Comparison between LTE and NLTE KELT-9b transmission spectra. Top: NLTE (black) and LTE (red) transmission spectra of KELT-9b ranging between the far-ultraviolet and the near-infrared. The transmission spectra were computed considering a spectral resolution of 100 000. The NLTE transmission spectrum was computed on the basis of the NLTE TP profile, while the LTE transmission spectrum was computed on the basis of the LTE TP profile. Bottom panel: deviation from LTE (in %). The strongest deviation from LTE is found at ultraviolet wavelengths. Bottom: transit depth difference between the NLTE and LTE transmission spectra.