The Hot Neptune Initiative (HONEI)

Context. The formation of Neptune planets with orbital periods shorter than ten days remains uncertain. They might have developed similarly to their longer-period counterparts, emerged from rare collisions between smaller planets, or be the remnant cores of stripped giant planets. Characterising a large number of such planets is important for advancing our understanding of how they form and evolve. Aims. We aim to confirm the planetary nature and characterise the physical and orbital properties of a close-in Neptune-type transiting exoplanet candidate revealed by TESS around the star TOI-5795 (V = 10.7 mag), 162 pc away from the Sun. Methods. We monitored TOI-5795 with the HARPS spectrograph for two months to quantify any periodic variations in its radial velocity (RV), necessary to estimate the mass of the smaller companion. We jointly analysed these RV measurements and the TESS photometry. We excluded contaminating sources as the origin of the detected signal using high-angular-resolution speckle and adaptive optical imaging. Results. We find that the parent star is a metal-poor ([Fe/H] = −0.27 ± 0.07) G3 V star (Teff = 5718 ± 50 K), with a radius of R* = 1.082 ± 0.026 R⊙, a mass of M⋆ = 0.901−0.037+0.055 M⊙ and an age of 10.2−3.3+2.5 Gyr. We confirm the planetary nature of the candidate, which can now be named TOI-5795 b. We estimate that the planet has an orbital period of Porb = 6.1406325 ± 0.0000054 days and an orbital eccentricity compatible with zero. With a mass of 23.66−4.60+4.09 M⊕, a radius of 5.62 ± 0.11 R⊕, and an equilibrium temperature of 1136 ± 18 K, it can be considered a hot super-Neptune at the edge of the so-called Neptune desert. The transmission spectroscopy metric of TOI-5795 b is ≈100, which makes it an interesting target for probing the chemical composition of its atmosphere. We simulated planet-formation processes but found almost no successful matches to the observed planet’s mass and orbit, suggesting that post-formation dynamical events may have shaped its current state. We also performed an atmospheric-evolution study of TOI-5795 b, finding that this planet likely experienced significant atmospheric stripping due to prolonged high-energy irradiation from its parent star.