THE MASS OF Kepler-93b AND THE COMPOSITION OF TERRESTRIAL PLANETS

Abstract

Kepler-93b is a 1.478 ± 0.019 R ⊕ 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 M ⊙ and a radius of 0.919 ± 0.011 R ⊙ . 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 M ⊕ . 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 M ⊕ , 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 M ⊕ : 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 M ⊕ planets. Subject headings: planetary systems -planets and satellites: composition -stars: individual (Kepler-93 = KOI 69 = KIC 3544595) -techniques: radial velocities * Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

SECURE MASS MEASUREMENTS FROM TRANSIT TIMING: 10 KEPLER EXOPLANETS BETWEEN 3 AND 8 M ⊕ WITH DIVERSE DENSITIES AND INCIDENT FLUXES

“…Radial velocity (RV)spectroscopy has provided the bulk of mass measurements of transiting exoplanets (Batalha et al 2011;Winn et al 2011;Fressin et al 2012;Gillon et al 2012;Gautier et al 2012;Howard et al 2013;Pepe et al 2013;Weiss et al 2013;Haywood et al 2014;Marcy et al 2014;Dressing et al 2015). However, among subNeptune-mass planets, RV mass detections are limited to planets on short orbital periods, because the RV signal depends on the strength of planet-star interactions and declines with orbital distance.…”

“…The strongest detections of dense exoplanets, mostly in RV but also including Kepler-36 b (Carter et al 2012), indicate primarily rocky compositions for planets up to ≈1.6 R ⊕ in size (Dressing et al 2015). However, strong mass upper limits on the weaker detections of slightly larger planets include several low-mass planets that are too low in bulk density to be rocky (e.g., Kepler-11 b and Kepler-11 f;Lissauer et al 2013).…”

The HARPS-N Rocky Planet Search

“…While waiting for the new generation of ground-and spacebased transit searches targeting bright stars, planet density estimates from the follow-up of the Kepler candidates with HARPS-N have already been presented in several papers (Pepe et al 2013;Dumusque et al 2014;Bonomo et al 2014;Dressing et al 2015). They are populating the low-radius regime of the mass-radius relation for small exoplanets with candidates for which precise mass and radius could be determined.…”

From stellar to planetary composition: Galactic chemical evolution of Mg/Si mineralogical ratio

“…Bond et al 2010;Thiabaud et al 2015;Dorn et al 2015). Recently, Dressing et al (2015) showed that all the terrestrial planets in their sample with masses lower than 6 M ⊕ lie in the same mass-radius area as Earth and Venus. Their suggestion that these planets can be explained by Earth-like composition was successfully tested by Santos et al (2015).…”