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The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age

Petigura, Erik A. and Rogers, James G. and Isaacson, Howard and Owen, James E. and Kraus, Adam L. and Winn, Joshua N. and MacDougall, Mason G. and Howard, Andrew W. and Fulton, Benjamin and Kosiarek, Molly R. and Weiss, Lauren M. and Behmard, Aida and Blunt, Sarah (2022) The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age. Astronomical Journal, 163 (4). Art. No. 179. ISSN 1538-3881. doi:10.3847/1538-3881/ac51e3.

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In 2017, the California-Kepler Survey (CKS) published its first data release (DR1) of high-resolution optical spectra of 1305 planet hosts. Refined CKS planet radii revealed that small planets are bifurcated into two distinct populations, super-Earths (smaller than 1.5 R_⊕) and sub-Neptunes (between 2.0 and 4.0 R_⊕), with few planets in between (the "radius gap"). Several theoretical models of the radius gap predict variation with stellar mass, but testing these predictions is challenging with CKS DR1 due to its limited M⋆ range of 0.8–1.4 M⊙. Here we present CKS DR2 with 411 additional spectra and derived properties focusing on stars of 0.5–0.8 M_⊙. We found that the radius gap follows R_p ∝ P^m with m = −0.10 ± 0.03, consistent with predictions of X-ray and ultraviolet- and core-powered mass-loss mechanisms. We found no evidence that m varies with M⋆. We observed a correlation between the average sub-Neptune size and M⋆. Over 0.5–1.4 M⊙, the average sub-Neptune grows from 2.1 to 2.6 R_⊕, following R_p ∝ M^α⋆ with α = 0.25 ± 0.03. In contrast, there is no detectable change for super-Earths. These M⋆–R_p trends suggest that protoplanetary disks can efficiently produce cores up to a threshold mass of M_c, which grows linearly with stellar mass according to M_c ≈ 10 M_⊕(M⋆/M_⊙). There is no significant correlation between sub-Neptune size and stellar metallicity (over −0.5 to +0.5 dex), suggesting a weak relationship between planet envelope opacity and stellar metallicity. Finally, there is no significant variation in sub-Neptune size with stellar age (over 1–10 Gyr), which suggests that the majority of envelope contraction concludes after ∼1 Gyr.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Petigura, Erik A.0000-0003-0967-2893
Rogers, James G.0000-0001-7615-6798
Isaacson, Howard0000-0002-0531-1073
Owen, James E.0000-0002-4856-7837
Kraus, Adam L.0000-0001-9811-568X
Winn, Joshua N.0000-0002-4265-047X
MacDougall, Mason G.0000-0003-2562-9043
Howard, Andrew W.0000-0001-8638-0320
Fulton, Benjamin0000-0003-3504-5316
Kosiarek, Molly R.0000-0002-6115-4359
Weiss, Lauren M.0000-0002-3725-3058
Behmard, Aida0000-0003-0012-9093
Blunt, Sarah0000-0002-3199-2888
Additional Information:© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 November 23; revised 2022 January 9; accepted 2022 January 23; published 2022 March 18. We thank Natalie Batalha, Konstantin Batygin, Akash Gupta, Daniel Huber, Eric Lopez, Hilke Schlichting, Joshua Schlieder, Samuel Yee, and Jon Zink for valuable conversations that improved this manuscript. The data presented herein were obtained at the W. M. Keck Observatory from telescope time allocated to the National Aeronautics and Space Administration through the agency's scientific partnership with the California Institute of Technology and the University of California. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Kepler was competitively selected as the 10th NASA Discovery mission. Funding for this mission is provided by the NASA Science Mission Directorate. We thank the Kepler Science Office, the Science Operations Center, the Threshold Crossing Event Review Team (TCERT), and the Follow-up Observations Program (FOP) working group for their work on all steps in the planet discovery process, ranging from selecting target stars to curating planet catalogs. E.A.P. acknowledges support from the following sources: 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 NASA Keck PI Data Award (80NSSC20K0308), administered by the NASA Exoplanet Science Institute; a NASA Astrophysics Data Analysis Program (ADAP) grant (80NSSC20K0457); and the Alfred P. Sloan Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has long had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: Keck:I (HIRES) - , Kepler. - Software: The software used in this analysis is available on GitHub ( and archived on Zenodo (Petigura & Rogers 2022). We used the following additional packages: Astropy (Astropy Collaboration et al. 2013), emcee (Foreman-Mackey et al. 2013), isoclassify (Huber et al. 2017; Berger et al. 2020b), Lmfit (Newville et al. 2014), Matplotlib (Hunter 2007), Numpy (van der Walt et al. 2011), Scipy (Jones et al. 2001), H5py (Collette et al. 2021), Pandas (McKinney 2010), Seaborn (Waskom 2021), and Xarray (Hoyer & Hamman 2017).
Group:Astronomy Department
Funding AgencyGrant Number
W. M. Keck FoundationUNSPECIFIED
NASA Hubble FellowshipHST-HF2-51365.001-A
NASANAS 5-26555
Alfred P. Sloan FoundationUNSPECIFIED
Subject Keywords:Exoplanet astronomy; Exoplanet formation; Transit photometry; Exoplanets; Super Earths; Mini Neptunes; High resolution spectroscopy
Issue or Number:4
Classification Code:Unified Astronomy Thesaurus concepts: Exoplanet astronomy (486); Exoplanet formation (492); Transit photometry (1709); Exoplanets (498); Super Earths (1655); Mini Neptunes (1063); High resolution spectroscopy (2096)
Record Number:CaltechAUTHORS:20220228-183210630
Persistent URL:
Official Citation:Erik A. Petigura et al 2022 AJ 163 179
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:113633
Deposited By: George Porter
Deposited On:01 Mar 2022 23:07
Last Modified:25 Mar 2022 23:37

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