The GAPS Programme at TNG. LVII. TOI-5076b: A Warm Sub-Neptune Planet Orbiting a Thin-to-thick-disk Transition Star in a Wide Binary System

We report the confirmation of a new transiting exoplanet orbiting the star TOI-5076. We present our vetting procedure and follow-up observations which led to the confirmation of the exoplanet TOI-5076b. In particular, we employed high-precision TESS photometry, high-angular-resolution imaging from several telescopes, and high-precision radial velocities from HARPS-N. From the HARPS-N spectroscopy, we determined the spectroscopic parameters of the host star: T$ eff $=(5070pm 143) K, log g=(4.6pm 0.3) Fe/H =(+0.20pm 0.08), and alpha /Fe =0.05pm 0.06. The transiting planet is a warm sub-Neptune with a mass m$ 2) M$ oplus $, a radius r$ 0.1) R$ oplus $ yielding a density $ 0.5) g $. It revolves around its star approximately every 23.445 days. The host star is a metal-rich, K2V dwarf, located at about 82 pc from the Sun with a radius of R$_ star $=(0.78pm 0.01) R$_ odot $ and a mass of M$_ star $=(0.80pm 0.07) M$_ odot $. It forms a common proper motion pair with an M-dwarf companion star located at a projected separation of 2178 au. The chemical analysis of the host-star and the Galactic-space velocities indicate that TOI-5076 belongs to the old population of thin-to-thick-disk transition stars. The density of TOI-5076b suggests the presence of a large fraction by volume of volatiles overlying a massive core. We found that a circular orbit solution is marginally favored with respect to an eccentric orbit solution for TOI-5076b.