Hot Jupiters in Binary Star Systems

Abstract

Radial velocity surveys find Jupiter-mass planets with semimajor axes a less than 0.1 AU around ~1% of solar-type stars; counting planets with a as large as 5 AU, the fraction of stars having planets reaches ~10% (as found by Marcy et al. and Butler et al.). An examination of the distribution of semimajor axes shows that there is a clear excess of planets with orbital periods around 3 or 4 days, corresponding to a ≈ 0.03 AU, with a sharp cutoff at shorter periods (see Fig. 1). It is believed that Jupiter-mass planets form at large distances from their parent stars; some fraction then migrates in to produce the short-period objects. We argue that a significant fraction of the hot Jupiters ( a < 0.1 AU) may arise in binary star systems in which the orbit of the binary is highly inclined to the orbit of the planet. Mutual torques between the two orbits drive down the minimum separation or periapsis r p between the planet and its host star (the Kozai mechanism). This periapsis collapse is halted when tidal friction on the planet circularizes the orbit faster than Kozai torque can excite it. The same friction then circularizes the planet orbit, producing hot Jupiters with the peak of the semimajor axis distribution lying around 3 days. For the observed distributions of binary separation, eccentricity, and mass ratio, roughly 2.5% of planets with initial semimajor axis a p ≈ 5 AU will migrate to within 0.1 AU of their parent star. Kozai migration could account for 10% or more of the observed hot Jupiters.