Parent-star V magnitude as a function of the expected transmission spectral signal of 1 atmospheric scale height for transiting exoplanets. Data are taken from TEPCat. The curves are adopted from Sing (2018) and give an indication of constant values for S/N and spectral resolution (see also Heng & Winn 2015). The numbers in red and blue refer to HST and JWST, respectively. The region of the diagram in which exoplanetary atmospheres are unobservable, due to low S/N, is marked in gray. The position of WASP-174 is highlighted together with those of other selected transiting exoplanets.