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

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, $\mathrm{log}\,g=4.11{8}_{-0.025}^{+0.015}$ , and $[{\rm{Fe/H}}]=0.12{4}_{-0.077}^{+0.079}$ 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 $78.{5}_{-9.9}^{+9.5}\,{M}_{\oplus }$ and a radius of $6.4{1}_{-0.16}^{+0.20}\,{R}_{\oplus }$ , 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 ${\rho }_{{\rm{P}}}=1.6{2}_{-0.24}^{+0.23}\,{{\rm{g}}\,{\rm{cm}}}^{-3}$ , 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.

Keywords

• Exoplanet systems
• Wide binary stars
• Transit photometry
• Radial velocity
• Extrasolar gaseous planets
• Exoplanet dynamics