Theoretical profile of HD 189733 Lyman-α line. The black thin line shows the theoretical intrinsic stellar emission line profile as seen by hydrogen atoms escaping the planetary atmosphere, scaled to the Earth distance. It also corresponds to the profile of the ratio β between radiation pressure and stellar gravity (values reported on the right axis). The black thick line shows the Lyman-α line profile after absorption by the interstellar hydrogen (1215.6 Å) and deuterium (1215.25 Å). The line profile convolved with the STIS G140M instrumental line spread function (red line) is compared with the observations (blue histogram), yielding a good fit with a χ² of 36.1 for 40 degrees of freedom. Note that β is higher than 1 for velocities in the range −120 to 120 km s^-1.