Alves, Douglas R.; Jenkins, James S.; Vines, José I.; Moyano, Maximilano; Anderson, David R.; Magliano, Christian; Covone, G.; Stassun, Keivan Guadalupe; Soubkiou, Abderahmane; Gillen, Edward C. P.; Battley, Matthew P.; Hughes, Alexander; Armstrong, David J.; Saha, Suman; Hawthorn, Faith; Wheatley, Peter J.; Collins, Karen A.; Schwarz, Richard P.; Srdoč, Gregor; Apergis, Ioannis; Zivave, Tafadzwa; Lendl, Monika A.; Tofflemire, Benjamin M.; Doty, John P.; Hedges, Christina L.; Mireles, Ismael; Burleigh, Matthew R.; Kendall, Alicia; Harvey, George T.; Goad, Michael R.; Casewell, Sarah L.; & Edkins, Troy. (2026). TOI-333b: A Neptune-desert planet around an F7V star. Astronomy & Astrophysics, 705, A210. https://doi.org/10.1051/0004-6361/202557014
Astronomers have observed that planets similar in size to Neptune are rarely found orbiting Sun-like stars with very short orbital periods of about four days or less. This region is known as the Neptune desert. Because such planets are uncommon, each new discovery provides important clues about how these planets form and evolve.
We report the discovery of TOI-333b, a planet located in the Neptune desert. It has a mass of about 20 times that of Earth (20.1 ± 2.4 Earth masses), a radius about 4.3 times Earth’s, and a bulk density of 1.42 g/cm³. The planet orbits an F7V-type star every 3.78 days. Its host star is slightly more massive and hotter than the Sun, with a mass of 1.2 solar masses and an effective temperature of about 6240 K. The system is likely younger than 1 billion years, based on the strength of the lithium absorption line near 6708 angstroms, which is commonly used as an age indicator in stars.
Models suggest that TOI-333b likely has a relatively small hydrogen and helium (H/He) gas envelope, making up only about 8 to 19 percent of its total mass. Other models, such as those for irradiated ocean worlds, suggest it could instead contain a significant amount of water, with about 20 percent of its mass in H2O and a rocky core making up roughly one third of the planet. Overall, the planet is likely dominated either by a mostly rocky interior with very little gas or by a rocky world with a large water component.
Compared with other known planets in the Neptune desert, TOI-333b is more massive than about 77 percent of them and larger than about 82 percent. Its host star is also among the hottest known for planets in this region. Because of these properties, the TOI-333 system provides a valuable opportunity to study how Neptune-sized planets evolve in close orbits around hot stars.
Fig 1: Left : TESS-detrended light curve phase-folded to the best-fitting period listed in Table 2 and zoomed to show the transit event. The blue and white circles correspond to modelled photometric data and binned data with the associated photon noise error. The blue line and shaded region show the median transit model and its 1σ confidence interval. Centre : same as the left panel for the LCOGT-SAAO telescope. Right : same as the left panel for the NGTS mission. Bottom : Residuals of the best-fit model.