Pathways of survival for exomoons and inner exoplanets

Abstract

Context. It is conceivable that a few thousand confirmed exoplanets initially harboured satellites similar to the moons of the Solar System or larger. We ask the question of whether some of them have survived over the æons of dynamical evolution to the present day. The dynamical conditions are harsh for exomoons in such systems because of the greater influence of the host star and of the tidal torque it exerts on the planet. Aims. We investigate the stability niches of exomoons around hundreds of innermost exoplanets for which the needed parameters are known today, and we determine the conditions of these moons’ long-term survival. General lower and upper bounds on the exomoon survival niches are derived for orbital separations, periods, and masses. Methods. The fate of an exomoon residing in a stability niche depends on the initial relative rate of the planet’s rotation and on the ability of the moon to synchronise the planet by overpowering the tidal action from the star. State-of-the-art models of tidal dissipation and secular orbital evolution are applied to a large sample of known exoplanet systems, which have the required estimated physical parameters. Results. We show that in some plausible scenarios, exomoons can prevent close exoplanets from spiralling into their host stars, thus extending these planets’ lifetimes. This is achieved when exomoons synchronise the rotation of their parent planets, overpowering the tidal action from the stars. Conclusions. Massive moons are more likely to survive and help their host planets maintain a high rotation rate (higher than these planets’ mean motion).