Tidal evolution of rocky (exo)planets

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 modelling of secular spin-orbital evolution of multilayered planets [1,2,3] with parameterised 1d mantle convection [4,5], we illustrate the feedback between the thermal state of a planet, its spin rate, and the rate of orbit circularisation. As one of the results, we also show how a single planet orbiting a single star can retain nonzero orbital eccentricity by forming a subsurface magma ocean. In addition to the primary topic of this work, we further explore the dependence of stable spin states (spin-orbit resonances) on the planet's interior structure and orbital eccentricity, as well as the parameter dependence of tidal heating. [1] Kaula (1964), Rev. Geophys. and Space Phys., 2:661-685, doi:10.1029/RG002i004p00661. [2] Boué & Efroimsky (2019), CM&DA, 131(7):30, doi:10.1007/s10569-019-9908-2. [3] Sabadini & Vermeersen (2004), Kluwer Academic Publishers, Dodrecht, the Netherlands, ISBN: 9781402012686. [4] Grott and Breuer (2008), Icarus, 193(2):503-515, doi:10.1016/j.icarus.2007.08.015. [5] Tosi et al. (2017), A&A, 605:A71, doi:10.1051/0004-6361/201730728