The GAPS Programme at TNG

LXXIII. Confirmation of the hot sub-Neptune TOI-4602 b (HD 25295 b), a key target for future atmospheric characterization^★

¹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy
² ESO – European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile
³ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monte Porzio Catone (RM), Italy
⁴ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁵ Institute for Particle Physics and Astrophysics, ETH Zürich, Otto-Stern-Weg 5, 8093 Zürich, Switzerland
⁶ Space Research Institute, Austrian Academy of Sciences, Schmiedl-strasse 6, 8042, Graz, Austria
⁷ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁸ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁹ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via Archirafi 36, Palermo, Italy
¹⁰ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹¹ Centro di Ateneo di Studi e Attività Spaziali “G. Colombo” – Università di Padova, Via Venezia 15, 35131 Padova, Italy
¹² Dipartimento di Matematica e Fisica, Università Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
¹³ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja (TF), Spain
¹⁴ Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁵ Center for Astrophysics, Harvard & Smithsonian, Cambridge, MA 02138, USA

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

Received: 10 February 2026
Accepted: 10 April 2026

Abstract

Context. Precise mass and radius measurements of small, transitional exoplanets, such as super-Earths and sub-Neptunes, are essential to constrain their bulk density and formation history, serving as prerequisites for atmospheric characterization.

Aims. The ArMS Large Programme, carried out within GAPS using the HARPS-N spectrograph at the Telescopio Nazionale Galileo, aims to confirm and characterize transitional planets in the radius valley through high-precision radial-velocity (RV) measurements. The ultimate goal is to identify ideal targets for atmospheric follow-up observations with next-generation facilities such as the James Webb Space Telescope and the future ESA Ariel satellite. We present the first mass determination of a sub-Neptune planet using data entirely collected within the ArMS programme, focusing on the validated planet TOI-4602 b.

Methods. We monitored TOI-4602, which hosts a close-in validated sub-Neptune (P ∼ 3.98 d) detected by the Transiting Exoplanet Survey Satellite (TESS), searching for planet-induced RV variations. We then performed a joint analysis of these RV measurements together with the TESS photometric data.

Results. We determined that TOI-4602 b is a sub-Neptune with a radius of R_p = 2.5 ± 0.2 R_⊕ and a mass of M_p = 5.5 ± 0.9 M_⊕. The resulting bulk density (ρ_p = 2.1 ± 0.6 g cm⁻³) and atmospheric evolution modelling suggest the planet is retaining a tenuous envelope while evolving towards a bare core, consistent with a position immediately above the radius valley.

Conclusions. Given its bright (V = 8.4) and quiet host star and the high transmission spectroscopy metric (TSM) value (140 ± 54), TOI-4602 b is a prime target for atmospheric characterization. Simulated retrievals indicate that JWST and Ariel can effectively constrain its atmospheric composition, offering a unique window into the physical processes driving the sub-Neptune to super-Earth transition.