Variable and super-sonic winds in the atmosphere of an ultra-hot giant planet

Hot Jupiters (HJs) present an extreme case for exploring the conditions that regulate planetary atmospheres because they experience intense irradiation from their host stars that induces global-scale winds. General circulation models (GCMs) of HJ atmospheres predict day-to-nightside winds (winds flowing from the side of the planet facing its host star to the side facing away from the star) and equatorial jets (winds that entirely circulate around the planet) with speeds on the order of a few km/s. We apply high-resolution transmission spectroscopy using the PEPSI spectrograph on the Large Binocular Telescope to empirically constrain supersonic ~10 km/s day-to-nightside winds traced by Fe II features in the atmosphere of KELT-9 b, an ultra-hot Jupiter (UHJ) that remains to-date the hottest known planet. Reconciling our findings with archival HARPS-N datasets suggests multi-epoch variability ~5-8 km/s over timescales between weeks to years. We compare with the UHJ KELT-20 b to demonstrate the exceptional nature of KELT-9 b’s atmospheric dynamics. A qualitative evaluation of our measured wind velocities and variability against current UHJ GCMs reveals that KELT-9 b poses unique challenges for validating giant planet atmospheric models.