The Hot-Neptune Initiative (HONEI)

I. Two hot sub-Neptunes on a close-in eccentric orbit (TOI-5800 b) and a farther-out circular orbit (TOI-5817 b)

¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
² Carnegie Science Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
³ Institute for Particle Physics and Astrophysics, ETH Zürich, Otto-Stern-Weg 5, 8093 Zürich, Switzerland
⁴ INAF – Osservatorio Astronomico di Roma, Monte P. Catone, Italy
⁵ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁶ La Sapienza University of Rome, Department of Physics, Piazzale Aldo Moro 2, 00185 Rome, Italy
⁷ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ Fulbright Visiting Research Scholar, Dept. of Astronomy, The University of Texas at Austin, Speedway 2515, Austin, TX, USA
⁹ Max-Planck-Institut für Astronomie, Heidelberg, Germany
¹⁰ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹¹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹² Institut d’astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
¹³ Observatoire de Haute-Provence, CNRS, Université d’Aix-Marseille, 04870 Saint-Michel-l’Observatoire, France
¹⁴ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁵ NASA Exoplanet Science Institute, IPAC, California Institute of Technology, Pasadena, CA 91125, USA
¹⁶ Centro di Ateneo di Studi e Attività Spaziali “G. Colombo” – Università di Padova, Via Venezia 15, 35131 Padova, Italy
¹⁷ Physics Department, Austin College, Sherman, TX 75090, USA
¹⁸ Observatoire Astronomique de l’Université de Genéve, Chemin Pegasi 51, Versoix 1290, Switzerland
¹⁹ DTU Space, Technical University of Denmark, Elektrovej 328, DK-2800 Kgs. Lyngby, Denmark
²⁰ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
²¹ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
²² Bay Area Environmental Research Institute, Moffett Field, CA 94035, USA
²³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁴ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
²⁵ Società Astronomica Lunae, Castelnuovo Magra, Italy
²⁶ Dept. of Physics, Engineering and Astronomy, Stephen F. Austin State University, 1936 North St, Nacogdoches, TX 75962, USA

^★ Corresponding author: luca.naponiello@inaf.it

Received: 15 May 2025
Accepted: 28 July 2025

Abstract

Context. Neptune-sized exoplanets are key targets for atmospheric studies, yet their formation and evolution remain poorly understood due to their diverse characteristics and limited sample size. The so-called Neptune desert, a region of parameter space with a dearth of short-period sub- to super-Neptunes, is a critical testbed for theories of atmospheric escape and migration.

Aims. The HONEI programme aims to confirm and characterise the best Neptune-sized candidates for composition, atmospheric, and population studies. By measuring planetary masses with high precision, we want to provide the community with optimal targets whose atmosphere can be effectively explored with the James Webb Space Telescope or by ground-based high-resolution spectroscopy.

Methods. For this purpose, we started a radial velocity follow-up campaign, using the twin high-precision spectrographs HARPS and HARPS-N to measure the masses of TESS Neptune-sized candidates and confirm their planetary nature.

Results. In this first paper of the series, we confirm the planetary nature of two candidates: TOI-5800 b and TOI-5817 b. TOI-5800 b is a hot sub-Neptune (R_p = 2.46_−0.16^+0.18 R_⊕, M_p = 9.5_−1.9^+1.7 M_⊕, ρ = 3.46_−0.90^+1.02 g cm⁻³, T_eq = 1108 ± 20 K) located at the lower edges of the Neptune desert (P = 2.628 days) and is the most eccentric planet (e ~ 0.3) ever found with P < 3 d. TOI-5800 b is expected to still be in the tidal migration phase with its parent star, a K3 V dwarf (V = 9.6 mag), although its eccentricity could arise from interactions with another object in the system. Having a high transmission spectroscopy metric (TSM = 103₋₂₂⁺³⁵), it represents a prime target for future atmospheric characterisation. TOI-5817 b is a relatively hot sub-Neptune (R_p = 3.08 ± 0.14 R_⊕, M_p = 10.3_−1.3^+1.4 M_⊕, ρ = 1.93_−0.34^+0.41 g cm⁻³, T_eq = 950₋₁₈⁺²¹ K) located in the Neptune savanna (P = 15.610 d), on a circular orbit around a bright G2 IV-V star (V = 8.7 mag). Despite a lower TSM = 56₋₉⁺¹¹, it is a potential target for atmospheric follow-up in the context of sub-Neptunes with P > 15 days. Finally, we find that if the difference in the planet densities are mainly due to different gas mass fractions, there will be an order of magnitude difference in the predicted atmospheric carbon-to-oxygen ratios, a prediction that can be tested with atmospheric follow-up observations.