Origins of Kepler-1656b’s Extreme Eccentricity

Highly eccentric orbits are one of the major surprises of exoplanets relative to the Solar System, and are typically indicative of a rich dynamical history. One system of particular interest is Kepler-1656, which hosts a single known planet on a close-in, highly eccentric (e=0.8) orbit. This orbital configuration places Kepler-1656b on the extreme upper envelope of the e-a diagram and is not a typical outcome of planet formation. Instead, planets formed in a near-circular orbit can be driven to much higher eccentricities via pathways such as planet-planet scattering, perturbation from a stellar flyby, or Kozai evolution induced by an outer stellar or planetary companion in the system. To investigate the possibility of these scenarios, we use Gaia, radial velocities, and ground-based imaging data to place observational constraints on the properties of a potential companion or flyby perturber to Kepler-1656b. We then model the secular evolution of the system in the presence of a third, outer planet using dynamical simulations to assess the likelihood of a high eccentricity excited via the Eccentric Kozai-Lidov effect.