High Orbital Eccentricities of Extrasolar Planets Induced by the Kozai Mechanism

One of the most remarkable properties of extrasolar planets is their highorbital eccentricities. Observations have shown that at least 20% of theseplanets, including some with particularly high eccentricities, are orbiting acomponent of a wide binary star system. The presence of a distant binarycompanion can cause significant secular perturbations to the orbit of a planet.In particular, at high relative inclinations, a planet can undergo alarge-amplitude eccentricity oscillation. This so-called "Kozai mechanism" iseffective at a very long range, and its amplitude is purely dependent on therelative orbital inclination. In this paper, we address the following simplequestion: assuming that every host star with a detected giant planet also has a(possibly unseen, e.g., substellar) distant companion, with reasonabledistributions of orbital parameters and masses, how well could secularperturbations reproduce the observed eccentricity distribution of planets? Ourcalculations show that the Kozai mechanism consistently produces an excess ofplanets with very high (e >0.6) and very low (e < 0.1) eccentricities. Thepaucity of near-circular orbits in the observed sample cannot be explainedsolely by the Kozai mechanism, because, even with high enough inclinations, theKozai mechanism often fails to produce significant eccentricity perturbationswhen there are other competing sources of orbital perturbations on seculartimescales, such as general relativity. On the other hand, the Kozai mechanismcan produce many highly eccentric orbits. Indeed the overproduction of higheccentricities observed in our models could be combined with plausiblecircularizing mechanisms (e.g., friction from residual gas) to create moreintermediate eccentricities (e=0.1-0.6).Comment: 24 pages, 6 figures, ApJ, in press, minor changes to reflect the accepted versio