The GAPS Programme at the TNG

LXXII. TOI-5734b: A hot sub-Neptune orbiting a relatively young K dwarf with an Earth-like density^★

¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
² Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
³ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Department of Astronomy, Sofia University “St Kliment Ohridski”, 5 James Bourchier Blvd, 1164 Sofia, Bulgaria
⁶ Landessternwarte, Zentrum für Astronomie der Universtät Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁷ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁸ ESO – European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile
⁹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁰ Centre for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹¹ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja (TF), Spain
¹² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹³ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹⁴ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁵ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
¹⁶ DISAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
¹⁷ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹⁸ NASA Exoplanet Science Institute-Caltech/IPAC, Pasadena, CA 91125, USA
¹⁹ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁰ National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Avenue, Tucson, AZ 85719, USA
²¹ Center for Space and Habitability, University of Bern, Bern, Switzerland
²² Sternberg Astronomical Institute, Lomonosov Moscow State University, 119992 Universitetskii prospekt 13, Moscow, Russia
²³ Planetary Discoveries in Fredericksburg, VA 22405, USA
²⁴ NASA Goddard Space Flight Centre, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
²⁵ Società Astronomica Lunae, Castelnuovo Magra, Italy

^★★ Corresponding author. This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 13 June 2025
Accepted: 15 February 2026

Abstract

Context. Increasing interest in young exoplanets is leading to a growing effort to understand the formation and evolutionary processes responsible for their different architectures. One interesting target is TOI-5734, a relatively young K3–K4 dwarf star (500₋₁₅₀⁺³⁰⁰ Myr) showing a transiting candidate in photometric observations followed up with high-resolution spectroscopic data.

Aims. Using both Transiting Exoplanet Survey Satellite (TESS) photometry and High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) radial-velocity (RV) data, we aim to validate the presence of the companion TOI-5734b, measure its planetary mass, size, and its orbital parameters after having characterised its host star. We then aim to study its possible planetary composition and atmospheric evolution.

Methods. By simultaneously modelling photometry and high-cadence RVs, we measured the radius, mass, and density of TOI-5734b precisely, which were needed in order to reconstruct the evolution of its atmosphere under the high-energy irradiation of its young host. In particular, we employed Gaussian processes (GPs) with a flexible kernel to discriminate between the stellar activity of the young host and planetary signals.

Results. We confirmed the planetary nature of TOI-5734b and measured its orbital period (P_b ~ 6.18 d), radius (R_b = 2.10_−0.12^+0.12 R_⊕), and mass (M_b = 9.1_−2.6^+2.6 M_⊕). By measuring its density (ρ_b = 0.98_−0.30^+0.36 ρ_⊕) and running atmospheric evolution modelling, we infer that TOI-5734b is close to having a rocky composition and an almost completely depleted primary envelope. We also modelled stellar activity, which points to a rotational period of the star of P_rot = 11.09_−0.08^+0.07 d; this is compatible with its young age. Our results point toward the possibility of considering the target for atmospheric studies with present and future ground- and space-based facilities.