Extrasolar Planet Eccentricities from Scattering in the Presence of Residual Gas Disks

TL;DR

This study uses hydrodynamic simulations to explore how residual gas disks influence the outcomes of two-planet scattering, affecting eccentricities and orbital configurations of extrasolar planets.

Contribution

It demonstrates that gas disks can significantly alter scattering dynamics, especially by damping eccentricities and modifying planet-disk interactions during instability.

Findings

Outer planet circularization by disk interaction.

High fraction of low eccentricity planets at larger radii.

Observable near-IR excess during late accretion bursts.

Abstract

Gravitational scattering between massive planets has been invoked to explain the eccentricity distribution of extrasolar planets. For scattering to occur, the planets must either form in -- or migrate into -- an unstable configuration. In either case, it is likely that a residual gas disk, with a mass comparable to that of the planets, will be present when scattering occurs. Using explicit hydrodynamic simulations, we study the impact of gas disks on the outcome of two-planet scattering. We assume a specific model in which the planets are driven toward instability by gravitational torques from an outer low mass disk. We find that the accretion of mass and angular momentum that occurs when a scattered planet impacts the disk can strongly influence the subsequent dynamics by reducing the number of close encounters. The eccentricity of the innermost surviving planet at the epoch when the system becomes Hill stable is not substantially altered from the gas-free case, but the outer planet is circularized by its interaction with the disk. The signature of scattering initiated by gas disk migration is thus a high fraction of low eccentricity planets at larger radii accompanying known eccentric planets. Subsequent secular evolution of the two planets in the presence of damping can further damp both eccentricities, and tends to push systems away from apsidal alignment and toward anti-alignment. We note that the late burst of accretion when the outer planet impacts the disk is in principle observable, probably via detection of a strong near-IR excess in systems with otherwise weak disk and stellar accretion signatures.