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

Creators: 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⊕. This gap splits the population of close-in (P < 100 days) small planets into two size regimes: R_p < 1.5 R⊕ and R_p = 2.0-3.0 R⊕, 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⊕ supports the emerging picture that close-in planets smaller than Neptune are composed of rocky cores measuring 1.5 R⊕ or smaller with varying amounts of low-density gas that determine their total sizes.

Additional Information

© 2017 The American Astronomical Society. Received 2017 March 30; revised 2017 June 1; accepted 2017 June 4; published 2017 August 24. Facilities: Keck:I (HIRES) - , Kepler - . The CKS project was conceived, planned, and initiated by A.W.H., G.W.M., J.A.J., H.T.I., and T.D.M. A.W.H., G.W.M., J.A.J. acquired Keck telescope time to conduct the magnitude-limited survey. Keck time for the other stellar samples was acquired by J.N.W., L.A.R., and G.W.M. The observations were coordinated by H.T.I. and A.W.H. and carried out by A.W.H., H.T.I., G.W.M., J.A.J., T.D.M., B.J.F., L.M.W., E.A.P., E.S., and L.A.H. A.W.H. secured CKS project funding. SpecMatch was developed and run by EAP and SME@XSEDE was developed and run by L.H. and P.A.C. Downstream data products were developed by E.A.P., H.T.I., and B.J.F. Results from the two pipelines were consolidated and the integrity of the parameters were verified by A.W.H., H.T.I., E.A.P., G.W.M., with assistance from B.J.F., L.M.W., E.S., L.A.H., and I.J.M.C. E.A.P. computed derived planetary and stellar properties with assistance from B.J.F. B.J.F. performed the analysis in this paper, with assistance from E.A.P., A.W.H., and G.W.M. This manuscript was largely written by B.J.F. with assistance from E.A.P., A.W.H., G.W.M., J.N.W., and L.M.W. We thank Josh Winn, Jason Rowe, Eric Lopez, Jeff Valenti, Daniel Huber, and Leslie Rogers for contributing insight during many helpful conversations and providing comments on early drafts of the manuscript. Most of the data presented here were determined directly from observations at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. We are grateful to the time assignment committees of the University of Hawaii, the University of California, the California Institute of Technology, and NASA for their generous allocations of observing time that enabled this large project. Kepler was competitively selected as the tenth NASA Discovery mission. Funding for this mission is provided by the NASA Science Mission Directorate. B.J.F. acknowledges that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship under grant No. 2014184874. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. E.A.P. acknowledges support from Hubble Fellowship grant HST-HF2-51365.001-A 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. A.W.H. acknowledges NASA grant NNX12AJ23G. T.D.M. acknowledges NASA grant NNX14AE11G. P.A.C. acknowledges National Science Foundation grant AST-1109612. L.H. acknowledges National Science Foundation grant AST-1009810. L.M.W. acknowledges support from Gloria and Ken Levy and from the Trottier Family. E.S. is supported by a post-graduate scholarship from the Natural Sciences and Engineering Research Council of Canada. Finally, the authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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