The Mass of Kepler-93b and The Composition of Terrestrial Planets

TL;DR

This study precisely measures the mass and composition of Kepler-93b, revealing it is a dense, rocky planet with a composition consistent with other planets in the 1-6 Earth mass range, and explores implications for planetary formation.

Contribution

The paper provides a precise mass measurement of Kepler-93b and demonstrates a consistent iron to silicate ratio among dense planets in the 1-6 Earth mass range, suggesting a common compositional model.

Findings

Kepler-93b has a mass of 4.02 +/- 0.68 Earth masses.

Kepler-93b's density indicates a rocky composition with iron and magnesium silicate.

Dense planets between 1-6 Earth masses share a similar fixed ratio of iron to silicate.

Abstract

Kepler-93b is a 1.478 +/- 0.019 Earth radius planet with a 4.7 day period around a bright (V=10.2), astroseismically-characterized host star with a mass of 0.911+/-0.033 solar masses and a radius of 0.919+/-0.011 solar radii. Based on 86 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 32 archival Keck/HIRES observations, we present a precise mass estimate of 4.02+/-0.68 Earth masses. The corresponding high density of 6.88+/-1.18 g/cc is consistent with a rocky composition of primarily iron and magnesium silicate. We compare Kepler-93b to other dense planets with well-constrained parameters and find that between 1-6 Earth masses, all dense planets including the Earth and Venus are well-described by the same fixed ratio of iron to magnesium silicate. There are as of yet no examples of such planets with masses > 6 Earth masses: All known planets in this mass regime have lower densities requiring significant fractions of volatiles or H/He gas. We also constrain the mass and period of the outer companion in the Kepler-93 system from the long-term radial velocity trend and archival adaptive optics images. As the sample of dense planets with well-constrained masses and radii continues to grow, we will be able to test whether the fixed compositional model found for the seven dense planets considered in this paper extends to the full population of 1-6 Earth mass planets.