The Fate of Scattered Planets

TL;DR

This paper investigates how gas-driven dynamical friction can cause scattered super-Earths to settle into distant, eccentric orbits, providing insights into the formation of remote planets and their relation to disk evolution.

Contribution

It introduces a simple numerical model showing how dynamical friction in evolving gas disks can circularize and position scattered planets at large distances.

Findings

Super-Earths can settle in outer regions via dynamical friction.

Longer-lived, more massive disks damp eccentricities more effectively.

Orbital parameters of distant planets can reveal disk evolutionary history.

Abstract

As gas giant planets evolve, they may scatter other planets far from their original orbits to produce hot Jupiters or rogue planets that are not gravitationally bound to any star. Here, we consider planets cast out to large orbital distances on eccentric, bound orbits through a gaseous disk. With simple numerical models, we show that super-Earths can interact with the gas through dynamical friction to settle in the remote outer regions of a planetary system. Outcomes depend on planet mass, the initial scattered orbit, and the evolution of the time-dependent disk. Efficient orbital damping by dynamical friction requires planets at least as massive as the Earth. More massive, longer-lived disks damp eccentricities more efficiently than less massive, short-lived ones. Transition disks with an expanding inner cavity can circularize orbits at larger distances than disks that experience a global (homologous) decay in surface density. Thus, orbits of remote planets may reveal the evolutionary history of their primordial gas disks. A remote planet with an orbital distance ~100 AU from the Sun is plausible and might explain correlations in the orbital parameters of several distant trans-Neptunian objects.