Plausibility of Capture into High-obliquity States for Exoplanets in the M Dwarf Habitable Zone

Abstract For temperate exoplanets orbiting M-dwarf hosts, the proximity of the habitable zone to the star necessitates careful consideration of tidal effects. Spin synchronization of the planetary orbital period and rotation period, tidal locking, and the subsequent impact on surface conditions frames common assumptions about M-dwarf planets. We investigate the plausibility of capture into Cassini State 2 (CS2) for a known sample of 280 multiplanet systems orbiting M-dwarf hosts. This resonance of the spin precession and orbital precession frequencies can excite planets into stable nonzero rotational obliquities, breaking tidal locking and inducing a version of “day” and “night.” Considering each planetary pair and estimating the spin and orbital precession frequencies, we find that 75% of detected planets orbiting M dwarfs may be plausibly excited to a high obliquity and maintain it through subsequent tidal dissipation over long timescales. We also investigate two possible mechanisms for capture into CS2: quantifying the orbital migration or primordial obliquity necessary for CS2. We find orbital migrations by a factor of ≲2 and an isotropic initial spin distribution can produce high-obliquity planets, aligning with similar findings for planets orbiting close-in to FGK dwarfs. Many of the planets in our sample reside in both CS2 and within their stellar habitable zone. Over half of the planets with T eq < 400 K around host stars with T eff < 3000 K could possess nonzero obliquity due to residence in CS2. This overlap renders the potential capture into Cassini States extremely relevant to understanding the galaxy’s most common temperate planets.