Expected transmission signals (see Sect. 8.4 for details) of currently known transiting terrestrial exoplanets (R_p < 1.6 R_⊕) as a function of their incident stellar flux. Data were taken from NASA Exoplanet Archive on 29 March 2022. We only show planets with mild incident irradiations, lower than five times that received by the Earth (S_p < 5 S_⊕). The symbols are colour-coded as a function of their S/N relative to TRAPPIST-1 b, obtained by scaling the signal amplitude with the hosts’ brightness in the J-band and using TRAPPIST-1 b’s S/N as a reference. The size of the symbols is proportional to the planetary radius. Transmission signals less than 5 ppm have been removed to enhance the readability of the figure.