Migration-driven diversity of super-Earth compositions

TL;DR

This paper demonstrates that super-Earths close to their stars can be rocky despite forming beyond the snow line, due to inward migration and resonant shepherding of icy embryos, challenging previous water-rich predictions.

Contribution

It shows that inward migration and resonant shepherding can produce rocky super-Earths, providing a new explanation for their composition diversity.

Findings

Inner super-Earths are often purely rocky.

Migration accelerates growth of rocky planets.

Simulation matches Kepler-36 system configuration.

Abstract

A leading model for the origin of super-Earths proposes that planetary embryos migrate inward and pile up on close-in orbits. As large embryos are thought to preferentially form beyond the snow line, this naively predicts that most super-Earths should be very water-rich. Here we show that the shortest-period planets formed in the migration model are often purely rocky. The inward migration of icy embryos through the terrestrial zone accelerates the growth of rocky planets via resonant shepherding. We illustrate this process with a simulation that provided a match to the Kepler-36 system of two planets on close orbits with very different densities. In the simulation, two super-Earths formed in a Kepler-36-like configuration; the inner planet was pure rock while the outer one was ice-rich. We conclude from a suite of simulations that the feeding zones of close-in super-Earths are likely to be broad and disconnected from their final orbital radii.