The Hot Neptune Initiative (HONEI)

II. TOI-5795 b: A hot super-Neptune orbiting a metal-poor star

¹ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy
² INAF – Turin Astrophysical Observatory, via Osservatorio 20, 10025 Pino Torinese, Italy
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁶ INAF – Padova Astronomical Observatory, Vicolo dell’Osservatorio 5, Padova 35122, Italy
⁷ Department of Physics and Astronomy “Galileo Galilei”, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁸ NASA Exoplanet Science Institute-Caltech/IPAC, Pasadena, CA 91125, USA
⁹ Department of Physics, Engineering and Astronomy, S. F. Austin State University, 1936 North St, Nacogdoches, TX 75962, USA
¹⁰ Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
¹¹ Department of Physics and Astronomy, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA
¹² Brierfield Observatory, Bowral, NSW 2576, Australia
¹³ INAF – Palermo Astronomical Observatory, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁴ Carnegie Science Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
¹⁵ Centro di Ateneo di Studi e Attività Spaziali “G. Colombo” – Università degli Studi di Padova, Via Venezia 15, 35131 Padova, Italy
¹⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

^★ Corresponding author: francesca.manni@inaf.it

Received: 24 June 2025
Accepted: 26 July 2025

Abstract

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 (T_eff = 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 P_orb = 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.