Kepler-1656b: a Dense Sub-Saturn With an Extreme Eccentricity

TL;DR

Kepler-1656b is a dense, sub-Saturn-sized exoplanet with an exceptionally high eccentricity, revealing insights into planetary composition and dynamical history through precise radial velocity measurements.

Contribution

This study provides the first precise mass and eccentricity measurements of Kepler-1656b, highlighting its unique high density and extreme orbit among similar-sized exoplanets.

Findings

Kepler-1656b has a mass of 48 ± 4 Earth masses.

The planet's eccentricity is 0.84 ± 0.01, the highest for sub-100 Earth mass planets.

Approximately 80% of the planet's mass is in high-density material.

Abstract

Kepler-1656b is a 5 $R_E$ 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 $48 \pm 4 M_E$, 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 $0.84 \pm 0.01$, the largest measured eccentricity for any planet less than 100 $M_E$. 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.