Formation of Super-Earth Mass Planets at 125-250 AU from a Solar-type Star

TL;DR

This study demonstrates that super-Earth mass planets can form at 125-250 AU from a solar-type star within 1-3 billion years through coagulation of planetesimals, involving collisional damping and potential second runaway growth phases.

Contribution

It provides new insights into the physical conditions and processes enabling in situ formation of super-Earths at large orbital distances, supported by detailed coagulation simulations.

Findings

Super-Earths can form at 125-250 AU in 1-3 Gyr.

Collisional damping triggers second runaway growth phases.

Formation of a hypothetical Planet X at >1000 AU requires a very massive nebula.

Abstract

We investigate pathways for the formation of icy super-Earth mass planets orbiting at 125-250 AU around a 1 solar mass star. An extensive suite of coagulation calculations demonstrates that swarms of 1 cm to 10 m planetesimals can form super-Earth mass planets on time scales of 1-3 Gyr. Collisional damping of 0.01-100 cm particles during oligarchic growth is a highlight of these simulations. In some situations, damping initiates a second runaway growth phase where 100-3000 km protoplanets grow to super-Earth sizes. Our results establish the initial conditions and physical processes required for in situ formation of super-Earth planets at large distances from the host star. For nearby dusty disks in HD 107146, HD 202628, and HD 207129, ongoing super-Earth formation at 80-150 AU could produce gaps and other structures in the debris. In the solar system, forming a putative planet X at a < 300 AU (a > 1000 AU) requires a modest (very massive) protosolar nebula.