Kepler-68: Three Planets, One With a Density Between That of Earth and Ice Giants

TL;DR

This paper reports the discovery and detailed characterization of a three-planet system around Kepler-68, including a planet with an intermediate density, using Kepler photometry, Doppler measurements, and asteroseismology.

Contribution

It provides the first detailed analysis of Kepler-68's planetary system, including mass, radius, density, and stellar properties, with novel asteroseismic modeling for precise stellar characterization.

Findings

Kepler-68b has a density between Earth and ice giants.

Kepler-68c is an Earth-sized transiting planet.

Kepler-68d is a Jovian-mass planet with a 580-day orbit.

Abstract

NASA's Kepler Mission has revealed two transiting planets orbiting Kepler-68. Follow-up Doppler measurements have established the mass of the innermost planet and revealed a third jovian-mass planet orbiting beyond the two transiting planets. Kepler-68b, in a 5.4 day orbit has mass 8.3 +/- 2.3 Earth, radius 2.31 +/- 0.07 Earth radii, and a density of 3.32 +/- 0.92 (cgs), giving Kepler-68b a density intermediate between that of the ice giants and Earth. Kepler-68c is Earth-sized with a radius of 0.953 Earth and transits on a 9.6 day orbit; validation of Kepler-68c posed unique challenges. Kepler-68d has an orbital period of 580 +/- 15 days and minimum mass of Msin(i) = 0.947 Jupiter. Power spectra of the Kepler photometry at 1-minute cadence exhibit a rich and strong set of asteroseismic pulsation modes enabling detailed analysis of the stellar interior. Spectroscopy of the star coupled with asteroseismic modeling of the multiple pulsation modes yield precise measurements of stellar properties, notably Teff = 5793 +/- 74 K, M = 1.079 +/- 0.051 Msun, R = 1.243 +/- 0.019 Rsun, and density 0.7903 +/- 0.0054 (cgs), all measured with fractional uncertainties of only a few percent. Models of Kepler-68b suggest it is likely composed of rock and water, or has a H and He envelope to yield its density of about 3 (cgs).