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.
Highlights
Small planets are abundant in the galaxy, but the compositional diversity of small planets is not well understood
Recent studies have explored the compositional diversity of small planets using hierarchical Bayesian modeling of the observed planet radii
We reduced the data with the standard HARPS-N pipeline by cross-correlating the observed spectra with a numerical mask based on the spectrum of a G2V star (Baranne et al 1996; Pepe et al 2002)
Summary
Small planets are abundant in the galaxy, but the compositional diversity of small planets is not well understood. Using 37 months of Kepler short cadence data, Ballard et al (2014) conducted an asteroseismic investigation to characterize Kepler-93 in exquisite detail They estimated an average stellar density of 1.652 ± 0.006 g cm−3, a stellar mass of 0.911 ± 0.033 M , and a stellar radius of 0.919 ± 0.011 R. They note that the infrared transit depth they measured with the Spitzer Space Telescope is consistent with the planetary interpretation of Kepler-93b They place stringent limits on the presence of nearby stars based on Keck AO images (Marcy et al 2014). We refine the mass measurement of Kepler-93b from 2.5σ to 6σ by analyzing two seasons of HARPS-N radial velocities in addition to the publicly available HIRES data
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