Planet-Planet Scattering and Stellar Flybys Generate Oort Planets

Wide-orbit exoplanets are starting to be detected, and planetary formation models are under development to understand their properties. We propose that a population of “Oort” planets around other stars can form by a mechanism analogous to how the Solar System's Oort comet cloud was populated. Gravitational scattering among planets is inferred from the eccentricity distribution of gas-giant exoplanets measured by the Doppler technique. This scattering is thought to commence while the protoplanetary disk is dissipating, 106 – 107 yr after formation of the star, or perhaps soon thereafter, when the majority of stars are expected to be part of a natal cluster. Previous calculations of planet-planet scattering have shown that one or more planets can spend 104 – 107 yr at distances >100 AU before ultimately being ejected. During that time, a close flyby from another star in the cluster may dynamically lift the periastron of the planet, staving off further scattering with the inner planets. We present numerical simulations demonstrating this mechanism as well as an analysis of the efficiency.

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