellar mass ratios of μ = 1/3, 1/2, and 2/3 (for primary stars with masses of either 0.5 M_\odot or 1 M_\odot), and the stellar separations (a_B) and eccentricities (e_B) are varied such that the systems have periastron values of q_B = 5 AU, 7.5 AU, or 10 AU. We find that when the periastron of the binary is larger than about q_B = 10 AU, even for the extreme case of equal mass stars, terrestrial planets can form over essentially the entire range of orbits allowed for single stars. When periastron q_B < 10 AU, however, the distributions of orbital parameters are strongly affected by the presence of the binary companion. Specifically, the number of terrestrial planets and the spatial extent of the terrestrial planet region both decrease with decreasing binary periastron, while the masses and eccentricities of those planets are much less affected. When the periastron value becomes as small as 5 AU, planets no longer form with a = 1 AU orbits and the mass distribution tilts toward M < 1 M\oplus, i.e., the formation of Earth-like planets is compromised. Based on these dynamical simulations, we estimate that about 50% of binaries are wide enough to allow both the formation and the long term stability of Earth-like planets.