Coupled thermal and orbital evolution of tidally-loaded exoplanets

Abstract

The long-term dynamics of close-in terrestrial exoplanets are strongly influenced by tidal interaction with the host star. Periodic tidal loading results in time-varying deformation, which is typically accompanied by energy dissipation. This phenomenon has two important consequences: First, the produced heat might enhance the interior dynamics of the planet and even trigger structural changes in its interior, such as partial melting. Second, the lost (or transferred) energy fuels orbital evolution, i.e., it leads to secular changes in the semi-major axis and eccentricity. Along with the orbital evolution, the spin rate of the planet evolves as well, such that the total angular momentum in the system is conserved. Here, we focus on coupling the effects introduced above. Combining semi-analytical modeling of the long-term spin-orbital evolution of anelastic multilayered planets with parameterised 1d approach to mantle convection, we illustrate the feedback between the thermal state of a planet, its spin rate, and the rate of orbit circularisation. In addition to the primary topic of this work, we explore the parameter dependence of stable spin states (spin-orbit resonances) for layered bodies, the parameter dependence of tidal heat rate, and the possible contribution of inter-planetary tides to the spin rate evolution in tightly-packed multi-planetary systems.