Global Instability of Exo-Moon System Triggered by Photo-Evaporation

TL;DR

This study investigates how photo-evaporation affects the stability and survival of moons around close-in exoplanets, revealing that it can lead to moon loss, system instability, and potential moon escape, especially around X-ray luminous stars.

Contribution

It provides the first numerical analysis of photo-evaporation's impact on exo-moon orbital stability and survival, highlighting its destructive effects on moon systems.

Findings

Photo-evaporation causes significant mass loss of host planets.

Moon systems become globally unstable as moons approach critical semi-major axes.

Close-in exoplanets, especially around M-type stars, likely have fewer surviving moons.

Abstract

Many exoplanets have been found in orbits close to their host stars and thus they are subject to the effects of photo-evaporation. Previous studies have shown that a large portion of exoplanets detected by the Kepler mission have been significantly eroded by photo-evaporation. In this paper, we numerically study the effects of photo-evaporation on the orbital evolution of a hypothesized moon system around a planet. We find that photo-evaporation is crucial to the stability of the moon system. Photo-evaporation can erode the atmosphere of the planet thus leading to significant mass loss. As the planet loses mass, its Hill radius shrinks and its moons increase their orbital semi-major axes and eccentricities. When some moons approach their critical semi-major axes, global instability of the moon system would be triggered, which usually ends up with two, one or even zero surviving moons. Some lost moons could escape from the moon system to become a new planet orbiting the star or run away further to become a free-floating object in the Galaxy. Given the destructive role of photo-evaporation, we speculate that exo-moons are less common for close-in planets ($<0.1$ AU), especially those around M-type stars, because they are more X-ray luminous and thus enhancing photo-evaporation. The lessons we learn in this study may be helpful for the target selection of on-going/future exomoon searching programs.