Climatic peculiarities of spin-orbit resonant exoplanets orbiting low-mass stars

M dwarfs are the most promising candidates for finding habitable planets. While the habitable zone is a starting point for assessing a planet’s potential for hosting life, detailed climate studies are needed to more accurately evaluate habitability. Oceans play a crucial role in shaping planetary climate, making their impact essential for modelling terrestrial exoplanets and interpreting future observations.
Potentially habitable planets orbiting M dwarfs experience particularly strong tidal forcings and are often tidally locked. Despite the majority of research being centered on the climate dynamics of synchronously rotating planets in this scenario, synchronous rotation is not an inevitable outcome of tidal locking. Several different circumstances can result in an asynchronous rotation, and in some instances can lead to spin-orbit resonances.

We have carried out studies using a coupled atmosphere-ocean general circulation model (FORTE2.0), which allows us to incorporate the effects of ocean circulation through a dynamic ocean. We investigated the case of terrestrial planets residing in the habitable zone of their M host star characterized by two different rotational states: an Earth-like rotation period and a spin-orbit resonance.
The results of the spin-orbit resonant case significantly differ from the Earth-like rotation case, but also from the synchronous case analysed in the literature. Interestingly, we found that climatic features in the resonant case, such as clouds and rainfall, present peculiar patterns with respect to the substellar point. The evolution of these quantities during a planetary orbit is noteworthy from the observational point of view, as observable features are shaped differently in each rotational scenario.