Making Planet Nine: Pebble Accretion at 250--750 AU in a Gravitationally   Unstable Ring
  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. 
Scott J. Kenyon and Benjamin C. Bromley
astro-ph.EP astro-ph.SR