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Kepler-1656b's Extreme Eccentricity: Signature of a Gentle Giant

Highly eccentric orbits are one of the major surprises of exoplanets relative to the Solar System and indicate rich and tumultuous dynamical histories. One system of particular interest is Kepler-1656, which hosts a sub-Jovian planet with an eccentricity of 0.8. Sufficiently eccentric orbits will shrink in semi-major axis due to tidal dissipation of orbital energy during periastron passage. Here our goal was to assess whether Kepler-1656b is currently undergoing such high-eccentricity migration, and to further understand the system's origins and architecture. We confirm a second planet in the system with $M_{\rm c}= 0.40 \pm 0.09M_{\rm jup}$ and P$_{\rm c}= 1919\pm 27\,$days. We simulated the dynamical evolution of planet b in the presence of planet c and find a variety of possible outcomes for the system, such as tidal migration and engulfment. The system is consistent with an in situ dynamical origin of planet b followed by subsequent Eccentric Kozai Lidov (EKL) perturbations that excite Kepler-1656b's eccentricity gently, i.e. without initiating tidal migration. Thus, despite its high eccentricity, we find no evidence that planet b is or has migrated through the high-eccentricity channel. Finally, we predict the outer orbit to be mutually inclined in a nearly perpendicular configuration with respect to the inner planet orbit based on the outcomes of our simulations, and make observable predictions for the inner planet's spin-orbit angle. Our methodology can be applied to other eccentric or tidally locked planets to constrain their origins, orbital configurations and properties of a potential companion.