Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. III. A High Mass and Low Envelope Fraction for the Warm Neptune K2-55b

Creators: Dressing, Courtney D.; Sinukoff, Evan; Fulton, Benjamin J.; Lopez, Eric D.; Beichman, Charles A.; Howard, Andrew W.; Knutson, Heather A.; Werner, Michael; Benneke, Björn; Crossfield, Ian J. M.; Isaacson, Howard; Krick, Jessica; Gorjian, Varoujan; Livingston, John; Petigura, Erik A.; Schlieder, Joshua E.; Akeson, Rachel L.; Batygin, Konstantin; Christiansen, Jessie L.; Ciardi, David R.; Crepp, Justin R.; Gonzales, Erica J.; Hardegree-Ullman, Kevin K.; Hirsch, Lea A.; Kosiarek, Molly; Weiss, Lauren M.

Abstract

K2-55b is a Neptune-sized planet orbiting a K7 dwarf with a radius of , a mass of 0.688 ± 0.069 M⊙, and an effective temperature of 4300^(+107)_(-100) K. Having characterized the host star using near-infrared spectra obtained at IRTF/SpeX, we observed a transit of K2-55b with Spitzer/Infrared Array Camera (IRAC) and confirmed the accuracy of the original K2 ephemeris for future follow-up transit observations. Performing a joint fit to the Spitzer/IRAC and K2 photometry, we found a planet radius of 4.41^(+0.32)_(-0.28) R⊕, an orbital period of 2.84927265^(+6.87 x 10^(-6))_(-6.42 x 10^(-6)) days, and an equilibrium temperature of roughly 900 K. We then measured the planet mass by acquiring 12 radial velocity (RV) measurements of the system using the High Resolution Echelle Spectrometer on the 10 m Keck I Telescope. Our RV data set precisely constrains the mass of K2-55b to 43.13^(+5.98)_(-5.80) M⊕, indicating that K2-55b has a bulk density of 2.8^(+0.8)_(-0.6) g cm^(−3) and can be modeled as a rocky planet capped by a modest H/He envelope (M_(envelope) = 12 ± 3% M_p ). K2-55b is denser than most similarly sized planets, raising the question of whether the high planetary bulk density of K2-55b could be attributed to the high metallicity of K2-55. The absence of a substantial volatile envelope despite the high mass of K2-55b poses a challenge to current theories of gas giant formation. We posit that K2-55b may have escaped runaway accretion by migration, late formation, or inefficient core accretion, or that K2-55b was stripped of its envelope by a late giant impact.

Additional Information

© 2018 The American Astronomical Society. Received 2018 April 13; revised 2018 June 15; accepted 2018 June 25; published 2018 July 27. 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 the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. C.D.D., A.W.H., and I.J.M.C. acknowledge support from the K2 Guest Observer Program. A.W.H. acknowledges support for our K2 team through a NASA Astrophysics Data Analysis Program grant and observing support from NASA at Keck Observatory. E.A.P. acknowledges support from Hubble Fellowship grant. We thank the anonymous referee for providing helpful comments that improved the quality of the paper. This paper includes data collected by the K2 mission, which is funded by the NASA Science Mission directorate. The W.M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. 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. 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. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. 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: IRTF (SpeX) - , Keck:I (HIRES) - , Spitzer (IRAC). - Software: emcee (Foreman-Mackey et al. 2013), ExoTransmit (Kempton et al. 2017), RadVel (Fulton et al. 2018).

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