Mg ii stellar absorption line profile of HD 209458 at 2796.3518 Å  (Mg ii-k line; top panel) and 2803.5305 Å  (Mg ii-h line; bottom panel). The blue line shows the theoretical self-reversed emission profile as seen by ionized magnesium atoms around the planet. The red line shows the line core profile after absorption by the interstellar Mg⁺, to be compared with the observations (black histogram). Outside of the line core, observations (black line) can be used directly to calculate radiation pressure.