Surprising Spin-orbit Resonances of Rocky Planets

Recent works suggest that, in multiplanetary systems, a close-in exoplanet can sometimes avoid becoming tidally locked to its host star if it is captured into a secular spin-orbit resonance with a companion planet. In such a resonance, the planet remains at a sub-synchronous spin rate and an appreciable obliquity (the planet's spin-orbit misalignment angle). However, many of these works have only considered planets with fluid-like rheologies. Recent observations suggest that planets up to a few Earth masses may be rocky and thus may have an appreciable rigidity. In this work, we study the spin-orbit dynamics of such rigid planets using a linear dissipative tidal model and not enforcing principal axis rotation about the body's shortest principal axis. We identify a new class of spin-orbit resonances when the planet spins at twice its orbital frequency. These resonances exist at nonzero obliquity and spontaneously excite non-principal-axis rotation upon resonance capture. While these resonances eventually disappear as tidal dissipation damps the obliquity to zero (and the body returns to principal-axis rotation), they still modify the spin evolutionary history of the planet. Such resonances may enhance the prevalence of secular spin-orbit resonances in exoplanetary systems.