2024A&A...691A..12B

We report the discovery and characterization of a transiting sub-Saturn exoplanet TOI-6651b using PARAS-2 spectroscopic observations. The host, TOI-6651 (m_V ≈ 10.2), is a sub-giant, metal-rich G-type star with [Fe / H] = 0.225_–0.045^0.044$\[[\mathrm{Fe} / \mathrm{H}]=0.225_{-0.045}^{+0.044}\]$, T_eff = 5940 ± 110 K, and log $\[g=4.087_{-0.032}^{+0.035}\]$. Joint fitting of the radial velocities from PARAS-2 spectrograph and transit photometric data from Transiting Exoplanet Survey Satellite (TESS) reveals a planetary mass of $\[61.0_{-7.9}^{+7.6} ~M_{\oplus}\]$ and radius of $\[5.09_{-0.26}^{+0.27} ~R_{\oplus}\]$, in a $\[5.056973_{-0.000018}^{+0.000016}\]$ day orbit with an eccentricity of $\[0.091_{-0.062}^{+0.096}\]$. TOI-6651b has a bulk density of $\[2.52_{-0.44}^{+0.52}\]$ g cm^–3, positioning it among the select few known dense sub-Saturns and making it notably the densest detected with TESS. TOI-6651b is consistent with the positive correlation between planet mass and the host star's metallicity. We find that a considerable portion ≈87% of the planet's mass consists of dense materials such as rock and iron in the core, while the remaining mass comprises a low-density envelope of H/He. TOI-6651b lies at the edge of the Neptunian desert, which will be crucial for understanding the factors shaping the desert boundaries. The existence of TOI-6651b challenges conventional planet formation theories and could be a result of merging events or significant atmospheric mass loss through tidal heating, highlighting the complex interplay of dynamical processes and atmospheric evolution in the formation of massive dense sub-Saturns.