The GAPS Programme at the TNG: LXX. TOI-5734b: A hot sub-Neptune orbiting a relatively young K dwarf with an Earth-like density

TL;DR

This study confirms TOI-5734b as a young, dense, hot sub-Neptune with a near-rocky composition, measured precisely through combined photometry and radial velocity data, highlighting its potential for atmospheric characterization.

Contribution

First detailed characterization of a young sub-Neptune's mass, radius, and density using Gaussian processes to separate stellar activity from planetary signals.

Findings

Confirmed TOI-5734b as a planet with 6.18-day orbit

Measured radius of 2.10 Earth radii and mass of 9.1 Earth masses

Derived density indicating a near-rocky composition

Abstract

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. Using 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. By simultaneously modelling photometry and high-cadence RVs, we measured the radius, mass, and density of TOI-5734b precisely. In particular, we employed Gaussian processes (GPs) with a flexible kernel to discriminate between the stellar activity of the young host and planetary signals. We confirmed the planetary nature of TOI-5734b and measured its orbital period ($P_{\rm b}\sim6.18$ d), radius ($R_{\rm b} = 2.10^{+0.12}_{-0.12}$ $R_\oplus$), and mass ($M_{\rm b}=9.1^{+2.6}_{-2.6}$ $M_\oplus$). By measuring its density ($\rho_{\rm b}=0.98_{-0.30}^{+0.36}$ $\rho_\oplus$), we infer that TOI-5734b is close to having a rocky composition and an almost completely depleted primary envelope. Our results point toward the possibility of considering the target for atmospheric studies with present and future ground- and space-based facilities.