The GAPS programme at TNG

XLIX. TOI-5398, the youngest compact multi-planet system composed of an inner sub-Neptune and an outer warm Saturn^★,★★

¹ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
e-mail: giacomo.mantovan@phd.unipd.it
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁴ INAF – Osservatorio Astronomico di Trieste, Via Giambattista Tiepolo, 11, 34131 Trieste (TS), Italy
⁵ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁷ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (Roma), Italy
⁸ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
⁹ Department of Physics, Fisk University, Nashville, TN 37208, USA
¹⁰ SUPA, School of Physics & Astronomy, University of St Andrews; Centre for Exoplanet Science, University of St Andrews, North Haugh, St Andrews KY169SS, UK
¹¹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹² Space Science and Astrobiology Division, NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
¹³ SETI Institute; USA/NASA Ames Research Center, Mountain View; Moffett Field, CA 94043; 94035, USA
¹⁴ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁵ INAF – Osservatorio Astrofisico di Catania, Oss. Astr. Catania, via S. Sofia 78, 95123 Catania, Italy
¹⁶ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
¹⁷ ESO, Karl-Schwarzschild-str. 2, 85748 Garching, Germany
¹⁸ Sternberg Astronomical Institute Lomonosov Moscow State University, Moscow, Russia
¹⁹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
²⁰ Departamento de Astrofísica, Universidad de La Laguna (ULL), Avd. Astrofisico Francisco Sánchez s/n, 38206 La Laguna, Tenerife, Spain
²¹ Astrobiology Research Unit, Université de Liège, 19C Allée du 6 Août, 4000 Liège, Belgium
²² Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
²³ Acton Sky Portal (private observatory), Acton, MA, USA
²⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
²⁵ McDonald Observatory and Center for Planetary Systems Habitability; The University of Texas, Austin Texas, USA
²⁶ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
²⁷ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁸ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
²⁹ Astronomical Institute, Czech Academy of Sciences, Fričova 298, 251 65, Ondřejov, Czech Republic
³⁰ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
³¹ Department of Astronomy, Wellesley College, Wellesley, MA 02481, USA
³² Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
³³ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³⁴ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudzia̧dzka 5, 87-100 Toruń, Poland
³⁵ Royal Astronomical Society, Burlington House, Piccadilly, London W1J 0BQ, UK
³⁶ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³⁷ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
³⁸ Dipartimento di Matematica e Fisica, Università Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
³⁹ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
⁴⁰ Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy

Received: 14 July 2023
Accepted: 25 October 2023

Abstract

Context. Short-period giant planets (P ≲ 10 days, M_p >0.1 M_J) are frequently found to be solitary compared to other classes of exo-planets. Small inner companions to giant planets with P ≲ 15 days are known only in five compact systems: WASP-47, Kepler-730, WASP-132, TOI-1130, and TOI-2000. Here, we report the confirmation of TOI-5398, the youngest known compact multi-planet system composed of a hot sub-Neptune (TOI-5398 c, P_c = 4.77271 days) orbiting interior to a short-period Saturn (TOI-5398 b, P_b = 10.590547 days) planet, both transiting around a 650 ± 150 Myr G-type star.

Aims. As part of the Global Architecture of Planetary Systems (GAPS) Young Object project, we confirmed and characterised this compact system, measuring the radius and mass of both planets, thus constraining their bulk composition.

Methods. Using multi-dimensional Gaussian processes, we simultaneously modelled stellar activity and planetary signals from the Transiting Exoplanet Survey Satellite (TESS) Sector 48 light curve and our High Accuracy Radial velocity Planet Searcher (HARPS-N) radial velocity (RV) time series. We confirmed the planetary nature of both planets, TOI-5398 b and TOI-5398 c, and obtained a precise estimation of their stellar parameters.

Results. Through the use of astrometric, photometric, and spectroscopic observations, our findings indicate that TOI-5398 is a young, active G dwarf star (650 ± 150 Myr) with a rotational period of P_rot = 7.34 days. The transit photometry and RV measurements enabled us to measure both the radius and mass of planets b, R_b = 10.30 ± 0.40 R_⊕, M_b = 58.7 ± 5.7 M_⊕, and c, R_c = 3.52 ± 0.19 R_⊕, M_c = 11.8 ± 4.8 M_⊕. TESS observed TOI-5398 during sector 48 and no further observations are planned in the current Extended Mission, making our ground-based light curves crucial for improvement of the ephemeris. With a transmission spectroscopy metric (TSM) value of around 300, TOI-5398 b is the most amenable warm giant (10 < P < 100 days) for JWST atmospheric characterisation.