Spin evolution for four HZ planet–moon systems orbiting an M2 dwarf. Evolution of planetary spin period for two a_p cases in the M2 parameter space (0.17 au, 0.34 au) at (a) 1.0 M_⊕ and (b) 2.0 M_⊕ from rebound simulations where curve lengths represent lifetimes. The bottom x-axis, plotted on a logarithmic scale, shows time normalized by the orbital period of a planet at the inner edge of the HZ (0.17 au) while the top x-axis shows time (in years).