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

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.