Sporadic Spin-orbit Variations in Compact Multiplanet Systems and Their Influence on Exoplanet Climate

Climate modeling has shown that tidally influenced terrestrial exoplanets, particularly those orbiting M-dwarfs, have unique atmospheric dynamics and surface conditions that may enhance their likelihood to host viable habitats. However, sporadic libration and rotation induced by planetary interactions, such as those due to mean motion resonances (MMR) in compact planetary systems, may destabilize attendant exoplanets away from synchronized states (1:1 spin-orbit ratios). Here, we use a three-dimensional N-rigid-body integrator and an intermediately complex general circulation model to simulate the evolving climates of TRAPPIST-1 e and f with different orbital- and spin-evolution pathways. Planet f scenarios perturbed by MMR effects with chaotic spin variations are colder and dryer compared to their synchronized counterparts due to the zonal drift of the substellar point away from open ocean basins of their initial eyeball states. On the other hand, the differences between perturbed and synchronized planet e are minor due to higher instellation, warmer surfaces, and reduced climate hysteresis. This is the first study to incorporate the time-dependent outcomes of direct gravitational N-rigid-body simulations into 3D climate modeling of extrasolar planets, and our results show that planets at the outer edge of the habitable zones in compact multiplanet systems are vulnerable to rapid global glaciations. In the absence of external mechanisms such as orbital forcing or tidal heating, these planets could be trapped in permanent snowball states.

Automated summary

Climate modeling shows that tidally influenced terrestrial exoplanets, especially those orbiting M-dwarfs, have unique atmospheric dynamics and surface conditions that may increase their chances of hosting habitable environments. However, planetary interactions, such as mean motion resonances in compact planetary systems, can destabilize these exoplanets away from synchronized states. This study uses a three-dimensional N-rigid-body integrator and a general circulation model to simulate the climates of TRAPPIST-1 e and f under different orbital and spin evolution pathways. The results show that planets perturbed by mean motion resonances experience colder and drier conditions compared to their synchronized counterparts, while the differences in perturbed and synchronized planet e are minimal. These findings highlight the vulnerability of planets at the outer edge of habitable zones in compact multiplanet systems to global glaciations.