Detection of atmospheric species and dynamics in the bloated hot Jupiter WASP-172 b with ESPRESSO

Context. The population of strongly irradiated Jupiter-sized planets has no equivalent in the Solar System. It is characterised by strongly bloated atmospheres and large atmospheric scale heights. Recent space-based observations of SO2 photochemistry have demonstrated the knowledge that can be gained about Earth's uniqueness from detailed atmospheric studies of these unusual planets. Aims. Here we explore the atmosphere of WASP-172 b, a planet similar in terms of temperature and bloating to the recently studied HD 149026 b. We characterise the atmospheric composition and subsequently the atmospheric dynamics of this prime target. Methods. We observed a particular transit of WASP-172 b in front of its host star with ESO's ESPRESSO spectrograph and analysed the spectra obtained before, during, and after transit. Results. We detect the absorption of starlight by WASP-172 b's atmosphere by sodium (5.6 sigma) and hydrogen (19.5 sigma) and obtained a tentative detection of iron (4.1 sigma). We detect strong - yet varying - blueshifts, relative to the planetary rest frame, of all of these absorption features. This allows for a preliminary study of the atmospheric dynamics of WASP-172 b. Conclusions. With only one transit, we were able to detect a wide variety of species that clearly track different atmospheric layers with possible jets. WASP-172 b is a prime follow-up target for a more in-depth characterisation with both ground- and space-based observatories. If the detection of Fe is confirmed, this may suggest that radius inflation is an important determinant for the detectability of Fe in hot Jupiters, as several non-detections of Fe have been published for planets that are hotter but less inflated than WASP-172 b.