http://arxiv.org/abs/1803.01971
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 relatively large moon may be important for stabilizing conditions on a planet so that life may develop. 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 is found to depend sensitively 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. 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 parameters of the system. This indicates that the moon’s tidal destruction may occur in older planetary systems and not just young ones. 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.
A. Piro
Wed, 7 Mar 18
50/65
Comments: 10 pages, 9 figures, submitted for publication in ApJ, comments appreciated
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