The California-Kepler Survey. V. Peas in a Pod: Planets in a Kepler Multi-planet System Are Similar in Size and Regularly Spaced

Creators: Weiss, Lauren M.; Marcy, Geoffrey W.; Petigura, Erik A.; Fulton, Benjamin J.; Howard, Andrew W.; Winn, Joshua N.; Isaacson, Howard T.; Morton, Timothy D.; Hirsch, Lea A.; Sinukoff, Evan J.; Cumming, Andrew; Hebb, Leslie; Cargile, Phillip A.

Abstract

We have established precise planet radii, semimajor axes, incident stellar fluxes, and stellar masses for 909 planets in 355 multi-planet systems discovered by Kepler. In this sample, we find that planets within a single multi-planet system have correlated sizes: each planet is more likely to be the size of its neighbor than a size drawn at random from the distribution of observed planet sizes. In systems with three or more planets, the planets tend to have a regular spacing: the orbital period ratios of adjacent pairs of planets are correlated. Furthermore, the orbital period ratios are smaller in systems with smaller planets, suggesting that the patterns in planet sizes and spacing are linked through formation and/or subsequent orbital dynamics. Yet, we find that essentially no planets have orbital period ratios smaller than 1.2, regardless of planet size. Using empirical mass–radius relationships, we estimate the mutual Hill separations of planet pairs. We find that 93% of the planet pairs are at least 10 mutual Hill radii apart, and that a spacing of ~20 mutual Hill radii is most common. We also find that when comparing planet sizes, the outer planet is larger in 65% ± 0.4% of cases, and the typical ratio of the outer to inner planet size is positively correlated with the temperature difference between the planets. This could be the result of photo-evaporation.

Additional Information

© 2018. The American Astronomical Society. Received 2017 June 18; revised 2017 December 4; accepted 2017 December 4; published 2018 January 3. 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. Keck time for the CKS multis project was acquired by G.W.M., with assistance from L.M.W. 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 E.A.P., 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. L.M.W. performed the analysis in this paper, with assistance from G.W.M., J.W., and A.C. This manuscript was largely written by L.M.W. with assistance from J.W. and G.W.M. We thank John Asher Johnson, a PI and originator of the magnitude-limited CKS survey. Conversations with Rebekah Dawson, Daniel Fabrycky, Eve Lee, Doug Lin, Jack Lissauer, Jason Rowe, and Scott Tremaine influenced this paper. We thank the referee for a thorough and insightful discussion. The results presented herein were made possible by 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. L.M.W. acknowledges support from Gloria and Ken Levy and from the Trottier Family 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. B.J.F. acknowledges National Science Foundation Graduate Research Fellowship under grant No. 2014184874. A.W.H. acknowledges NASA grant NNX12AJ23. G.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. 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. Facilities: Keck:I (HIRES) - KECK I Telescope, Kepler - The Kepler Mission.

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