UCLA

One longstanding problem for the potential habitability of Earth-like planets around very small, cool dwarf stars is their perceived likelihood of being tidally locked into synchronous spin states wherein the planet's spin rate equals its orbital rate resulting in a perpetual day and corresponding night side on the planet. This problem had thus far largely been addressed by only considering two objects in the modeling: the planet under consideration and the star around which it orbits. However, many systems have been found to harbor multiple planets, with some in or near mean motion resonances. One prime example is the TRAPPIST-1 system, which contains at least seven terrestrial planets arranged compactly in a long resonant chain around a small ultra-cool dwarf star. The new Transiting Exoplanet Survey Satellite (TESS) also promises to reveal many new habitable zone planets around small stars. The presence of one or more nearby planetary companions near a mean motion resonance can induce oscillatory variations in the mean motion of the planet, which we show can have potentially dramatic influences on the effective spin state of a planet. In particular, we find that planetary companions can excite the spin states of planets in the habitable zone of small, cool stars, pushing otherwise synchronously rotating planets into higher amplitude librations in the spin state or even fully circulating states resulting in full stellar days. In other cases, especially those with multiple companion planets, we find that spin states are unable to sustain long-term stability, switching among quasi-stable attractor states. We also explore effects that atmospheric tides can have on the resultant spin states, finding that they can work to increase the spin rates in many cases. Overall, we find that these effects can result in an increased illumination area on such planets, having potentially drastic consequences for habitability.