The GAPS Programme at the TNG

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.