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

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