TOI-1296b and TOI-1298b observed with TESS and SOPHIE: two hot Saturn-mass exoplanets with different densities around metal-rich stars^★

¹ Université de Toulouse, CNRS, IRAP, 14 avenue Belin, 31400 Toulouse, France
e-mail: claire.moutou@irap.omp.eu
² Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ Institut d’Astrophysique de Paris, CNRS, UMR 7095, Sorbonne Université, 98 bis bd Arago, 75014 Paris, France
⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ Vanderbilt University, Department of Physics & Astronomy, 6301 Stevenson Center Ln., Nashville, TN 37235, USA
⁶ Acton Sky Portal (Private Observatory), Acton, MA, USA
⁷ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁸ Aix-Marseille Université, CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille), Marseille, France
⁹ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁰ Physics Department, Austin College, Sherman, TX 75090, USA
¹¹ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹² Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹³ Grand Pra Observatory, 1984 Les Hauderes, Switzerland
¹⁴ Observatori Astronòmic Albanyà, Camì de Bassegoda S/N, Albanyà 17733, Girona, Spain
¹⁵ NASA Ames Research Center, Moffett Field, CA, 94035, USA
¹⁶ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, Meudon, France
¹⁷ SAF, Observatoire Privé du Mont, 40280, Saint-Pierre-du-Mont, France
¹⁸ NCCR/PlanetS, Centre for Space & Habitability, University of Bern, Bern, Switzerland
¹⁹ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
²⁰ SETI Institute, Mountain View, CA 94043, USA
²¹ Department of Physics, Engineering and Astronomy, Stephen F. Austin State University, 1936 North St, Nacogdoches, TX 75962, USA
²² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
²³ Department of Physics, Shahid Beheshti University, Tehran, Iran
²⁴ Laboratoire J.-L. Lagrange, Observatoire de la Côte d’Azur (OCA), Universite de Nice-Sophia Antipolis (UNS), CNRS, Campus Valrose, 06108 Nice Cedex 2, France
²⁵ Laboratório Nacional de Astrofísica, Rua Estados Unidos 154, 37504-364, Itajubá, MG, Brazil
²⁶ Observatoire de Haute Provence, 04870 Saint Michel l’Observatoire, France

Received: 22 April 2021
Accepted: 28 June 2021

Abstract

We present the discovery of two new transiting extrasolar planet candidates identified as TOI-1296.01 and TOI-1298.01 by the Transiting Exoplanet Survey Satellite (TESS). The planetary nature of these candidates has been secured with the SOPHIE high-precision spectrograph through the measurement of the companion’s mass with the radial velocity method. Both planets are similar to Saturn in mass and have similar orbital periods of a few days. They, however, show discrepant radii and therefore different densities. The radius discrepancy might be explained by the different levels of irradiation by the host stars. The subgiant star TOI-1296 hosts a low-density planet with 1.2 R_Jup while the less luminous, lower-size star TOI-1298 hosts a much denser planet with a 0.84 R_Jup radius, resulting in bulk densities of 0.198 and 0.743 g cm⁻³, respectively.In addition, both stars are strongly enriched in heavy elements, having metallicities of +0.44 and +0.49 dex, respectively. The planet masses and orbital periods are 0.298 ± 0.039 M_Jup and 3.9443715 ± 5.8 ± 10⁻⁶ days for TOI-1296b, and 0.356 ± 0.032 M_Jup and 4.537164 ± 1.2 ± 10⁻⁵ days for TOI-1298b. The mass measurements have a relative precision of better than 13%.