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Tidal Inflation Reconciles Low-Density Sub-Saturns with Core Accretion

Millholland, Sarah and Petigura, Erik and Batygin, Konstantin (2020) Tidal Inflation Reconciles Low-Density Sub-Saturns with Core Accretion. Astrophysical Journal, 897 (1). Art. No. 7. ISSN 1538-4357. doi:10.3847/1538-4357/ab959c.

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While the solar system contains no planets between the sizes of Uranus and Saturn, our current exoplanet census includes several dozen such planets with well-measured masses and radii. These sub-Saturns exhibit a diversity of bulk densities, ranging from ~0.1 to 3 g cm⁻³. When modeled simply as hydrogen/helium envelopes atop rocky cores, this diversity in densities translates to a diversity in planetary envelope fractions, f_(env) = M_(env)/M_p , ranging from ~10% to ~50%. Planets with f_(env) ≈ 50% pose a challenge to traditional models of giant planet formation by core-nucleated accretion, which predict the onset of runaway gas accretion when M_(env) ~ M_(core). Here, we show that many of these apparent f_(env) ≈ 50% planets are less envelope-rich than they seem, after accounting for tidal heating. We present a new framework for modeling sub-Saturn interiors that incorporates envelope inflation due to tides, which are driven by the observed nonzero eccentricities, as well as potential obliquities. Consequently, when we apply our models to known sub-Saturns, we infer lower f_(env) than tides-free estimates. We present a case study of K2-19 b, a moderately eccentric sub-Saturn. Neglecting tides, K2-19 b appears to have f_(env) ≈ 50%, poised precariously near the runaway threshold; by including tides, however, we find f_(env) ≈ 10%, resolving the tension. Through a systematic analysis of 4–8 R⊕ planets, we find that most (but not all) of the similarly envelope-rich planets have more modest envelopes of f_(env) ≈ 10%–20%. Thus, many sub-Saturns may be understood as sub-Neptunes that have undergone significant radius inflation, rather than a separate class of objects. Tidally induced radius inflation likely plays an important role in other size classes of planets including ultra-low-density Jupiter-size planets like WASP-107 b.

Item Type:Article
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URLURL TypeDescription Paper
Millholland, Sarah0000-0003-3130-2282
Petigura, Erik0000-0003-0967-2893
Batygin, Konstantin0000-0002-7094-7908
Additional Information:© 2020 The American Astronomical Society. Received 2020 March 20; revised 2020 May 11; accepted 2020 May 20; published 2020 June 26. We are grateful to the anonymous referee for the helpful comments and suggestions. We thank Chris Spalding for insightful conversations. We also thank Howard Chen & Leslie Rogers for their publicly available MESA model. S.M. is supported by the NSF Graduate Research Fellowship Program under Grant DGE-1122492. E.P. acknowledges the generous support of the Alfred P. Sloan Foundation. K.B. is grateful to the David and Lucile Packard Foundation and the Alfred P. Sloan Foundation for their generous support. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.
Group:Astronomy Department
Funding AgencyGrant Number
NSF Graduate Research FellowshipDGE-1122492
Alfred P. Sloan FoundationUNSPECIFIED
David and Lucile Packard FoundationUNSPECIFIED
Subject Keywords:Exoplanets ; Extrasolar gas giants ; Exoplanet atmospheres ; Exoplanet structure ; Exoplanet tides
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Exoplanets (498); Extrasolar gas giants (509); Exoplanet atmospheres (487); Exoplanet structure (495); Exoplanet tides (497)
Record Number:CaltechAUTHORS:20200611-113646322
Persistent URL:
Official Citation:Sarah Millholland et al 2020 ApJ 897 7
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:103841
Deposited By: George Porter
Deposited On:12 Jun 2020 19:17
Last Modified:16 Nov 2021 18:25

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