Dynamical Outcomes of Planet–planet Scattering

Observations in the past decade have revealed extrasolar planets with a wide range of semi-major axes and eccentricities. Based on the present understanding of planet formation via core accretion and oligarchic growth, we expect that giant planets often form in closely packed configurations. While the protoplanets are embedded in a protoplanetary nebula, dissipation prevents eccentricity growth and can suppress instabilities from becoming manifest. However, once the disk dissipates, eccentricities can grow rapidly, leading to close encounters between the planets. In this study we explore strong gravitational scattering in a gas-free multi-planet system as a mechanism to explain the orbital properties of exoplanets. We numerically investigate the long-term stability of representative multi-planet systems containing three giant planets in orbit around a solar-like central star. We assign the planet masses in a realistic manner following the core accretion scenario of planet formation. In contrast to the case of two planets, there is no sharp stability boundary for 3-planet systems, so numerical integrations of 3-planet systems can approach instability naturally, even without including dissipation, mass growth, or migration. We characterize the timescale to reach instability as a function of the initial planet–planet separation. We discuss strong gravitational scattering as a possible mechanism to create high eccentricities as well as the close-in planetary orbits in the observed exoplanet population. We find that this mechanism can reasonably reproduce the observed eccentricity distribution. Our results also make testable predictions for the inclinations of short-period giant planets that are formed via strong planet scattering followed by tidal circularization.