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K2-19b and c are in a 3:2 Commensurability but out of Resonance: A Challenge to Planet Assembly by Convergent Migration

Petigura, Erik A. and Livingston, John and Batygin, Konstantin and Mills, Sean M. and Werner, Michael and Isaacson, Howard and Fulton, Benjamin J. and Howard, Andrew W. and Weiss, Lauren M. and Espinoza, Néstor and Jontof-Hutter, Daniel and Shporer, Avi and Bayliss, Daniel and Barros, S. C. C. (2020) K2-19b and c are in a 3:2 Commensurability but out of Resonance: A Challenge to Planet Assembly by Convergent Migration. Astronomical Journal, 159 (1). Art. No. 2. ISSN 1538-3881.

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K2-19b and c were among the first planets discovered by NASA's K2 mission and together stand in stark contrast with the physical and orbital properties of the solar system planets. The planets are between the size of Uranus and Saturn at 7.0 ± 0.2 R⊕ and 4.1 ± 0.2 R⊕, respectively, and reside a mere 0.1% outside the nominal 3:2 mean-motion resonance. They represent a different outcome of the planet formation process than the solar system, as well as the vast majority of known exoplanets. We measured the physical and orbital properties of these planets using photometry from K2, Spitzer, and ground-based telescopes, along with radial velocities from Keck/HIRES. Through a joint photodynamical model, we found that the planets have moderate eccentricities of e ≈ 0.20 and well-aligned apsides Δϖ ≈ 0°. The planets occupy a strictly nonresonant configuration: the resonant angles circulate rather than librate. This defies the predictions of standard formation pathways that invoke convergent or divergent migration, both of which predict Δ ≈ 180° and eccentricities of a few percent or less. We measured masses of M_(p,b) = 32.4 ± 1.7 M⊕ and M_(p,c) = 10.8 ± 0.6 M⊕. Our measurements, with 5% fractional uncertainties, are among the most precise of any sub-Jovian exoplanet. Mass and size reflect a planet's core/envelope structure. Despite having a relatively massive core of M_(core) ≈ 15 M⊕, K2-19b is envelope-rich, with an envelope mass fraction of roughly 50%. This planet poses a challenge to standard models of core-nucleated accretion, which predict that cores ≳10 M⊕ will quickly accrete gas and trigger runaway accretion when the envelope mass exceeds that of the core.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Petigura, Erik A.0000-0003-0967-2893
Livingston, John0000-0002-4881-3620
Batygin, Konstantin0000-0002-7094-7908
Mills, Sean M.0000-0002-4535-6241
Isaacson, Howard0000-0002-0531-1073
Fulton, Benjamin J.0000-0003-3504-5316
Howard, Andrew W.0000-0001-8638-0320
Weiss, Lauren M.0000-0002-3725-3058
Espinoza, Néstor0000-0001-9513-1449
Jontof-Hutter, Daniel0000-0002-6227-7510
Shporer, Avi0000-0002-1836-3120
Bayliss, Daniel0000-0001-6023-1335
Barros, S. C. C.0000-0003-2434-3625
Additional Information:© 2019 The American Astronomical Society. Received 2019 April 6; revised 2019 October 10; accepted 2019 October 24; published 2019 December 10. We thank the anonymous reviewer for helpful suggestions that improved this manuscript. E.A.P. acknowledges support for this work by NASA through the NASA Hubble Fellowship grant HST-HF2-51417 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. S.C.C.B. acknowledges support from Fundação para a Ciência e a Tecnologia (FCT) through Investigador FCT contract IF/01312/2014/CP1215/CT0004 and by FEDER through COMPETE2020 and POCI in the framework of the project POCI-01-0145-FEDER-028953. This work made use of NASA's Astrophysics Data System Bibliographic Services. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. This work makes use of observations from the LCOGT network. Some of the data presented herein were obtained 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. 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: Kepler - The Kepler Mission, Spitzer - , Keck:I (HIRES) - , LCOGT. - Software: batman (Kreidberg 2015), emcee (Foreman-Mackey et al. 2013), Phodymm (Mills et al. 2016), REBOUND (Rein & Liu 2012), EVEREST2.0 (Luger et al. 2018), RadVel (Fulton et al. 2018).
Group:Infrared Processing and Analysis Center (IPAC), Astronomy Department
Funding AgencyGrant Number
NASA Hubble FellowshipHST-HF2-51417
Fundação para a Ciência e a Tecnologia (FCT)IF/01312/2014/CP1215/CT0004
Fondo Europeo de Desarrollo Regional (FEDER)COMPETE2020
Fundação para a Ciência e a Tecnologia (FCT)POCI-01-0145-FEDER-028953
Subject Keywords:Exoplanet astronomy; Exoplanet dynamics; Exoplanet formation; Exoplanet evolution; Extrasolar gas giants; Exoplanet structure; Radial velocity
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Exoplanet astronomy (486); Exoplanet dynamics (490); Exoplanet formation (492); Exoplanet evolution (491); Extrasolar gas giants (509); Exoplanet structure (495); Radial velocity (1332)
Record Number:CaltechAUTHORS:20191120-101536409
Persistent URL:
Official Citation:Erik A. Petigura et al 2020 AJ 159 2
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99952
Deposited By: Tony Diaz
Deposited On:20 Nov 2019 18:33
Last Modified:09 Mar 2020 13:18

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