A hot Saturn orbiting an oscillating late subgiant discovered by TESS

Daniel Huber, William J. Chaplin, Ashley Chontos, Hans Kjeldsen, Joergen Christensen-Dalsgaard, Timothy R. Bedding, Warrick Ball, Rafael Brahm, Nestor Espinoza, Thomas Henning, Andres Jordan, Paula Sarkis, Emil Knudstrup, Simon Albrecht, Frank Grundahl, Mads Fredslund Andersen, Pere L. Palle, Ian Crossfield, Benjamin Fulton, Andrew W. HowardHoward T. Isaacson, Lauren M. Weiss, Rasmus Handberg, Mikkel N. Lund, Aldo M. Serenelli, Jakob Mosumgaard, Amalie Stokholm, Allyson Bierlya, Lars A. Buchhave, David W. Latham, Samuel N. Quinn, Eric Gaidos, Teruyuki Hirano, George R. Ricker, Roland K. Vanderspek, Sara Seager, Jon M. Jenkins, Joshua N. Winn, H. M. Antia, Thierry Appourchaux, Sarbani Basu, Keaton J. Bell, Othman Benomar, Alfio Bonanno, Derek L. Buzasi, Tiago L. Campante, Z. Celik Orhan, Enrico Corsaro, Margarida S. Cunha, Guy R. Davies, Sebastien Deheuvels, Samuel K. Grunblatt, Amir Hasanzadeh, Maria Pia Di Mauro, Rafael A. Garcia, Patrick Gaulme, Leo Girardi, Joyce A. Guzik, Marc Hon, Chen Jiang, Thomas Kallinger, Steven D. Kawaler, James S. Kuszlewicz, Yveline Lebreton, Tanda Li, Miles Lucas, Mia S. Lundkvist, Stephane Mathis, Savita Mathur, Anwesh Mazumdar, Travis S. Metcalfe, Andrea Miglio, Mario J. Monteiro, Benoit Mosser, Anthony Noll, Benard Nsamba, Andrew W. Mann, Jia Mian Joel Ong, S. Ortel, Filipe Pereira, Pritesh Ranadive, Clara Regulo, Thaise S. Rodrigues, Ian W. Roxburgh, Victor Silva Aguirre, Barry Smalley, Mathew Schofield, Sergio G. Sousa, Keivan G. Stassun, Dennis Stello, Jamie Tayar, Timothy R. White, Kuldeep Verma, Mathieu Vrard, M. Yildiz, David Baker, Michael Bazot, Charles Beichmann, Christoph Bergmann, Lisa Bugnet, Bryson Cale, Roberto Carlino, Scott M. Cartwright, Jessie L. Christiansen, David R. Ciardi, Orlagh Creevey, Jason A. Dittmann, Jose Dias Do Nascimento, Vincent van Eylen, Gabor Furesz, Jonathan Gagne, Peter Gao, Kosmas Gazeas, Frank Giddens, Oliver Hall, Saskia Hekker, Michael J. Ireland, Natasha Latouf, Danny LeBrun, Alan M Levine, William Matzko, Eva Natinsky, Emma Page, Peter Plavchan, Masoud Mansouri-Samani, Sean McCauliff, Susan E Mullally, Brendan Orenstein, Aylin Soto, Martin Paegert, Jennifer L. van Saders, Chloe Schnaible, David R. Soderblom, Robert Szabo, Angelle Tanner, C. G. Tinney, Johanna Teske, Alexandra Thomas, Regner Trampedach, Duncan Wright, Thomas T. Yuan, Farzaneh Zohrabi

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We present the discovery of HD 221416 b, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. HD 221416 b (HIP 116158, TOI-197) is a bright (V = 8.2 mag), spectroscopically classified subgiant that oscillates with an average frequency of about 430 μHz and displays a clear signature of mixed modes. The oscillation amplitude confirms that the redder TESS bandpass compared to Kepler has a small effect on the oscillations, supporting the expected yield of thousands of solar-like oscillators with TESS 2 minute cadence observations. Asteroseismic modeling yields a robust determination of the host star radius (R* = 2.943 ± 0.064 R), mass (M* = 1.212 ± 0.074 M), and age (4.9 ± 1.1 Gyr), and demonstrates that it has just started ascending the red-giant branch. Combining asteroseismology with transit modeling and radial-velocity observations, we show that the planet is a "hot Saturn" (Rp = 9.17 ± 0.33 R) with an orbital period of ~14.3 days, irradiance of F = 343 ± 24 F, and moderate mass (Mp = 60.5 ± 5.7 M) and density (ρ p = 0.431 ± 0.062 g cm−3). The properties of HD 221416 b show that the host-star metallicity–planet mass correlation found in sub-Saturns (4–8 R) does not extend to larger radii, indicating that planets in the transition between sub-Saturns and Jupiters follow a relatively narrow range of densities. With a density measured to ~15%, HD 221416 b is one of the best characterized Saturn-size planets to date, augmenting the small number of known transiting planets around evolved stars and demonstrating the power of TESS to characterize exoplanets and their host stars using asteroseismology.
Original languageEnglish
Article number245
Number of pages14
JournalThe Astronomical Journal
Issue number6
Publication statusPublished - 30 May 2019

Bibliographical note

12 pages (excluding author list and references), 9 figures, 4 tables, accepted for publication in AJ. An electronic version of Table 3 is available as an ancillary file (sidebar on the right)


  • astro-ph.EP
  • astro-ph.SR
  • asteroseismology
  • techniques: photometric
  • planets and satellites: individual (HD 221416 b)
  • stars: fundamental parameters
  • planets and satellites: fundamental parameters


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