Discovery and characterization of a dense sub-Saturn TOI-6651b

Abstract

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[Fe/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_⊕ and radius of 5.09_‑0.26^+0.27 R_⊕, 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.