TOI-333b: A Neptune Desert planet around a F7V star

Observations have shown that planets similar to Neptune are rarely found orbiting Sun-like stars with periods up to ~4 days, defining the so-called Neptune desert region. Therefore, the detection of each individual planet in this region holds a high value, providing detailed insights into how such a population came to form and evolve. Here we report the detection of TOI-333b, a Neptune desert planet with a mass, radius, and bulk density of 20.1 $\pm$ 2.4 M${\oplus}$, 4.26 $\pm$ 0.11 R${\oplus}$, and 1.42 $\pm$ 0.21 \gccc, respectively. The planet orbits a F7V star every 3.78 d, whose mass, radius and effective temperature are of 1.2 $\pm$ 0.1 \msun, 1.10 $\pm$ 0.03 \rsun, and 6241$^{+73}{-62}$ K, respectively. TOI-333b is likely younger than 1 Gyr, which is supported by the presence of the doublet Li line around 6707.856 textup{~Å} and its comparison to Li abundances in open clusters with well constrained ages. The planet is expected to host only 8.5$^{+10.9}{-8.3}%$ gas-to-core mass ratio for a H/He envelope. On the other hand, irradiated ocean world models predict 20$^{+11}_{-10}%$ H$2$O mass fraction with a core fraction of 35$^{+20}{-23}%$. Therefore, we expect that TOI-333b internal composition may be 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-333b is more massive and larger than 77$%$ and 82$%$ of its Neptune desert counterparts, respectively, while its host ranks among the hottest known for Neptune Desert planets, making this system a unique laboratory to study the evolution of such planets around hot stars.

💡 Research Summary

The authors present the discovery and comprehensive characterization of TOI‑333b, a planet that resides deep within the so‑called “Neptune desert” – a region of orbital period < 4 days, radius ≈ 2–10 R⊕ and mass ≈ 0.03–0.1 MJ where few planets are found. The planet was first identified in TESS data (sectors 02, 29, 69) using the PDC‑SAP light curves and the Transit Least Squares algorithm, revealing a 3.785 day periodic transit with a depth of ~0.3 % (≈ 300 ppm). Ground‑based follow‑up with LCOGT (SAAO) and NGTS confirmed the transit on‑target, and centroid analysis ruled out background eclipsing binaries.

High‑resolution spectroscopy was obtained with HARPS (44 spectra) and FEROS (7 spectra). Using the HARPS G2 mask, the authors achieved an RV precision of ~8 m s⁻¹ and measured a semi‑amplitude of ~20 m s⁻¹, corresponding to a planetary mass of 20.1 ± 2.4 M⊕. No significant periodicities were found in the bisector span, FWHM, or CCF contrast, and the Pearson correlation coefficients between RVs and these activity indicators were low (0.07, 0.21, –0.4), indicating that stellar activity does not dominate the RV signal.

Spectroscopic analysis of the host star (F7 V) yields Teff = 6241 K, log g = 4.33,