The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets

September 2017 • 2017AJ....154..109F

Authors • Fulton, Benjamin J. • Petigura, Erik A. • Howard, Andrew W. • Isaacson, Howard • Marcy, Geoffrey W. • Cargile, Phillip A. • Hebb, Leslie • Weiss, Lauren M. • Johnson, John Asher • Morton, Timothy D. • Sinukoff, Evan • Crossfield, Ian J. M. • Hirsch, Lea A.

Abstract • The size of a planet is an observable property directly connected to the physics of its formation and evolution. We used precise radius measurements from the California-Kepler Survey to study the size distribution of 2025 Kepler planets in fine detail. We detect a factor of ≥2 deficit in the occurrence rate distribution at 1.5-2.0 {R}_\oplus. This gap splits the population of close-in (P < 100 days) small planets into two size regimes: {R}_{{P}}< 1.5 {R}_\oplus and {R}_{{P}}=2.0{--}3.0 {R}_\oplus, with few planets in between. Planets in these two regimes have nearly the same intrinsic frequency based on occurrence measurements that account for planet detection efficiencies. The paucity of planets between 1.5 and 2.0 {R}_\oplus supports the emerging picture that close-in planets smaller than Neptune are composed of rocky cores measuring 1.5 {R}_\oplus or smaller with varying amounts of low-density gas that determine their total sizes.

Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology. Keck time was granted for this project by the University of California, and California Institute of Technology, the University of Hawaii, and NASA.

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Benjamin Fulton

Assistant Scientist