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 launched from HD 189733b. The maximum velocity of ~140 km s^-1 is reached in about 6 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. Because HD 189733b is closer to its star than HD 209458b, ionization lifetimes are shorter.