Two Transiting Low Density Sub-Saturns from K2

Creators: Petigura, Erik A.; Howard, Andrew W.; Lopez, Eric D.; Deck, Katherine M.; Fulton, Benjamin J.; Crossfield, Ian J. M.; Ciardi, David R.; Chiang, Eugene; Lee, Eve J.; Isaacson, Howard; Beichman, Charles A.; Hansen, Brad M. S.; Schlieder, Joshua E.; Sinukoff, Evan

Abstract

We report the discovery and confirmation of K2-24 b and c, two sub-Saturn planets orbiting a bright (V = 11.3), metal-rich ([Fe/H] = 0.42 ± 0.04 dex) G3 dwarf in the K2 Campaign 2 field. The planets are 5.68 ± 0.56 R_⊕ and 7.82 ± 0.72 R_⊕ and have orbital periods of 20.8851 ± 0.0003 days and 42.3633 ± 0.0006 days, near the 2:1 mean-motion resonance. We obtained 32 radial velocities with Keck/HIRES and detected the reflex motion due to K2-24 b and c. These planets have masses of 21.0 ± 5.4 M_⊕ and 27.0 ± 6.9 M_⊕, respectively. With low densities of 0.63 ± 0.25 g cm^(−3) and 0.31 ± 0.12 g cm^(−3), respectively, the planets require thick envelopes of H/He to explain their large sizes and low masses. Interior structure models predict that the planets have fairly massive cores of 17.6 ± 4.3 M_⊕ and 16.1 ± 4.2 M_⊕, respectively. They may have formed exterior to their present locations, accreted their H/He envelopes at large orbital distances, and migrated in as a resonant pair. The proximity to resonance, large transit depths, and host star brightness offers rich opportunities for TTV follow-up. Finally, the low surface gravities of the K2-24 planets make them favorable targets for transmission spectroscopy by Hubble Space Telescope, Spitzer, and James Webb Space Telescope.

Additional Information

© 2016 American Astronomical Society. Received 2015 November 13; accepted 2015 December 26; published 2016 February 4. We thank Geoffrey Marcy, Konstantin Batygin, and Leslie Rogers for helpful discussions. We thank an anonymous referee for valuable comments. E. A. P. acknowledges support from a Hubble Fellowship grant HST-HF2-51365.001-A 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. 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. E. D. L. received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement number 313014 (ETAEARTH). B. J. F. acknowledges support from a National Science Foundation Graduate Research Fellowship under grant No. 2014184874. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This work made use of the SIMBAD database (operated at CDS, Strasbourg, France), NASA's Astrophysics Data System Bibliographic Services, and data products from the Two Micron All Sky Survey (2MASS), the APASS database, the SDSS-III project, and the Digitized Sky Survey. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. Some of the data presented herein were obtained at the W. M. Keck Observatory (which is operated as a scientific partnership among Caltech, UC, 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. Facility: Kepler - , Keck: I - (HIRES - ), Keck: II - (NIRC2 - ).

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Submitted - 1511.04497v1.pdf

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