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 $T_{\rm{eff}}=6110\pm100~\mathrm{K}$, $\log{g}=4.118^{+0.015}_{-0.025}$, 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.

• The paper presents the discovery and detailed characterization of TOI-6038 A b, a dense sub-Saturn located in the transition regime between the Neptunian ridge and savanna, using TESS photometry and PARAS-2 spectroscopy.
• TOI-6038 A b is characterized as a dense sub-Saturn with a high core mass fraction, making it a valuable case study for planetary formation and evolution in this transitional region.
• This exoplanet system, including a binary companion, serves as a prime target for future follow-up observations to investigate atmospheric properties and orbital dynamics.

Overview of "TOI-6038~A~b: A Dense Sub-Saturn in the Transition Regime Between the Neptunian Ridge and Savanna"

The paper presents the discovery and detailed characterization of a sub-Saturn exoplanet, TOI-6038~A~b. This exoplanet was identified and characterized using the combination of spectroscopic data from the PARAS-2 spectrograph and photometric data from the Transiting Exoplanet Survey Satellite (TESS). The planet orbits the star TOI-6038~A, a late F-type star with metallic enrichment, and is accompanied by a wide-orbit companion star, TOI-6038~B, an early K-type star.

Key Findings

• Planetary Characteristics: TOI-6038~A~b is categorized as a dense sub-Saturn with a mass of approximately 78.5 Earth masses and a radius of around 6.41 Earth radii. The planet orbits its host star in a nearly circular path with a period of about 5.83 days.
• Stellar Characteristics: The host star TOI-6038~A is a bright star with an effective temperature of approximately 6110 K and a metallicity denoted by [Fe/H] of around 0.124 dex. The system is part of a wide binary, with the secondary component residing at a projected distance of approximately 3217 AU.
• Internal Structure: Modeling suggests that the planet comprises about 74% dense materials forming its core, while the remainder is a lighter hydrogen-helium envelope. Such composition aligns it with high-density planets within the Neptunian ridge region.
• Orbital Architecture: The location of the planet lies at the boundary between the Neptunian ridge and savanna—a key region for studying transitional planetary characteristics. The high density of TOI-6038~A~b reflects attributes typical of planets formed via high-eccentricity tidal migration, although this requires further validation.

Implications and Future Prospects

• Evolutionary Pathways: The paper addresses the unusual evolutionary path that might have led TOI-6038~A~b to its present orbit, differentiating between possibilities such as high-eccentricity tidal migration and disk-driven migration. Each proposed mechanism draws on theoretical models of planet dynamics influenced by stellar companions or other planetary bodies.
• Atmospheric and Orbital Follow-up: The bright host star and favorable characteristics make TOI-6038~A~b a promising candidate for follow-up studies that assess atmospheric escape and orbital dynamics. Such studies could refine models of planetary evolution in transitional regions of the exoplanetary landscape.
• Broader Context in Exoplanetary Science: TOI-6038~A~b adds an important data point to the study of sub-Saturns, enhancing the understanding of their diversity in formation and characteristics. As observations and detection techniques continue to evolve, the exoplanet could provide insights into the factors that define various planetary regimes across different stellar environments.
• Technique and Instrumentation Utilization: The integration of PARAS-2 and TESS data underscores the utility of combining high-precision radial velocity measurements with extensive photometric surveys for robust planet characterization. This synergy is crucial in enhancing the sensitivity and accuracy of parameter estimation for exoplanets in complex star systems.

In conclusion, TOI-6038~A~b represents a significant addition to the exoplanetary catalog, serving as a focal case for studying the dynamics of close-in gas giants and their interaction with host stars, potentially influencing theoretical models of planet formation and migration. The discoveries and hypotheses paved by this study open avenues for future observational campaigns and theoretical investigations into transitional exoplanetary phenomena.