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Hot Jupiters receive intense irradiation from their stellar hosts. Theresulting extreme environments in their atmospheres allow us to study theconditions that drive planetary atmospheric dynamics, e.g., global-scale winds.General circulation models predict day-to-nightside winds and equatorial jetswith speeds on the order of a few km $\mathrm{s^{-1}}$. To test these models,we apply high-resolution transmission spectroscopy using the PEPSI spectrographon the Large Binocular Telescope to study the atmosphere of KELT-9 b, anultra-hot Jupiter and currently the hottest known planet. We measure $\sim$10km $\mathrm{s^{-1}}$ day-to-nightside winds traced by Fe II features in theplanet's atmosphere. This is at odds with previous literature (including datataken with PEPSI), which report no significant day-to-nightside winds on KELT-9b. We identify the cause of this discrepancy as due to an inaccurate ephemerisfor KELT-9 b in previous literature. We update the ephemeris, which shifts themid-transit time by up to 10 minutes for previous datasets, resulting inconsistent detections of blueshifts in all the datasets analyzed here.Furthermore, a comparison with archival HARPS-N datasets suggests temporal windvariability $\sim$5-8 km $\mathrm{s^{-1}}$ over timescales between weeks toyears. Temporal variability of atmospheric dynamics on hot Jupiters is aphenomenon anticipated by certain general circulation models that has not beenobserved over these timescales until now. However, such large variability as wemeasure on KELT-9 b challenges general circulation models, which predict muchlower amplitudes of wind variability over timescales between days to weeks.

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