Exomoons in Systems with a Strong Perturber: Applications to $\alpha$ Cen AB

TL;DR

This paper revises orbital stability limits for exomoons in systems with strong stellar perturbers, especially in binary star systems like $ ext{α}$ Cen AB, and explores implications for exomoon observability via transit timing variations.

Contribution

It provides updated stability criteria for exomoons in binary systems considering orbital eccentricity effects and analyzes their impact on exomoon detection prospects.

Findings

Outer stability limits can be significantly less than half the Hill Radius.

Oscillations in planetary eccentricity affect exomoon stability boundaries.

Transit timing variations induced by exomoons are typically less than 10 minutes.

Abstract

The presence of a stellar companion can place constraints on occurrence and orbital evolution of satellites orbiting exoplanets, i.e., exomoons. In this work we revise earlier orbital stability limits for retrograde orbits in the case of a three body system consisting of star-planet-satellite. The latter reads $a_{\rm sat}^{\rm crit} \approx 0.668(1-1.236e_{\rm p})$ for $e_p \leq 0.8$ in units of the Hill Radius and represents the lower critical orbit as a function of the planetary eccentricity $e_{\rm p}$. A similar formula is determined for exomoons hosted by planets in binary star systems, where $e_{\rm p}$ is replaced with the components of free and forced eccentricity from secular orbit evolution theory. By exploring the dynamics of putative exomoons in $\alpha$ Centauri AB we find that the outer stability limit can be much less than half the Hill Radius due to oscillations in the planetary orbital eccentricity caused by the gravitational interaction with the binary star. We show, furthermore, how the resulting truncation of the outer stability limit can affect the outward tidal migration and potential observability of exomoons through transit timing variations (TTVs). Typical TTV (RMS) amplitudes induced by exomoons in binary systems are $\lesssim$10 min and appear more likely for planets orbiting the less massive stellar component. A GitHub repository (saturnaxis/exomoon-in-binaries) is available to reproduce figures.