Rotational States and Tidal Heating of Habitable Exoplanets
Abstract: Are tides important in the context of habitability of exoplanets? In the overall energy balance on Earth’s surface today, the tidal dissipation is too small to be of significance. In the long term, the tidal dissipation is bound to increase due to the lengthening of the day, but it is a very slow process. The rate of lunar orbit expansion is too fast for the assumed age of the Moon, which testifies to a complex tidal evolution of the Earth-Moon system in the past. Still, advanced analytical computations, as well as simple first-principle estimation, show that the greater lunar tides could hardly compensate for the deficient solar irradiation 3.7 Gyr ago when first living organisms emerged. Comparatively, proposed habitable exoplanet worlds orbiting M dwarfs are subject to much stronger tidal interactions. Because the spin-down has the shortest time scale in tidal evolution, rocky planets in close habitable zones are expected to be locked in spin-orbit resonances. Resonances higher than 1:1 are more likely than synchronous rotation for systems with measurable eccentricities. This further increases the rate of tidal heating, inevitably resulting in the emergence of partial melt in the mantle. We describe two self-regulation mechanisms, which provide a negative feedback to tidal heating and prevent terrestrial planets from turning into magma balls. Partially molten or semiliquid bodies of smaller viscosity have only one state of stable equilibrium above the 1:1 resonance, which is called pseudosynchronous rotation. This can be the long-term stable equilibrium state of large solivagant (free-floating) planets with subsurface oceans, which can be habitable when heated by close, massive moons.