Kepler-1656b: A Dense Sub-Saturn with an Extreme Eccentricity

Kepler-1656b is a 5 {R}_\oplus planet with an orbital period of 32 days initially detected by the prime Kepler mission. We obtained precision radial velocities of Kepler-1656 with Keck/HIRES in order to confirm the planet and to characterize its mass and orbital eccentricity. With a mass of 48 ± 4 {M}_\oplus , Kepler-1656b is more massive than most planets of comparable size. Its high mass implies that a significant fraction, roughly 80%, of the planet’s total mass is in high-density material such as rock/iron, with the remaining mass in a low-density H/He envelope. The planet also has a high eccentricity of 0.84 ± 0.01, the largest measured eccentricity for any planet less than 100 {M}_\oplus . The planet’s high density and high eccentricity may be the result of one or more scattering and merger events during or after the dispersal of the protoplanetary disk.