A Hot Saturn Orbiting An Oscillating Late Subgiant Discovered by TESS

Abstract: We present the discovery of TOI-197.01, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. TOI-197 (HIP116158) is a bright (V=8.2 mag), spectroscopically classified subgiant which oscillates with an average frequency of about 430 muHz and displays a clear signature of mixed modes. The oscillation amplitude confirms that the redder TESS bandpass compared to Kepler has a small effect on the oscillations, supporting the expected yield of thousands of solar-like oscillators with TESS 2-minute cadence observations. Asteroseismic modeling yields a robust determination of the host star radius (2.943+/-0.064 Rsun), mass (1.212 +/- 0.074 Msun) and age (4.9+/-1.1 Gyr), and demonstrates that it has just started ascending the red-giant branch. Combining asteroseismology with transit modeling and radial-velocity observations, we show that the planet is a "hot Saturn" (9.17+/-0.33 Rearth) with an orbital period of ~14.3 days, irradiance of 343+/-24 Fearth, moderate mass (60.5 +/- 5.7 Mearth) and density (0.431+/-0.062 gcc). The properties of TOI-197.01 show that the host-star metallicity - planet mass correlation found in sub-Saturns (4-8 Rearth) does not extend to larger radii, indicating that planets in the transition between sub-Saturns and Jupiters follow a relatively narrow range of densities. With a density measured to ~15%, TOI-197.01 is one of the best characterized Saturn-sized planets to date, augmenting the small number of known transiting planets around evolved stars and demonstrating the power of TESS to characterize exoplanets and their host stars using asteroseismology.

Discovery and Characterization of TOI-197.01: A Transiting Exoplanet Orbiting an Oscillating Subgiant

This paper discusses the discovery and characterization of TOI-197.01, a transiting exoplanet identified by the Transiting Exoplanet Survey Satellite (TESS) with the unique feature of permitting asteroseismology of its host star, TOI-197. The study leverages the synergy between exoplanet science and stellar oscillation analysis to provide robust estimations of stellar and planetary parameters.

Asteroseismology and Stellar Analysis

TOI-197 is classified spectroscopically as a late subgiant, and its oscillations exhibit a mixed mode signature with an average frequency of around 430 µHz. The host star is characterized using asteroseismic modeling, yielding precise measurements of stellar parameters: a radius of 2.943 R_{\odot}, mass of 1.212 M_{\odot}, and age of 4.9 Gyr. These results highlight that the star has just begun ascending the red-giant branch. The capability to determine such parameters with high precision underscores asteroseismology's power as a tool for stellar characterization, especially within the framework of TESS's observational capabilities.

Planetary Properties

TOI-197.01 is characterized as a "hot Saturn," with a radius of 9.17 R_{\oplus}, mass of 60.5 M_{\oplus}, and density of 0.431 g cm^{-3}. Its orbital period is approximately 14.3 days, and it receives an irradiance of 343 F_{\oplus}. The comprehensive analysis combines transit modeling and radial-velocity observations, providing one of the most precisely characterized Saturn-sized planets. TOI-197.01 exemplifies the transition field between sub-Saturns and Jupiters, exhibiting a uniformly moderate density suggestive of planetary mass-radius relations driven by interior structural transitions.

Implications and Future Prospects

The authors highlight several implications of this discovery. The density measurement of TOI-197.01, compared to the mass and radius discovered in sub-Saturns, does not support the previously observed correlation between host-star metallicity and planet mass for planets larger than 8 R_{\oplus}. This contributes to an understanding of planetary formation dynamics within different stellar environments and builds a case for distinct mass-radius relations across planet classes.

Furthermore, the radial expansion and increased luminosity of the host star as it ascends the red-giant branch offer a natural laboratory for studying the mechanisms of planetary inflation due to stellar evolution. TOI-197.01's modest eccentricity addresses theoretical predictions on tidal interactions and dissipation, providing insights into the broad diversity of dissipation processes in giant planets.

The study presents the methodology and implications of such precise measurements as promising for exploring the demographics of exoplanets around evolved stars. It suggests potential routes for future research, including ensemble studies of oscillating subgiants hosting close-in planets, which could illuminate the evolution and migration phenomena underpinning observed planetary architectures.

In conclusion, the paper demonstrates TESS's capability to extend the frontier of exoplanetary science through the integration of space-based asteroseismology and high-resolution follow-up observations, highlighting the prospects for further characterization of exoplanets in increasingly diverse stellar environments.