Discovery and Characterization of a Dense Sub-Saturn TOI-6651b

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^+0.044_-0.045, T_ eff = 5940±110K, and logg = 4.087^+0.035_-0.032. 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^+7.6_-7.9 M_⊕ and radius of 5.09^+0.27_-0.26 R_⊕, in a 5.056973^+0.000016_-0.000018 day orbit with an eccentricity of 0.091^+0.096_-0.062. TOI-6651b has a bulk density of 2.52^+0.52_-0.44 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 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.