Expected primary transit transmission signal per scale height plotted against the K-band magnitude of GJ 357 b (star) and all know planets (dots) with mass measurements and with radius between 0.5 and 3 R_⊕. The color scale provides the expected S/N for a single transit assuming the use of a ten-meter telescope and 50 nm wavelength integration bins. The gray lines indicate the pattern of the S/N assuming a single transit duration of 3 h. The relative S/N is maintained when extrapolated to other instrumentation. A few benchmark targets for JWST transmission spectroscopy studies are labeled.