TOI-333b: A Neptune-desert planet around an F7V star

¹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
² Centro de Astrofísica y Tecnologías Afines (CATA), Casilla 36-D, Santiago, Chile
³ Instituto de Estudios Astrofísicos, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
⁴ Instituto de Astronomía, Universidad Católica del Norte, Angamos 0610, Antofagasta 1270709, Chile
⁵ Department of Physics “Ettore Pancini”, University of Naples Federico II, Naples, Italy
⁶ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
⁷ Astrobiology Research Unit, Université de Liège, Allée du 6 août 19, Liège 4000, Belgium
⁸ Astronomy Unit, Queen Mary University of London, Mile End Road, London E1 4NS, UK
⁹ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹⁰ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹¹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge MA 02138, USA
¹² Kotizarovci Observatory, Sarsoni 90, 51216 Viskovo, Croatia
¹³ Observatoire astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁴ SETI Institute, Mountain View, CA 94043 USA/NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁵ Noqsi Aerospace Ltd., 15 Blanchard Avenue, Billerica, MA 01821, USA
¹⁶ NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
¹⁷ Department of Physics and Astronomy, University of New Mexico, 210 Yale Blvd NE, Albuquerque, NM 87106, USA
¹⁸ School of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 August 2025
Accepted: 12 November 2025

Abstract

Observations have shown that planets similar to Neptune are rarely found orbiting Sun-like stars with periods up to ∼4 days. This defines the so-called Neptune desert region. The detection of each individual planet in this region therefore holds a high value by providing detailed insights into the formation and evolution of this population. We report the detection of TOI-333b, a Neptune-desert planet with a mass, radius, and bulk density of 20.1 ± 2.4 M_⊕, 4.26 ± 0.11 R_⊕, and 1.42 ± 0.21 g cm⁻³. The planet orbits an F7V star every 3.78 d, whose mass, radius, and effective temperature are of 1.2 ± 0.1 M_⊙, 1.10 ± 0.03 R_⊙, and 6241₋₆₂⁺⁷³ K, respectively. TOI-333bis likely younger than 1 Gyr, which is supported by the doublet Li line around 6707.856 Å and its comparison to Li abundances in open clusters with well-constrained ages. The planet is expected to host only a 8.5_−8.3^+10.9% gas-to-core mass ratio for an H/He envelope. On the other hand, models of irradiated ocean worlds predict a 20₋₁₀⁺¹¹% H₂O mass fraction with a core fraction of 35₋₂₃⁺²⁰%. We therefore expect that the internal composition of TOI-333bis dominated by a pure rocky composition with almost no H/He envelope, or a rocky world with almost equal mass fraction of water. Finally, TOI-333bis more massive and larger than 77% and 82% of its Neptune-desert counterparts, and its host ranks among the hottest known stars for Neptune-desert planets. This makes this system a unique laboratory for studying the evolution of these planets around hot stars.