Evolution of the theoretical absorption profile as a function of time during the transit of HD 209458b, for the best-fit simulation of the G-scenario. Absorption in the blue side is generated by radiation-pressure accelerated magnesium atoms and always drops at about −60 km s^-1 (at mid-height). The central absorption peak comes from atoms close to the planet (dotted lines show the part of the absorption profile generated by the main atmosphere) and shifts redward of the line with the orbital motion. This is similar to the spectral extension of the observed absorption signature toward positive velocities during and after the transit (Fig. 1). The simulated planetary disk is the source for an absorption depth of ~1.4% at all wavelengths during the transit.