TOI-3862 b: A dense super-Neptune deep in the hot Neptune desert

TL;DR

TOI-3862 b is a dense sub-Saturn in a 1.56-day orbit around a Sun-like star, with a precisely measured mass of $53.7_{-2.9}^{+2.8}$ $M_⊕$ and radius of $5.53±0.18$ $R_⊕$, yielding a density of $1.75±0.20$ g cm⁻³. Through joint transit and radial velocity modeling, the study confirms the planet and shows it lies deep in the hot Neptune desert, suggesting a metal-rich interior with a relatively small H/He envelope (envelope mass fraction ≈ 15–17%). Internal structure and atmospheric-evolution modeling with jade indicates a maximum initial mass around $70.6^{+2.7}_{-2.6}$ $M_⊕$, with substantial mass loss over ≈7.5 Gyr leading to its current configuration of ~0.22 $M_J$ and ~0.37 of the mass in the envelope at formation. The work situates TOI-3862 b as a rare desert survivor that informs planetary formation, migration, and atmospheric-retention theories and highlights opportunities for future atmospheric escape and spin–orbit studies through high-precision observations.

Abstract

The structure and evolution of close-in exoplanets are shaped by atmospheric loss and migration processes, which give rise to key population features such as the hot Neptune desert, ridge, and savanna - regions of the period-radius space whose boundaries offer critical insights into planetary formation and survival. As part of the KESPRINT collaboration, we selected the TESS transiting planet candidate TOI-3862.01 for radial velocity follow-up to confirm its planetary nature and characterize its mass and bulk properties. This planet candidate is of particular interest due to its position in the middle of the hot Neptune desert, making it a valuable probe for testing theories of planet migration and atmospheric loss. We confirmed the planetary nature and determined the mass of TOI-3862.01 (hereinafter TOI-3862b) by performing a joint fit with both transit and radial velocity data, precisely characterizing the bulk properties of this planet. TOI-3862b is a super-Neptune on a 1.56-day orbit around a Sun-like star with an effective temperature of 5300$\pm$50K. It has a mass of 53.7$_{-2.9}^{+2.8}$ M$_{\oplus}$ and a radius of 5.53$\pm$0.18 R$_{\oplus}$, corresponding to a density of 1.7$\pm$0.2 g/cm^3. This places it among the rare population of hot and dense super-Neptune desert planets. TOI-3862b, residing deep in the hot Neptune desert, represents a rare occurrence in an otherwise sparsely populated region, offering a valuable opportunity to probe the processes that may allow planets to survive in such environments.

Figures (8)

• Figure 1: Left: TESS TPF of Sector 22 for TOI-3862. The color bar represents the electron counts for each pixel. The orange squares denote the pixels chosen by the TESS pipeline for aperture photometry. All sources from Gaia DR3 are overlaid on the plot and depicted as circles of varying sizes, corresponding to their G-mag difference relative to the target (as detailed in the legend). This visualization was generated using the tpfplotter code aller2020. Gray arrows indicate the proper motion directions for all sources shown in the plot. Right: TESS heat maps, generated through TESS-contCastroGonzalez2024b, showing the percentage of the flux in each pixel that comes from the target star. The five most contaminating Gaia DR3 sources are overlaid with sizes scaling with their emitted fluxes.
• Figure 2: Spectral energy distributions of TOI-3862. Red symbols represent the observed photometric measurements, where the horizontal bars represent the effective width of the pass-band. Blue symbols are the model fluxes from the best-fit PHOENIX atmosphere model (black). The absolute flux-calibrated Gaia spectrum is shown as a gray swathe in the inset figure.
• Figure 3: GLS periodograms for TOI-3862. The periodograms were performed for the RV dataset, its residuals (after subtracting the fit model), and all the activity indicators obtained through the HARPS-N DRS ($\rm log\,R^{\prime}_\mathrm{HK}$, S-index, Bisector, CCF Contrast, and CCF FWHM) and serval (chromospheric index CRX, dLw, H-alpha, and the sodium lines Na$_1$ and Na$_2$). The stellar rotation period, highlighted by the orange region in the periodograms, corresponds to the result of the joint fit using GPs with broad period boundaries (2–100 days), as described in Section \ref{['sec:planet']}. The red region indicates the rotation period estimated from the SED analysis (see Section \ref{['sec:sed']}). The dashed red line marks the period corresponding to the maximum power in the periodograms, while the green line denotes the orbital period of planets b.
• Figure 4: TESS and ground-based light curves with binned data for TOI-3862 b, together with the models obtained from the joint fit.
• Figure 5: HARPS-N RV data for TOI-3862, with the 1p+GP model overplotted.