The GJ 436 Lyα spectrum during the transit of GJ 436b. The star spectrum before or after the transit is shown by a thick grey line. The spectrum resulting from the absorption by the planetary exophere during the transit is shown for various escape rates. The blue dotted, the red dot-dashed, and black long-dashed lines show the resulting spectrum when the escape rate is 10⁸, 10⁹, and 10¹⁰ g s^-1, respectively. With this last escape rate, the blue side of the line is absorbed by about 17%, resulting in a decrease of about 11% in the total Lyman-α flux.