Distribution of the S/N of Fe I signals using the CCF method and synthetic spectra computed for different metallicities (stellar metallicity is [Fe/H] = 0.1 dex). The peak of the distribution lies at the 50th percentile (solid black line), while the 16th and 84th percentiles are shown by the dashed black lines. The measured S/N of the tentative Fe I detection in GJ 436 b’s atmosphere using the ESPRESSO observations (S/N = 3.4 ± 0.2) is indicated by the blue band.