2025A&A...701A..79N

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^+0.18R_⊕, M_p = 9.5_–1.9^+1.7M_⊕, 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^+1.4M_⊕, 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.