K2-66b and K2-106b: Two Extremely Hot Sub-Neptune-size Planets with High Densities

Creators: Sinukoff, Evan; Howard, Andrew W.; Petigura, Erik A.; Fulton, Benjamin J.; Crossfield, Ian J. M.; Isaacson, Howard; Gonzales, Erica; Crepp, Justin R.; Brewer, John M.; Hirsch, Lea; Weiss, Lauren M.; Ciardi, David R.; Schlieder, Joshua E.; Benneke, Bjoern; Christiansen, Jessie L.; Dressing, Courtney D.; Hansen, Brad M. S.; Knutson, Heather A.; Kosiarek, Molly; Livingston, John H.; Greene, Thomas P.; Rogers, Leslie A.; Lépine, Sébastien

Abstract

We report precise mass and density measurements of two extremely hot sub-Neptune-size planets from the K2 mission using radial velocities, K2 photometry, and adaptive optics imaging. K2-66 harbors a close-in sub-Neptune-sized (2.49_(-0.24)^(+0.34)R_⊕) planet (K2-66b) with a mass of 21.3 ± 3.6 M_⊕. Because the star is evolving up the subgiant branch, K2-66b receives a high level of irradiation, roughly twice the main-sequence value. K2-66b may reside within the so-called "photoevaporation desert," a domain of planet size and incident flux that is almost completely devoid of planets. Its mass and radius imply that K2-66b has, at most, a meager envelope fraction (<5%) and perhaps no envelope at all, making it one of the largest planets without a significant envelope. K2-106 hosts an ultra-short-period planet (P = 13.7 hr) that is one of the hottest sub-Neptune-size planets discovered to date. Its radius (1.82_(-0.14)^(+0.20) R_⊕) and mass (9.0 ± 1.6 M_⊕) are consistent with a rocky composition, as are all other small ultra-short-period planets with well-measured masses. K2-106 also hosts a larger, longer-period planet (R_p = 2.77_(-0.23^(+0.37)R_⊕, P = 13.3 days) with a mass less than 24.4 M_⊕ at 99.7% confidence. K2-66b and K2-106b probe planetary physics in extreme radiation environments. Their high densities reflect the challenge of retaining a substantial gas envelope in such extreme environments.

Additional Information

© 2017 American Astronomical Society. Received 2017 March 14. Accepted 2017 May 8. Published 2017 June 1. We thank the many observers who contributed to the measurements reported here. We gratefully acknowledge the efforts and dedication of the Keck Observatory staff. We thank Tabetha Boyajian for helpful discussions. This paper includes data collected by the K2 mission. Funding for the K2 mission is provided by the NASA Science Mission directorate. E.S. is supported by a postgraduate scholarship from the Natural Sciences and Engineering Research Council of Canada. E.A.P. acknowledges support by NASA through a Hubble Fellowship grant 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. B.J.F. was supported by the National Science Foundation Graduate Research Fellowship under grant no. 2014184874. A.W.H. acknowledges support for our K2 team through a NASA Astrophysics Data Analysis Program grant. A.W.H. and I.J.M.C. acknowledge support from the K2 Guest Observer Program. L.M.W. acknowledges the Trottier Family Foundation for their generous support. J.R.C. acknowledges support from the Kepler Participating Scientist program (NNX14AB85G). 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. 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. Finally, the authors extend special thanks to those of Hawai'ian ancestry on whose sacred mountain of Maunakea we are privileged to be guests. Without their generous hospitality, the Keck observations presented herein would not have been possible. Facilities: Kepler, Keck-HIRES.

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