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

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. 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. 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. We determined that TOI-4602,b is a sub-Neptune with a radius of R_p = 2.5 R_⊕ and a mass of M_p = 5.5 M_⊕. The resulting bulk density (̊ho_p = 2.1 ) 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. g cm^ -3 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.