Sculpting the circumbinary planet size distribution through resonant interactions with companion planets

TL;DR

This study investigates how resonant interactions between migrating circumbinary planets influence their final sizes and orbits, potentially explaining the observed scarcity of small circumbinary planets and the origin of interstellar objects.

Contribution

It introduces a model showing how planet-planet mass ratios affect the trapping or ejection of planets near circumbinary disc edges during migration.

Findings

Smaller outer planets tend to be parked in stable resonances outside the disc edge.

Equal or larger outer planets often push the inner planet inward, leading to ejection or exterior orbits.

The process may explain the dearth of small circumbinary planets and the origin of interstellar objects.

Abstract

Resonant locking of two planets is an expected outcome of convergent disc migration. The planets subsequently migrate together as a resonant pair. In the context of circumbinary planets, the disc is truncated internally by the binary. If there were only a single planet, then this inner disc edge would provide a natural parking location. However, for two planets migrating together in resonance there will be a tension between the inner planet stopping at the disc edge, and the outer planet continuing to be torqued inwards. In this paper we study this effect, showing that the outcome is a function of the planet-planet mass ratio. Smaller outer planets tend to be parked in a stable exterior 2:1 or 3:2 resonance with the inner planet, which itself remains near the disc edge. Equal or larger mass outer planets tend to push the inner planet past the disc edge and too close to the binary, causing it to be ejected or sometimes flipped to an exterior orbit. Our simulations show that this process may explain an observed dearth of small (< 3 Earth radii) circumbinary planets, since small planets are frequently ejected or left on long-period orbits, for which transit detection is less likely. This may also be an efficient mechanism for producing free-floating planets and interstellar interlopers like `Oumuamua.