2026ApJ...997L..40Z

High-resolution spectroscopy provides a unique opportunity to directly probe atmospheric dynamics by resolving Doppler shifts of planetary signals as a function of orbital phase. Using the optical spectrometer the Keck Planet Finder, we carry out a pilot study on high-resolution phase-curve spectra of the ultrahot Jupiter KELT-9 b. We spectrally and temporally resolve its dayside emission from posttransit to preeclipse (orbital phase ϕ = 0.1-0.45). The signal strength and width increase with orbital phase as the dayside rotates into view. The net Doppler shift varies progressively from -13.4 ± 0.6 to -0.4 ± 1.0 km s^–1, the extent of which exceeds its rotation velocity of 6.4 ± 0.1 km s^–1, providing unambiguous evidence of atmospheric winds. We devise a retrieval framework to fit the full time-series spectra, accounting for the variation of the line profiles due to the rotation and winds. We retrieve a supersonic day-to-night wind speed up to 11.7 ± 0.6 km s^–1 on the emerging dayside, representing the most extreme atmospheric winds in hot Jupiters to date. Comparison to 3D circulation models reveals weak atmospheric drag, consistent with relatively efficient heat recirculation, as also supported by space-based phase-curve measurements. Additionally, we retrieve the dayside chemistry (including Fe I, Fe II, Ti I, Ti II, Ca I, Ca II, Mg I, and Si I) and temperature structure, and we place constraints on the nightside thermal profile. Our high-resolution phase-curve spectra and the measured supersonic winds provide excellent benchmarks for extreme physics in circulation models, demonstrating the power of this technique in understanding the climates of hot Jupiters.