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Many exoplanets have been found in orbits close to their host stars and thusthey are subject to the effects of photo-evaporation. Previous studies haveshown that a large portion of exoplanets detected by the Kepler mission havebeen significantly eroded by photo-evaporation. In this paper, we numericallystudy the effects of photo-evaporation on the orbital evolution of ahypothesized moon system around a planet. We find that photo-evaporation iscrucial to the stability of the moon system. Photo-evaporation can erode theatmosphere of the planet thus leading to significant mass loss. As the planetloses mass, its Hill radius shrinks and its moons increase their orbitalsemi-major axes and eccentricities. When some moons approach their criticalsemi-major axes, global instability of the moon system would be triggered,which usually ends up with two, one or even zero surviving moons. Some lostmoons could escape from the moon system to become a new planet orbiting thestar or run away further to become a free-floating object in the Galaxy. Giventhe destructive role of photo-evaporation, we speculate that exo-moons are lesscommon 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. Thelessons we learn in this study may be helpful for the target selection ofon-going/future exomoon searching programs.

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