Exomoons of Circumbinary Planets

TL;DR

This study simulates the orbital evolution of exomoons around migrating circumbinary planets, revealing their potential survival, habitability, and pathways to becoming free-floating objects, thus informing moon formation theories in complex systems.

Contribution

It provides the first detailed simulation-based analysis of exomoon survival and evolution around migrating circumbinary planets, highlighting conditions for moon retention and habitability.

Findings

18% of surviving moons are within the habitable zone.

38% of moons become long-period circumbinary planets.

1% of moons are ejected from the system.

Abstract

Confirmation of the first exomoon remains elusive. Although several exomoon candidates exist around single stars, there are currently no candidates around circumbinary planets (CBPs). Most circumbinary planets are thought to form far from the host binary and migrate through the protoplanetary disc. Therefore, an exomoon of a CBP represents a fascinating yet complex and evolving four-body system. Their existence (or absence) would shed light on the robustness of moon formation and evolution in dynamically active planetary systems. In this work, we simulate the orbital evolutions of exomoons around migrating CBPs. We show that for fully migrated CBPs, a moon is capable of surviving the migration if it is formed within $\sim5-10\%$ of the planet's Hill Radius, well within the currently proposed range at which moons are thought to settle in the planetary disc for giant planets. Even though all known CBPs are gas giants, 18\% of the surviving moons in our sample are within the habitable zone, giving credence to circumbinary habitability, albeit hosted by moons rather than planets. $38\%$ of moons escape their host planet early in the migration and become long-period CBPs (i.e a multi-planet circumbinary system). Nearly one-third of exomoons collide with their host planet, and $1\%$ are ejected from the system entirely. This last class presents another pathway for producing free-floating planetary mass objects, like those discovered recently and expected from the Roman microlensing survey.