The GAPS programme at TNG -- LXIII. Photo-evaporating puzzle: Exploring the enigmatic nature of TOI-5398 b atmospheric signal

TL;DR

This study characterizes the atmosphere of the warm Saturn TOI-5398 b using high-resolution spectroscopy, detecting key photo-evaporative signatures like H alpha and He I triplets, and compares observations with models to understand atmospheric escape.

Contribution

It provides the first detailed atmospheric analysis of TOI-5398 b, demonstrating the effectiveness of high-resolution spectroscopy in studying photo-evaporating atmospheres of smaller exoplanets.

Findings

Detected H alpha and He I triplets indicating atmospheric escape

Estimated atmospheric heights of 2.33 Rp for H alpha and 1.65 Rp for He I

Supported observations with ATES models predicting similar He I absorption

Abstract

Atmospheric characterization is key to understanding exoplanetary systems, offering insights into the planets current and past conditions. By analyzing key lines like H alpha and the He I triplet, we can trace the evolution of planets through atmospheric photo-evaporation. While ultra-hot Jupiters have been the focus for years, attention is shifting toward smaller, colder planets, which are more challenging to study due to weaker signals, requiring more precise techniques. This study aims to characterize the atmosphere of TOI-5398 b, a warm Saturn with a 10.59-day orbit around a young (650 Myr) G-type star. The system also hosts a smaller inner planet, TOI-5398 c, with a 4.77-day orbit. Both planets are ideal for atmospheric studies due to their proximity to the host star, which drives strong photo-evaporation, especially in planet b, whose high transmission spectroscopy metric (288) makes it a prime target. We analyzed data from a transit observed with the HARPS-N and GIANO-B high-resolution spectrographs, using cross-correlation and single-line analysis to search for atomic species. During this observation, planet c was also transiting, so we investigated the source of the signals. Based on photo-evaporation models, we attribute the signal mainly to planet b, which is expected to lose more mass. We detected H alpha and He I triplets, key markers of photo-evaporation, corresponding to atmospheric heights of 2.33 Rp and 1.65 Rp, respectively. The ATES models supported our observations, predicting a similar He I absorption for planet b and suggesting an He/H ratio of 1/99. Additionally, we detected an Na I doublet via single-line analysis, though cross-correlation did not reveal other atomic species.