A Hot Saturn Orbiting an Oscillating Late Subgiant Discovered by TESS
Authors: Huber Daniel , Chaplin William J. , Chontos Ashley , Kjeldsen Hans , Christensen-Dalsgaard Jørgen , Bedding Timothy R. , Ball Warrick , Brahm Rafael , Espinoza Nestor , Henning Thomas , Jordán Andrés , Sarkis Paula , Knudstrup Emil , Albrecht Simon , Grundahl Frank , Andersen Mads Fredslund , Pallé Pere L. , Crossfield Ian , Fulton Benjamin , Howard Andrew W. , Isaacson Howard T. , Weiss Lauren M. , Handberg Rasmus , Lund Mikkel N. , Serenelli Aldo M. , Rørsted Mosumgaard Jakob , Stokholm Amalie , Bieryla Allyson , Buchhave Lars A. , Latham David W. , Quinn Samuel N. , Gaidos Eric , Hirano Teruyuki , Ricker George R. , Vanderspek Roland K. , Seager Sara , Jenkins Jon M. , Winn Joshua N. , Antia H. M. , Appourchaux Thierry , Basu Sarbani , Bell Keaton J. , Benomar Othman , Bonanno Alfio , Buzasi Derek L. , Campante Tiago L. , Çelik Orhan Z. , Corsaro Enrico , Cunha Margarida S. , Davies Guy R. , Deheuvels Sebastien , Grunblatt Samuel K. , Hasanzadeh Amir , Di Mauro Maria Pia , A. García Rafael , Gaulme Patrick , Girardi Léo , Guzik Joyce A. , Hon Marc , Jiang Chen , Kallinger Thomas , Kawaler Steven D. , Kuszlewicz James S. , Lebreton Yveline , Li Tanda , Lucas Miles , Lundkvist Mia S. , Mann Andrew W. , Mathis Stéphane , Mathur Savita , Mazumdar Anwesh , Metcalfe Travis S. , Miglio Andrea , F. G. Monteiro Mário J. P. , Mosser Benoit , Noll Anthony , Nsamba Benard , Joel Ong Jia Mian , Örtel S. , Pereira Filipe , Ranadive Pritesh , Régulo Clara , Rodrigues Thaíse S. , Roxburgh Ian W. , Aguirre Victor Silva , Smalley Barry , Schofield Mathew , Sousa Sérgio G. , Stassun Keivan G. , Stello Dennis , Tayar Jamie , White Timothy R. , Verma Kuldeep , Vrard Mathieu , Yıldız M. , Baker David , Bazot Michaël , Beichmann Charles , Bergmann Christoph , Bugnet Lisa , Cale Bryson , Carlino Roberto , Cartwright Scott M. , Christiansen Jessie L. , Ciardi David R. , Creevey Orlagh , Dittmann Jason A. , Nascimento Jose-Dias Do , Eylen Vincent Van , Fürész Gabor , Gagné Jonathan , Gao Peter , Gazeas Kosmas , Giddens Frank , Hall Oliver J. , Hekker Saskia , Ireland Michael J. , Latouf Natasha , LeBrun Danny , Levine Alan M. , Matzko William , Natinsky Eva , Page Emma , Plavchan Peter , Mansouri-Samani Masoud , McCauliff Sean , Mullally Susan E. , Orenstein Brendan , Soto Aylin Garcia , Paegert Martin , van Saders Jennifer L. , Schnaible Chloe , Soderblom David R. , Szabó Róbert , Tanner Angelle , Tinney C. G. , Teske Johanna , Thomas Alexandra , Trampedach Regner , Wright Duncan , Yuan Thomas T. , Zohrabi Farzaneh 
Published: May 2019

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” (R
p = 9.17 ± 0.33 R
⊕) with an orbital period of ∼14.3 days, irradiance of F = 343 ± 24 F
⊕, and moderate mass (M
p = 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.