The Hot-Neptune Initiative (HONEI)

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.

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Publication Details

Journal
Astronomy and Astrophysics
Published
2025-08-04
DOI
https://doi.org/10.1051/0004-6361/202555523
Citations
4
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
4.62

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article

The Hot-Neptune Initiative (HONEI)

G. Hébrard, D.R. Ciardi, L. Mancini, T. Forveille et al.
4 citations
Astronomy and Astrophysics
Stellar, planetary, and galactic studies
4.62
article

The Hot-Neptune Initiative (HONEI)

G. Hébrard, D.R. Ciardi, L. Mancini, T. Forveille, Shreyas Vissapragada, C. Dorn, Cristilyn N. Watkins, L. Naponiello, A. F. Lanza, M. Damasso, A. S. Bonomo, K.A. Collins, T. Zingales, Samuel W. Yee, A. Sozzetti, N. Heidari, K. Biazzo, V. Bourrier, Lars A. Buchhave, Roberto Zambelli, G. Mantovan, Colin Littlefield, J. Lissauer, Florent Manni, Catherine A. Clark, C. Ziegler, David Baker, M.B. Lund, A. Fiorenzano, R. Cosentino, M. López-Morales, S.B. Howell, D.W. Latham, V. D’Orazi, L. Malavolta, Marie-Luise Steinmeyer, S. Filomeno, M. Pinamonti, X. Dumusque, D. Nardiello
article en
4 citations

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.

Astronomy and AstrophysicsVol. 701
University of Geneva (CH), University of Rome Tor Vergata (IT), Austin College (US), California Institute of Technology (US), Ames Research Center (US), Centre National de la Recherche Scientifique (FR), Space Telescope Science Institute (US), University of Padua (IT), Stephen F. Austin State University (US), Aix-Marseille Université (FR), NASA Exoplanet Science Institute (US), Astronomical Observatory of Rome (IT), Observatoire de Haute-Provence (FR), Institut d'Astrophysique de Paris (FR), ETH Zurich (CH), Osservatorio Astrofisico di Torino (IT), Max Planck Institute for Astronomy (DE), Bay Area Environmental Research Institute (US), Université Bourgogne Franche-Comté (FR), Osservatorio Astrofisico di Catania (IT), Center for Astrophysics Harvard & Smithsonian (US), Carnegie Observatories (US), Osservatorio Astronomico di Padova (IT), Telescopio Nazionale Galileo (ES), Sapienza University of Rome (IT), The University of Texas at Austin (US), National Institute for Astrophysics (IT), Université Grenoble Alpes (FR), Technical University of Denmark (DK)
National Science Foundation, National Aeronautics and Space Administration, California Institute of Technology, Space Telescope Science Institute, European Space Agency, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Centre National d’Etudes Spatiales, Ministry of Science, ICT and Future Planning, Korea Astronomy and Space Science Institute, European Southern Observatory, Centre National de la Recherche Scientifique, Istituto Nazionale di Astrofisica, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Agencia Nacional de Investigación y Desarrollo, Ames Research Center, Jet Propulsion Laboratory, Science Mission Directorate
Openalex Percentile: Top 5%
Stellar, planetary, and galactic studies
4.62
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