The Biggest Eye on the Sky: the time-resolved winds of WASP-121b in ESPRESSO’s 4UT mode

The groundbreaking ESPRESSO 4UT mode, where light from all 4 UTs are fed into ESPRESSO, boasts the world’s largest photon-collecting power with a virtual 16-m class telescope simulating a mini-ELT. We present an observational study leveraging this mode of the ultra-hot Jupiter, WASP-121b. Existing observations of a partial transit obtained as part of the ESPRESSO GTO unveiled a time-resolved jet stream in the planet’s atmosphere providing the first dive into time-resolved exoplanetary atmospheric dynamics (see Seidel et al. 2023). This localized jet was retrieved due to its distinct imprint on the line shape of the sodium doublet with a notable difference in line shape between mid-transit data and data taken closer to egress. However, this transit was partial, thus allowing us to only draw conclusions about the viewing geometry of the jet for one side of the planet. Notably, the absence of ingress data has left the critical question of whether the shifts induced by atmospheric dynamics in the sodium doublet signature result from a super-rotational jet or day-to-night side winds unanswered. Here, we present the missing ingress dataset observed in September 2023 as part of a GTO follow-up (Seidel et al. 2024, in prep.). Our findings complete the first time-resolved dataset on an ultra-hot Jupiter crucially revealing the atmospheric dynamics resolved across the planet’s visible atmosphere. This one-of-a-kind dataset with unparalleled S/N will be crucial in the ramp-up for the ELT era, especially in the context of ANDES, as it allows to test new retrieval techniques and theoretical models on a real dataset as close to ANDES-like data as possible.