Making Planet Nine: Pebble Accretion at 250--750 AU in a Gravitationally Unstable Ring

TL;DR

This study models the formation of super-Earth planets at 250-750 AU around a star, showing that pebble accretion in a gravitationally unstable ring can produce such planets within a few hundred million years, with observable debris disks.

Contribution

It demonstrates that pebble accretion in a gravitationally unstable ring can form super-Earths at large orbital distances within realistic timescales, a novel insight into outer planetary system formation.

Findings

Super-Earths form within 100-200 Myr at 250 AU

Super-Earths form within 1-2 Gyr at 750 AU

Detectable debris disks result from destructive collisions

Abstract

We investigate the formation of icy super-Earth mass planets within a gravitationally unstable ring of solids orbiting at 250-750 AU around a 1 solar mass star. Coagulation calculations demonstrate that a system of a few large oligarchs and a swarm of pebbles generates a super-Earth within 100-200 Myr at 250 AU and within 1-2 Gyr at 750 AU. Systems with more than ten oligarchs fail to yield super-Earths over the age of the solar system. As these systems evolve, destructive collisions produce detectable debris disks with luminosities of $10^{-5}$ to $10^{-3}$ relative to the central star.