We present the discovery and characterization of a sub-Saturn exoplanet, TOI-6038~A~b, using the PARAS-2 spectrograph. The planet orbits a bright ($m_V=9.9$), metal-rich late F-type star, TOI-6038~A, with $T_{ \rm{ eff}}=6110\pm100~\mathrm{ K}$, $\log{ g}=4.118^{ +0.015}_{ -0.025}$, and $[{ \rm{ Fe/H}}]=0.124^{ +0.079}_{ -0.077}$ dex. The system also contains a wide-orbit binary companion, TOI-6038~B, an early K-type star at a projected separation of $\approx3217$ AU. We combined radial velocity data from PARAS-2 with photometric data from the Transiting Exoplanet Survey Satellite (TESS) for joint modeling. TOI-6038~A~b has a mass of $78.5^{ +9.5}_{ -9.9}~M_\oplus$ and a radius of $6.41^{ +0.20}_{ -0.16}~R_\oplus$, orbiting in a circular orbit with a period of $5.8267311^{ +0.0000074}_{ -0.0000068}$ days. Internal structure modeling suggests that $\approx74\%$ of the planet's mass is composed of dense materials, such as rock and iron, forming a core, while the remaining mass consists of a low-density H/He envelope. TOI-6038~A~b lies at the transition regime between the recently identified Neptunian ridge and savanna. Having a density of $\rho_{ \rm{ P}}=1.62^{ +0.23}_{ -0.24}\rm~g\,cm^{ -3}$, TOI-6038~A~b is compatible with the population of dense ridge planets ($\rho_{ \rm{ P}}\simeq$ 1.5-2.0 $\rm~g\,cm^{ -3}$), which have been proposed to have reached their close-in locations through high-eccentricity tidal migration (HEM). First-order estimates suggest that the secular perturbations induced by TOI-6038~B may be insufficient to drive the HEM of TOI-6038~A~b. Therefore, it is not clear whether HEM driven by a still undetected companion, or early disk-driven migration, brought TOI-6038~A~b to its present-day close-in orbit. Its bright host star makes TOI-6038~A~b a prime target for atmospheric escape and orbital architecture observations, which will help us to better understand its overall evolution.
Categories: astro-ph.EP