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WASP-107b’s Density Is Even Lower: A Case Study for the Physics of Planetary Gas Envelope Accretion and Orbital Migration

Piaulet, Caroline and Benneke, Björn and Rubenzahl, Ryan A. and Howard, Andrew W. and Lee, Eve J. and Thorngren, Daniel and Angus, Ruth and Peterson, Merrin and Schlieder, Joshua E. and Werner, Michael and Kreidberg, Laura and Jaouni, Tareq and Crossfield, Ian J. M. and Ciardi, David R. and Petigura, Erik A. and Livingston, John and Dressing, Courtney D. and Fulton, Benjamin J. and Beichman, Charles and Christiansen, Jessie L. and Gorjian, Varoujan and Hardegree-Ullman, Kevin K. and Krick, Jessica and Sinukoff, Evan (2021) WASP-107b’s Density Is Even Lower: A Case Study for the Physics of Planetary Gas Envelope Accretion and Orbital Migration. Astronomical Journal, 161 (2). Art. No. 70. ISSN 1538-3881.

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With a mass in the Neptune regime and a radius of Jupiter, WASP-107b presents a challenge to planet formation theories. Meanwhile, the planet's low surface gravity and the star's brightness also make it one of the most favorable targets for atmospheric characterization. Here, we present the results of an extensive 4 yr Keck/HIRES radial-velocity (RV) follow-up program of the WASP-107 system and provide a detailed study of the physics governing the accretion of the gas envelope of WASP-107b. We reveal that WASP-107b's mass is only 1.8 Neptune masses (M_b = 30.5 ± 1.7 M_⊕). The resulting extraordinarily low density suggests that WASP-107b has a H/He envelope mass fraction of >85% unless it is substantially inflated. The corresponding core mass of <4.6 M_⊕ at 3σ is significantly lower than what is traditionally assumed to be necessary to trigger massive gas envelope accretion. We demonstrate that this large gas-to-core mass ratio most plausibly results from the onset of accretion at gsim1 au onto a low-opacity, dust-free atmosphere and subsequent migration to the present-day a_b = 0.0566 ± 0.0017 au. Beyond WASP-107b, we also detect a second, more massive planet (M_c sin i = 0.36 ± 0.04MJ ) on a wide eccentric orbit (e _c = 0.28 ± 0.07) that may have influenced the orbital migration and spin–orbit misalignment of WASP-107b. Overall, our new RV observations and envelope accretion modeling provide crucial insights into the intriguing nature of WASP-107b and the system's formation history. Looking ahead, WASP-107b will be a keystone planet to understand the physics of gas envelope accretion.

Item Type:Article
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URLURL TypeDescription Paper
Piaulet, Caroline0000-0002-2875-917X
Benneke, Björn0000-0001-5578-1498
Rubenzahl, Ryan A.0000-0003-3856-3143
Howard, Andrew W.0000-0001-8638-0320
Lee, Eve J.0000-0002-1228-9820
Thorngren, Daniel0000-0002-5113-8558
Angus, Ruth0000-0003-4540-5661
Peterson, Merrin0000-0002-2787-5451
Schlieder, Joshua E.0000-0001-5347-7062
Werner, Michael0000-0003-4990-189X
Kreidberg, Laura0000-0003-0514-1147
Crossfield, Ian J. M.0000-0002-1835-1891
Ciardi, David R.0000-0002-5741-3047
Petigura, Erik A.0000-0003-0967-2893
Livingston, John0000-0002-4881-3620
Dressing, Courtney D.0000-0001-8189-0233
Fulton, Benjamin J.0000-0003-3504-5316
Beichman, Charles0000-0002-5627-5471
Christiansen, Jessie L.0000-0002-8035-4778
Gorjian, Varoujan0000-0002-8990-2101
Hardegree-Ullman, Kevin K.0000-0003-3702-0382
Krick, Jessica0000-0002-2413-5976
Sinukoff, Evan0000-0002-5658-0601
Additional Information:© 2021. The American Astronomical Society. Received 2020 May 8; revised 2020 November 20; accepted 2020 November 22; published 2021 January 18. We thank the anonymous reviewer, whose comments greatly improved the paper. C.P. wishes to thank D. Lai and M. ali-Dib for useful discussions regarding the formation and orbital dynamics of this unique system, as well as S. Pelletier, P. Gupta, J. Chan, L.-P. Coulombe, P.-A. Roy, S. Delisle, and M. Papillon for useful comments on the first manuscript of this paper. We thank D. Anderson for providing us the CORALIE RVs of WASP-107. C.P. and B.B. acknowledge financial support by the Natural Sciences and Engineering Research Council (NSERC) as part of B.B.'s NSERC Discovery Grant and the NSERC CREATE Technologies for Exo-planetary Science (TEPS) program. C.P. and B.B. furthermore acknowledge funding from the Fonds de Recherche Québécois Nature et Technologies (FRQNT). R.A.R. is supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1745301. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. A.W.H. acknowledges support from the K2 Guest Observer Program and NASA's Key Strategic Mission Support program. This research was enabled in part by support provided by Calcul Québec (, Compute Canada (, and the Savio computational cluster resource provided by the Berkeley Research Computing program at the University of California, Berkeley (supported by the UC Berkeley chancellor, vice chancellor for research, and chief information officer). This work has made use of data from the European Space Agency (ESA) mission Gaia (, processed by the 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. This research has made use of NASA's Astrophysics Data System and 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. This work was based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology. 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: Astropy (Astropy Collaboration et al. 2013), bayev (, corner (Foreman-Mackey 2016), emcee (Foreman-Mackey et al. 2013), isoclassify (Huber et al. 2017), Matplotlib (Hunter 2007), Numpy/Scipy (van der Walt et al. 2011), RadVel (Fulton et al. 2018), smint (, SpecMatch-emp (Yee et al. 2017).
Group:Astronomy Department, Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Fonds de recherche du Québec – Nature et technologies (FRQNT)UNSPECIFIED
NSF Graduate Research FellowshipDGE-1745301
University of California, BerkeleyUNSPECIFIED
Gaia Multilateral AgreementUNSPECIFIED
Subject Keywords:Exoplanet formation ; Planetary structure ; Exoplanet atmospheres
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Exoplanet formation (492); Planetary structure (1256); Exoplanet atmospheres (487)
Record Number:CaltechAUTHORS:20210119-143307701
Persistent URL:
Official Citation:Caroline Piaulet et al 2021 AJ 161 70
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107560
Deposited By: George Porter
Deposited On:19 Jan 2021 23:17
Last Modified:19 Jan 2021 23:17

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