Plot of the theoretical profile of GJ 436’s Lyα line. The black thin line shows the intrinsic stellar emission line as seen by hydrogen atoms escaping the planetary atmosphere. The black thick line shows the resulting profile after absorption by the ISM hydrogen and deuterium. 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 34.5 for 39 degrees of freedom. The location of the (subtracted) air glow is indicated by green hatches.