Radial velocity (blue line), maximum observed velocity (black upper line) and minimum observed velocity (black lower line) as a function of time for a hydrogen atom escaped from HD 209458b. The maximum velocity of ~130 km s^-1 is reached in about 10 h. The vertical black lines show the characteristic lifetimes of a hydrogen atom for stellar ionizing EUV fluxes of 0.5 (dotted line), 1 (dashed line) and 5 (solid line) times the solar value.