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Hydrogen and helium transmission signals trace the upper atmospheres of hotgas-giant exoplanets, where the incoming stellar extreme ultraviolet and X-rayfluxes are deposited. Further, for the hottest stars, the near-ultravioletexcitation of hydrogen in the Balmer continuum may play a dominant role incontrolling the atmospheric temperature and driving photoevaporation. KELT-9 bis the archetypal example of such an environment as it is the hottest gas-giantexoplanet known to date (T$_{eq}$ $\sim$ 4500 K) and orbits an A0V-type star.Studies of the upper atmosphere and escaping gas of this ultra-hot Jupiter havetargeted the absorption in the Balmer series of hydrogen (n$_1$ = 2$\rightarrow$ n$_2$ $>$ 2). Unfortunately, the lowermost metastable heliumstate that causes the triplet absorption at 108.3 nm is not sufficientlypopulated for detection. Here, we present evidence of hydrogen absorption inthe Paschen series in the transmission spectrum of KELT-9 b observed withCARMENES. Specifically, we focus on the strongest line covered by its NIRchannel, Paschen-$\beta$ at 1282.16 nm (n$_1$ = 3 $\rightarrow$ n$_2$ = 5). Theobserved absorption shows a contrast of (0.53 $^{+0.12}_{-0.13}$)%, a blueshiftof $-$14.8 $^{+3.5}_{-3.2}$ km/s, and a FWHM of 31.9$^{+11.8}_{-8.3}$ km/s. Theobserved blueshift in the absorption feature could be explained by day-to-nightcirculation within the gravitationally bound atmosphere or, alternatively, byPaschen-$\beta$ absorption originating in a tail of escaping gas moving towardthe observer as a result of extreme atmospheric evaporation. This detectionopens a new window for investigating the atmospheres of ultra-hot Jupiters,providing additional constraints of their temperature structure, mass-lossrates, and dynamics for future modeling of their scorching atmospheres.

Detection of Paschen $\beta$ absorption in the atmosphere of KELT-9 b: A new window into the atmospheres of ultra-hot Jupiters