Planet-planet scattering as the source of the highest eccentricity exoplanets

Abstract

Most giant exoplanets discovered by radial velocity surveys have much higher eccentricities than those in the solar system. The planet--planet scattering mechanism has been shown to match the broad eccentricity distribution, but the highest-eccentricity planets are often attributed to Kozai-Lidov oscillations induced by a stellar companion. Here we investigate whether the highly eccentric exoplanet population can be produced entirely by scattering. We ran 500 N-body simulations of closely packed giant-planet systems that became unstable under their own mutual perturbations. We find that the surviving bound planets can have eccentricities up to $e > 0.99$, with a maximum of 0.999017 in our simulations. This suggests that there is no maximum eccentricity that can be produced by planet--planet scattering. Importantly, we find that extreme eccentricities are not extremely rare; the eccentricity distribution for all giant exoplanets with $e > 0.3$ is consistent with all planets concerned being generated by scattering. Our results show that the discovery of planets with extremely high eccentricities does not necessarily signal the action of the Kozai-Lidov mechanism.