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Most 1.6 Earth-Radius Planets are not Rocky

Rogers, Leslie A. (2015) Most 1.6 Earth-Radius Planets are not Rocky. Astrophysical Journal, 801 (1). Art. No. L41. ISSN 0004-637X.

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The Kepler mission, combined with ground-based radial velocity (RV) follow-up and dynamical analyses of transit timing variations, has revolutionized the observational constraints on sub-Neptune-sized planet compositions. The results of an extensive Kepler follow-up program including multiple Doppler measurements for 22 planet-hosting stars more than doubles the population of sub-Neptune-sized transiting planets that have RV mass constraints. This unprecedentedly large and homogeneous sample of planets with both mass and radius constraints opens the possibility of a statistical study of the underlying population of planet compositions. We focus on the intriguing transition between rocky exoplanets (comprised of iron and silicates) and planets with voluminous layers of volatiles (H/He and astrophysical ices). Applying a hierarchical Bayesian statistical approach to the sample of Kepler transiting sub-Neptune planets with Keck RV follow-up, we constrain the fraction of close-in planets (with orbital periods less than ~50 days) that are sufficiently dense to be rocky, as a function of planet radius. We show that the majority of 1.6 R⊕ planets have too low density to be comprised of Fe and silicates alone. At larger radii, the constraints on the fraction of rocky planets are even more stringent. These insights into the size demographics of rocky and volatile-rich planets offer empirical constraints to planet formation theories, and guide the range of planet radii to be considered in studies of the occurrence rate of "Earth-like" planets, η⊕.

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URLURL TypeDescription DOIArticle Paper
Rogers, Leslie A.0000-0003-0638-3455
Additional Information:© 2015 The American Astronomical Society. Received 2014 January 17; accepted 2015 January 22; published 2015 March 2. I would like to especially thank Geoff Marcy for extensive discussions and advice, Howard Isaacson for sharing samples from his MCMC fits to the Keck RV data, and an anonymous reviewer whose comments helped to improve this manuscript. I am also grateful for helpful discussions with Lauren Weiss, Rebekah Dawson, Tim Morton, Brice-Olivier Demory, Brian Jackson, Jack Lissauer, Tom Loredo, Eric Ford, David Hogg, and John Johnson. I acknowledge support provided by NASA through Hubble Fellowship grant #HF-51313 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. This work also benefited from the Summer Program on Modern Statistical and Computational Methods for Analysis of Kepler Data, held at SAMSI, Research Triangle Park, NC in 2013 June. This research has made use of the Exoplanet Orbit Database and the Exoplanet Data Explorer at
Funding AgencyGrant Number
NASA Hubble FellowshipHF-51313
NASANAS 5-26555
Subject Keywords:methods: data analysis; methods: statistical; planetary systems; planets and satellites: composition; techniques: photometric; techniques: radial velocities
Issue or Number:1
Record Number:CaltechAUTHORS:20140804-082233937
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Official Citation:Most 1.6 Earth-radius Planets are Not Rocky Leslie A. Rogers 2015 ApJ 801 41
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:47867
Deposited By: Ruth Sustaita
Deposited On:04 Aug 2014 17:19
Last Modified:03 Oct 2019 06:57

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