Top: observed line profiles for one photospheric Fe I line in the planet-hosting star HD 209458. The thin red curves are observed profiles from various epochs during the exoplanet transit; the dotted curve is the reference profile from outside transit. Bottom: by averaging data from 11 different Fe I lines, the photometric signal-to-noise ratio approaches what is required to resolve the gradual line profile changes during transit. The line profile ratios are in the same format as the theoretical curves in Figs. 9 and 10.