Year: 2026
Type: article
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 Rp = 2.5 ± 0.2 R⊕ and a mass of Mp = 5.5 ± 0.9 M⊕. The resulting bulk density (ρp = 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.
Authors: 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
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7160084866
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