Origin 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. Here we investigate the possibility of the latter scenario, in which the eccentricity of Kepler-1565b is excited via the Eccentric Kozai Lidov (EKL) mechanism in the presence of an undetected outer companion. We find early evidence of a second planet in the system with a period of P∼600−2100 days, and model the secular evolution of Kepler-1656b’s orbit in the presence of such a companion. Our results suggest that Kepler-1656b formed in situ, with subsequent eccentricity excitations induced by an outer companion via EKL.