Day and Night in the Milky Way: Plausibility of Capture into High-Obliquity States for Exoplanets in the M Dwarf Habitable Zone

For temperate exoplanets orbiting M dwarf hosts, the proximity of the habitable zone to the star gives tidal effects an outsized importance. For example, spin synchronization of the planetary orbital period and rotation period is the commonly-assumed default for habitable zone planets across the entire M spectral type. This predicted tidal locking has important implications for conditions on the planetary surface. We investigate the plausibility of a dynamical state for which tidal locking may not be the norm for a known sample of 280 multiplanet systems orbiting M dwarf hosts. The potential for excitation into the Cassini State 2 resonance is capable of exciting planets into stable nonzero rotational obliquities: such a phenomenon could preclude synchronous rotation, inducing some version of "day" and "night." Considering each planetary pair and estimating the spin and orbital precession frequencies at an order-of-magnitude scale, we report that 75% of detected planets orbiting M dwarfs in compact configurations may be plausibly vulnerable to capture into Cassini States. This is consistent with similar findings for planets orbiting close-in to FGK dwarfs. However, it is only for M dwarf planets that the parameter space relevant for capture into Cassini State 2 overlaps with the stellar habitable zone. This overlap renders the potential capture into Cassini States very relevant for understanding the conditions for life on M dwarfs as the tidal effects of a stable, high-obliquity state have complex implications for both atmospheric circulation and planetary geology.