TOI-6038 A b: A Dense Sub-Saturn in the Transition Regime between the Neptunian Ridge and Savanna

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Abstract 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 eff = 6110 ± 100 K, log g = 4.11 8 − 0.025 + 0.015 , and [ Fe/H ] = 0.12 4 − 0.077 + 0.079 dex. The system also contains a wide-orbit binary companion, TOI-6038 B, an early K-type star at a projected separation of ≈3217 au. We combined radial velocity data from PARAS-2 with photometric data from the Transiting Exoplanet Survey Satellite for joint modeling. TOI-6038 A b has a mass of 78 . 5 − 9.9 + 9.5 M ⊕ and a radius of 6.4 1 − 0.16 + 0.20 R ⊕ , orbiting in a circular orbit with a period of 5.826731 1 − 0.0000068 + 0.0000074 days. Internal structure modeling suggests that ≈74% 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 ρ P = 1.6 2 − 0.24 + 0.23 g cm − 3 , TOI-6038 A b is compatible with the population of dense ridge planets (ρ P ≃ 1.5–2.0 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. Interestingly, 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.