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

C. Di Maio; S. Benatti; D. Locci; R. Spinelli; M. Baratella; K. Biazzo; J. Maldonado; A. F. Lanza; C. Dorn; P. E. Cubillos; A. Salmi; A. Maggio; L. Naponiello; F. Marzari; G. Micela; V. Fardella; L. Malavolta; M. Damasso; A. Sozzetti; G. Mantovan; D. Nardiello; I. Carleo; R. Claudi; R. Cosentino; M. Gonzalez; D. Muthukrishna; M. Pinamonti; T. Zingales

arXiv · 2604.11728

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

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Abstract

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 like 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-4602b. 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-4602b is a sub-Neptune with a radius of Rp = 2.5 Rearth and a mass of Mp = 5.5 Mearth. The resulting bulk density (rho_p = 2.1 ) and atmospheric evolution modelling suggest the planet is retaining a tenuous envelope while evolving toward 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.

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C. Di Maio, S. Benatti, D. Locci, R. Spinelli, M. Baratella, K. Biazzo, J. Maldonado, A. F. Lanza, C. Dorn, P. E. Cubillos, A. Salmi, A. Maggio, L. Naponiello, F. Marzari, G. Micela, V. Fardella, L. Malavolta, M. Damasso, A. Sozzetti, G. Mantovan, D. Nardiello, I. Carleo, R. Claudi, R. Cosentino, M. Gonzalez, D. Muthukrishna, M. Pinamonti, T. Zingales. 2026-04-13. 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. https://arxiv.org/abs/2604.11728

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