Exoplanets Torqued by the Combined Tides of a Moon and Parent Star

Abstract

In recent years, there has been interest in Earth-like exoplanets in the habitable zones of low mass stars ($\sim0.1-0.6\,M_\odot$). Furthermore, it has been argued that a large moon may be important for stabilizing conditions on a planet for life. If these two features are combined, then an exoplanet can feel a similar tidal influence from both its moon and parent star, leading to potentially interesting dynamics. The moon's orbital evolution depends on the exoplanet's initial spin period $P_0$. When $P_0$ is small, transfer of the exoplanet's angular momentum to the moon's orbit can cause the moon to migrate outward sufficiently to be stripped by the star. When $P_0$ is large, the moon migrates less and the star's tidal torques spin down the exoplanet. Tidal interactions then cause the moon to migrate inward until it is likely tidally disrupted by the exoplanet and potentially produces rings. While one may think that these findings preclude the presence of moons for the exoplanets of low mass stars, in fact a wide range of timescales are found for the loss or destruction of the moon; it can take $\sim10^6-10^{10}\,{\rm yrs}$ depending on the system parameters. When the moon is still present, the combined tidal torques force the exoplanet to spin asynchronously with respect to both its moon and parent star, which tidally heats the exoplanet. This can produce heat fluxes comparable to those currently coming through the Earth, arguing that combined tides may be a method for driving tectonic activity in exoplanets.