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Gravitational scattering between massive planets has been invoked to explainthe eccentricity distribution of extrasolar planets. For scattering to occur,the planets must either form in -- or migrate into -- an unstableconfiguration. In either case, it is likely that a residual gas disk, with amass comparable to that of the planets, will be present when scattering occurs.Using explicit hydrodynamic simulations, we study the impact of gas disks onthe outcome of two-planet scattering. We assume a specific model in which theplanets are driven toward instability by gravitational torques from an outerlow mass disk. We find that the accretion of mass and angular momentum thatoccurs when a scattered planet impacts the disk can strongly influence thesubsequent dynamics by reducing the number of close encounters. Theeccentricity of the innermost surviving planet at the epoch when the systembecomes Hill stable is not substantially altered from the gas-free case, butthe outer planet is circularized by its interaction with the disk. Thesignature of scattering initiated by gas disk migration is thus a high fractionof low eccentricity planets at larger radii accompanying known eccentricplanets. Subsequent secular evolution of the two planets in the presence ofdamping can further damp both eccentricities, and tends to push systems awayfrom apsidal alignment and toward anti-alignment. We note that the late burstof accretion when the outer planet impacts the disk is in principle observable,probably via detection of a strong near-IR excess in systems with otherwiseweak disk and stellar accretion signatures.

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