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

TL;DR

TOI-333b is a Neptune desert planet orbiting a hot F7V star with a period of $P \approx 3.785$ d, a mass of $M_p = 20.1 \pm 2.4\,M_\oplus$, a radius of $R_p = 4.26 \pm 0.11\,R_\oplus$, and a bulk density of $\rho_p = 1.42 \pm 0.21\,\mathrm{g\,cm^{-3}}$. The planet was confirmed through a multi-instrument campaign (TESS, LCOGT, NGTS photometry; HARPS/FEROS RVs; speckle imaging) and analyzed with a joint photometry–RV model using instrument-specific noise treatments and a Keplerian orbit, including a GP component for transits. Stellar properties derived from SPECIES+ARIADNE, independent SED fitting, and EXOFASTv2 place the host at $T_{\rm eff} \approx 6267$ K, $R_{\star} \approx 1.10\,R_\odot$, $M_{\star} \approx 1.2\,M_\odot$, with an age likely $\lesssim 1$ Gyr. The ND context is enriched by TOI-333b’s position in the mass–radius–period space, suggesting a primarily rocky interior with little or no H/He or a water-rich composition, and the system offers a promising target for JWST atmospheric studies due to favorable transmission and emission spectroscopy metrics.

Abstract

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.

Figures (19)

• Figure 1: Left: TESS detrended lightcurve phase-folded to the best-fitting period listed in Table \ref{['tab:planet']} and zoomed to show the transit event. 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 1-$\sigma$ confidence interval. Centre: Same as left figure for the LCOGT-SAAO telescope. Right: Same as left figure for the NGTS mission. Bottom: residuals to the best fit model.
• Figure 2: Periodogram of the line bisectors (top panel), CCF-FWHM (centre panel) and contrast (bottom panel) for HARPS (in black). The planet’s orbital period is highlighted by the grey vertical line with dashed lines showing the harmonics at 1/8, 1/4, 1/2, 2, and 3 from left to right, respectively. Top to bottom dashed black lines represent the FAP at 0.1$\%$ 1$\%$ and 10$\%$, respectively.
• Figure 3: Top: RV phase-folded to the best-fitting period listed in Table \ref{['tab:planet']}. RV data is colour-coded in brown, and green for HARPS and FEROS, respectively, with black circles representing the binned RVs. The red curve and light red shaded region shows the Keplerian model and its 1-$\sigma$ confidence interval. Bottom: residuals to the best fit model.
• Figure 4: TESS sector 29 full-frame image cutout (11 x 11 pixels) generated with the tpfplotter script described in aller2020planetary. TOI-333b is shown in the centre labeled number 1, followed by GAIA DR3 6535747602288702720 (number 2), a G = 14.11 mag, and 51" away from the planet. The star did not contribute significant flux to the aperture, thus negligible dilution was observed in TESS.
• Figure 5: Modelshift module of TOI 333b obtained in TESS sector 02. The first panel displays the phase-folded lightcurvewith the best-fit trapezoid transit model (black line). The second panel shows the convolved lightcurvewith the transit model and noise level (blue lines). The lower panels offer close-ups of primary and secondary events, both odd and even primaries, and any additional events. The upper table indicates the statistical significance of these features, highlighted in red when flagged as significant by the pipeline.