Running The Gauntlet -- Survival of Small Circumbinary Planets Migrating Through Destabilising Resonances

TL;DR

This paper investigates why small circumbinary planets are rare, proposing that resonant ejection during migration in turbulent discs may remove them, but finds this mechanism alone unlikely to explain the observed size distribution.

Contribution

It introduces a theoretical model and N-body simulations to explore how turbulent disc conditions affect the survival of migrating small planets near resonances.

Findings

Resonant ejection can occur in turbulent discs.

Small planets are more likely to be ejected during migration.

The mechanism alone cannot fully explain the scarcity of small planets.

Abstract

All of the known circumbinary planets are large (> 3 Earth radii). Whilst observational biases may account for this dearth of small planets, in this paper we propose a theoretical explanation. Most of the known planets are near the stability limit, interspersed between potentially unstable 5 : 1, 6 : 1 and 7 : 1 mean motion resonances with the binary. It is believed that these planets did not form in situ, but rather migrated from farther out in the disc, and hence passed through these resonances. Planets are expected to migrate at a speed proportional to their mass, and a slower rate makes resonant capture and subsequent ejection more likely. Therefore, whilst large planets may be able to successfully "run the gauntlet", small planets may be imperiled. This hypothesis is tested using N-body integrations of migration in a truncated and turbulent disc. We discover that surprisingly none of the known planets exist interior to a fully unstable resonance. We demonstrate that resonant ejection of migrating planets may occur in nature, and that it does indeed disproportionately affect small planets, but it requires a highly turbulent disc and its efficiency is likely too low to fully explain a dearth of < 3 Earth radii planets.