Making Planet Nine: A Scattered Giant in the Outer Solar System

TL;DR

This paper proposes a formation scenario for Planet Nine involving early scattering by giant planets and subsequent orbital settling through gas disk interactions, supported by numerical simulations.

Contribution

It introduces a new formation pathway for Planet Nine involving scattering and gas disk dynamics, supported by detailed simulations.

Findings

Scattered super-Earths can settle on distant, eccentric orbits.

Gas disk evolution influences the final orbit of Planet Nine.

Massive disks can shepherd minor planets as observed.

Abstract

Correlations in the orbits of several minor planets in the outer solar system suggest the presence of a remote, massive Planet Nine. With at least ten times the mass of the Earth and a perihelion well beyond 100 AU, Planet Nine poses a challenge to planet formation theory. Here we expand on a scenario in which the planet formed closer to the Sun and was gravitationally scattered by Jupiter or Saturn onto a very eccentric orbit in an extended gaseous disk. Dynamical friction with the gas then allowed the planet to settle in the outer solar system. We explore this possibility with a set of numerical simulations. Depending on how the gas disk evolves, scattered super-Earths or small gas giants settle on a range of orbits, with perihelion distances as large as 300 AU. Massive disks that clear from the inside out on million-year time scales yield orbits that allow a super-Earth or gas giant to shepherd the minor planets as observed. A massive planet can achieve a similar orbit in a persistent, low-mass disk over the lifetime of the solar system.