Plot of the 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. The black thick line shows the resulting profile after absorption by the interstellar hydrogen (1215.6 Å) and deuterium (1215.3 Å). The line profile convolved with the HST G140M instrumental line spread function (red line) is compared to the observations (blue histogram), yielding a good fit with a χ² of 36.1 for 40 degrees of freedom.