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K2-97b: A (Re-?)Inflated Planet Orbiting a Red Giant Star

Grunblatt, Samuel K. and Huber, Daniel and Gaidos, Eric J. and Lopez, Eric D. and Fulton, Benjamin J. and Vanderburg, Andrew and Barclay, Thomas and Fortney, Jonathan J. and Howard, Andrew W. and Isaacson, Howard T. and Mann, Andrew W. and Petigura, Erik A. and Silva Aguirre, Victor and Sinukoff, Evan J. (2016) K2-97b: A (Re-?)Inflated Planet Orbiting a Red Giant Star. Astronomical Journal, 152 (6). Art. No. 185. ISSN 0004-6256.

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Strongly irradiated giant planets are observed to have radii larger than thermal evolution models predict. Although these inflated planets have been known for over 15 years, it is unclear whether their inflation is caused by the deposition of energy from the host star or the inhibited cooling of the planet. These processes can be distinguished if the planet becomes highly irradiated only when the host star evolves onto the red giant branch. We report the discovery of K2-97b, a 1.31 ± 0.11 R_J, 1.10 ± 0.11 M_J planet orbiting a 4.20 ± 0.14 R⊙, 1.16 ± 0.12 M⊙ red giant star with an orbital period of 8.4 days. We precisely constrained stellar and planetary parameters by combining asteroseismology, spectroscopy, and granulation noise modeling along with transit and radial velocity measurements. The uncertainty in planet radius is dominated by systematic differences in transit depth, which we measure to be up to 30% between different light-curve reduction methods. Our calculations indicate the incident flux on this planet was 170^(+140)_(-60) times the incident flux on Earth, while the star was on the main sequence. Previous studies suggest that this incident flux is insufficient to delay planetary cooling enough to explain the present planet radius. This system thus provides the first evidence that planets may be inflated directly by incident stellar radiation rather than by delayed loss of heat from formation. Further studies of planets around red giant branch stars will confirm or contradict this hypothesis and may reveal a new class of re-inflated planets.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Grunblatt, Samuel K.0000-0003-4976-9980
Huber, Daniel0000-0001-8832-4488
Gaidos, Eric J.0000-0002-5258-6846
Fulton, Benjamin J.0000-0003-3504-5316
Vanderburg, Andrew0000-0001-7246-5438
Barclay, Thomas0000-0001-7139-2724
Fortney, Jonathan J.0000-0002-9843-4354
Howard, Andrew W.0000-0001-8638-0320
Isaacson, Howard T.0000-0002-0531-1073
Mann, Andrew W.0000-0003-3654-1602
Petigura, Erik A.0000-0003-0967-2893
Sinukoff, Evan J.0000-0002-5658-0601
Alternate Title:EPIC 211351816.01: A (Re-?)Inflated Planet Orbiting a Red Giant Star
Additional Information:© 2016 The American Astronomical Society. Received 2016 June 18; revised 2016 September 8; accepted 2016 September 16; published 2016 November 23. The authors would like to thank Jeffrey C. Smith, Suzanne Aigrain, and Travis Berger for helpful discussions. This research was supported by NASA Origins of Solar Systems grant NNX11AC33G to E.G. and by the NASA K2 Guest Observer Award NNX16AH45G to D.H. D.H. acknowledges support by the Australian Research Council's Discovery Projects funding scheme (project number DE140101364) and support by the National Aeronautics and Space Administration under Grant NNX14AB92G issued through the Kepler Participating Scientist Program. This research has made use of the Exoplanet Orbit Database and the Exoplanet Data Explorer at This work was based on observations at the W. M. Keck Observatory granted by the University of Hawaii, the University of California, and the California Institute of Technology. We thank the observers who contributed to the measurements reported here and acknowledge the efforts of the Keck Observatory staff. We extend special thanks to those of Hawaiian ancestry on whose sacred mountain of Mauna Kea we are privileged to be guests. SNIFS on the UH 2.2 m telescope is part of the Nearby Supernova Factory project, a scientific collaboration among the Centre de Recherche Astronomique de Lyon, Institut de Physique Nuclaire de Lyon, Laboratoire de Physique Nuclaire et des Hautes Energies, Lawrence Berkeley National Laboratory, Yale University, University of Bonn, Max Planck Institute for Astrophysics, Tsinghua Center for Astrophysics, and the Centre de Physique des Particules de Marseille. Based on data from the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative Agreement no. NNX-08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. Some/all of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. 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. This research has made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research made use of the SIMBAD and VIZIER Astronomical Databases, operated at CDS, Strasbourg, France (, and of NASAs Astrophysics Data System, of the Jean-Marie Mariotti Center Search service (, co-developed by FIZEAU and LAOG/IPAG. E.D.L. received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement number 313014 (ETAEARTH). B.J.F. notes that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship under grant No. 2014184874. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
Funding AgencyGrant Number
Australian Research CouncilDE140101364
European Research Council (ERC)313014 (ETAEARTH)
NSF Graduate Research Fellowship2014184874
Subject Keywords:asteroseismology – planets and satellites: detection – planets and satellites: gaseous planets – planets and satellites: physical evolution – planet–star interactions
Issue or Number:6
Record Number:CaltechAUTHORS:20161123-112352581
Persistent URL:
Official Citation:Samuel K. Grunblatt et al 2016 AJ 152 185
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:72279
Deposited By: Tony Diaz
Deposited On:23 Nov 2016 19:53
Last Modified:29 Nov 2019 02:58

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