Spin Dynamics of Planets in Resonant Chains: An Abundance of High Obliquities

About a dozen exoplanetary systems have been discovered with three or more planets participating in a chain of mean-motion resonances. The uniquely complex architectures of these so-called “resonant chains” motivate efforts to characterize their planets holistically. In this work, we perform a comprehensive exploration of the spin-axis dynamics of planets in resonant chains. Planetary spin states are closely linked with atmospheric dynamics and habitability and are thus especially relevant to resonant chains like TRAPPIST-1, which hosts several planets in the habitable zone. Considering the full set of observed resonant chains, we calculate the equilibrium states of the planetary axial tilts (“obliquities”). We show that high-obliquity states are stable for the majority of planets in resonant chain systems, even in the presence of strong tidal dissipation. We demonstrate how these high obliquity states could have been attained during the initial epoch of disk-driven orbital migration that established the resonant orbital architectures. Our results highlight that both the orbital and spin states of resonant chains are valuable relics of the early stages of planet formation and evolution.