Observed out-of-transit (highest blue curve) and observed in-transit (green curve) Lyman α spectra, reproduced from fig. 2 of Vidal-Madjar et al. (2003). In the line ‘core’ from −42 to +32 km s⁻¹, where interstellar absorption is too strong to extract a planetary transit signal, the flux is set to zero. Our theoretical in-transit spectrum (red curve) is computed by multiplying the observed out-of-transit spectrum by 1 − A, where A is plotted in Fig. 4. The agreement between the theoretical and observed in-transit spectra is good, supporting the idea that charge exchange between the stellar and planetary winds correctly explains the observed absorption at Doppler-shift velocities around ±100 km s⁻¹.