Long-lived Planetary Obliquities of Close-in Exoplanets: The Tricky Story for Rocky Bodies

Most simply, one would expect that the obliquities of exoplanets (the angles between their spin axes and orbit normal) that are orbiting close to their host stars are small due to tidal dissipation. In recent years, this theoretical expectation has been challenged by the proposal of secular spin-orbit resonances, induced by a companion planet, that can sustain substantial spin-orbit misalignment in spite of tidal obliquity damping. Large obliquities have been of great interest for their implications for habitability, atmospheric modeling, and potentially explaining underdense planets via tilted rings, and, as such, it is important to understand their robustness and universality. For instance, subsequent works have shown that this mechanism accommodates, and is sometimes even enhanced by, the presence of additional planets. In this talk, I will discuss a different complication when examining multiple possible rheologies for the planets. In particular, past works operate under a fluid planet model, where the planet’s rotation state and shape are simply related. On the other hand, observations suggest that many planets less massive than a few Earth masses appear to be consistent with predominantly rocky compositions. The shapes of such rocky planets likely decouple from their rotation when they spin sufficiently slowly. I present numerical and analytical results characterizing a class of previously-undiscovered resonances unique to rocky planets and discuss the impact of these new results on the prospect of oblique exoplanets.