Hydrogen, helium and sodium in the atmosphere of the Saturnian planet TOI-5398 b

The characterization of exoplanetary atmospheres is one of the most active research fields in modern astrophysics, made possible by the development of dedicated instruments and techniques. The basic idea of this research is quite simple: obtaining spectroscopic observations of stars with planets during the transit of the planet in front of its star, which is the moment when part of the stellar radiation passes through the planet atmosphere. In this way, it is possible to measure the effects caused by the atoms and molecules present in the planet atmosphere. The most suitable planets for these measurements are gas giants in close orbits around their star, as they present an extended atmosphere due to the intense stellar radiation they intercept and the evaporation of part of the atmosphere induced by the incoming radiation (photoevaporation).

An exoplanet of particular interest for these studies is TOI-5398 b, a Saturnian-type planet orbiting at about 0.1 AU from its star, a solar-type star only 650 million years old that hosts two known planets. The planet atmosphere has an equilibrium temperature of 950 K, resulting in an atmospheric scale height of over 1100 km (assuming an atmosphere primarily composed of hydrogen). The planet is also at the boundary of the region known as the “Neptunian savannah”: a transitional zone in the radius-orbital period diagram, sparsely populated, corresponding to orbital periods longer than five days and planetary radii between four and ten Earth radii.

The team led by astrophysicist M. d’Arpa (University of Palermo, INAF – Palermo Astronomical Observatory) analyzed spectroscopic observations of TOI-5398 b in both the optical and infrared bands, obtained with the GIARPS spectrograph on the Telescopio Nazionale Galileo, dating back to March 25, 2023, during the transit of both planets in the system. The data analysis detected signals from hydrogen, helium, and sodium atoms in TOI-5398 b atmosphere, from which the atmospheric size can be estimated using hydrogen and helium as “tracers”, obtaining values of 2.33 and 1.65 planetary radii, respectively. These estimates were verified using the ATES code, which generates 1D models of irradiated atmospheres. The study is detailed in the article “The GAPS programme at TNG. LXIII. Photo-evaporating puzzle: Exploring the enigmatic nature of TOI-5398 b’s atmospheric signal”, recently published in Astronomy & Astrophysics. The coauthors include astrophysicists R. Spinelli, D. Locci, A. Maggio, A. Petralia, C. Di Maio, S. Benatti, G. Micela, L. Affer from the INAF – Palermo Astronomical Observatory.

The cover image (click here to view it in full) shows the distribution of planetary radii as a function of orbital periods for known exoplanets. The diagram indicates the positions of the “desert” and “savannah” of Neptunian planets. The large diamond marks the current position of TOI-5398 b, with an arrow showing its possible future evolution as suggested by a recent study. A smaller diamond represents the position of the second planet in the TOI-5398 system.