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 ($m_V\approx 10.2$), is a sub-giant, metal-rich G-type star with $[Fe/H]=0.225_{-0.045}^{+0.044}$, $T_{\mathrm{eff}}=5940\pm 110\mathrm{K}$, 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 $2.52_{-0.44}^{+0.52}\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, 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 $\approx$ 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.

15 pages, 12 figures

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