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 (mV≈ 10.2), is a sub-giant, metal-rich G-type star with [ Fe/H] = 0.225+0.044-0.045, Teff = 5940±110\ K, 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% 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.