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Four Sub-Saturns with Dissimilar Densities: Windows into Planetary Cores and Envelopes

Petigura, Erik A. and Sinukoff, Evan and Lopez, Eric D. and Crossfield, Ian J. M. and Howard, Andrew W. and Brewer, John M. and Fulton, Benjamin J. and Isaacson, Howard T. and Ciardi, David R. and Howell, Steve B. and Everett, Mark E. and Horch, Elliott P. and Hirsch, Lea A. and Weiss, Lauren M. and Schlieder, Joshua E. (2017) Four Sub-Saturns with Dissimilar Densities: Windows into Planetary Cores and Envelopes. Astronomical Journal, 153 (4). Art. No. 142. ISSN 1538-3881.

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We present results from a Keck/HIRES radial velocity campaign to study four sub-Saturn-sized planets, K2-27b, K2-32b, K2-39b, and K2-108b, with the goal of understanding their masses, orbits, and heavy-element enrichment. The planets have similar sizes (R_P = 4.5–5.5 R⊕), but have dissimilar masses (M_P = 16–60 M⊕), implying a diversity in their core and envelope masses. K2-32b is the least massive (M_P = 16.5 ± 2.7 M⊕) and orbits in close proximity to two sub-Neptunes near a 3:2:1 period commensurability. K2-27b and K2-39b are significantly more massive at M_P = 30.9 ± 4.6 M⊕ and M_P = 39.8 ± 4.4 M⊕, respectively, and show no signs of additional planets. K2-108b is the most massive at M_P = 59.4 ± 4.4 M⊕, implying a large reservoir of heavy elements of about ≈50 M⊕. Sub-Saturns as a population have a large diversity in planet mass at a given size. They exhibit remarkably little correlation between mass and size; sub-Saturns range from ≈6–60 M⊕, regardless of size. We find a strong correlation between planet mass and host star metallicity, suggesting that metal-rich disks form more massive planet cores. The most massive sub-Saturns tend to lack detected companions and have moderately eccentric orbits, perhaps as a result of a previous epoch of dynamical instability. Finally, we observe only a weak correlation between the planet envelope fraction and present-day equilibrium temperature, suggesting that photo-evaporation does not play a dominant role in determining the amount of gas sub-Saturns accrete from their protoplanetary disks.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Petigura, Erik A.0000-0003-0967-2893
Sinukoff, Evan0000-0002-5658-0601
Crossfield, Ian J. M.0000-0002-1835-1891
Howard, Andrew W.0000-0001-8638-0320
Brewer, John M.0000-0002-9873-1471
Fulton, Benjamin J.0000-0003-3504-5316
Isaacson, Howard T.0000-0002-0531-1073
Ciardi, David R.0000-0002-5741-3047
Howell, Steve B.0000-0002-2532-2853
Everett, Mark E.0000-0002-0885-7215
Horch, Elliott P.0000-0003-2159-1463
Hirsch, Lea A.0000-0001-8058-7443
Weiss, Lauren M.0000-0002-3725-3058
Schlieder, Joshua E.0000-0001-5347-7062
Additional Information:© 2017 The American Astronomical Society. Received 2016 December 1; revised 2017 January 30; accepted 2017 January 30; published 2017 March 7. We thank Konstantin Batygin and John Livingston for helpful discussions. 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. This work has made use of data from the European Space Agency (ESA) mission Gaia, processed by Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Some of the data presented herein were obtained at the W M Keck Observatory (which is operated as a scientific partnership among Caltech, UC, and NASA). We thank the Caltech and NASA Keck Time Allocation Committees for providing HIRES time. This work included observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). 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. Software: Numpy/Scipy (Van Der Walt et al. 2011), Matplotlib (Hunter 2007), Pandas (McKinney 2010), Astropy (Astropy Collaboration et al. 2013), emcee (Goodman & Weare 2010; Foreman-Mackey et al. 2013), SME (Brewer et al. 2015), isochrones (Morton 2015), k2sc (Aigrain et al. 2015), batman (Kreidberg 2015), radvel (, k2phot (
Group:Infrared Processing and Analysis Center (IPAC), Astronomy Department
Funding AgencyGrant Number
NASA Hubble FellowshipHST-HF2-51365.001-A
NASANAS 5-26555
Subject Keywords:planets and satellites: composition – planets and satellites: detection – planets and satellites: dynamical evolution and stability – planets and satellites: formation – planets and satellites: gaseous planets – planets and satellites: interiors
Issue or Number:4
Record Number:CaltechAUTHORS:20170308-084328165
Persistent URL:
Official Citation:Erik A. Petigura et al 2017 AJ 153 142
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:74885
Deposited By: Tony Diaz
Deposited On:08 Mar 2017 17:08
Last Modified:14 Oct 2019 21:21

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