Distribution of hydrogen atoms in the orbital plane of HD 209458b using best-fit parameters (escape rate Ṁ = 10¹⁰ g s^-1; ionizing flux F_ion = 4  F_⊙). The color scale indicates the time since the atoms escaped the upper atmosphere. Without self-shielding (left panel), the atoms are homogeneously accelerated away from the star by radiation pressure. With self-shielding (right panel), the internal regions of the cloud are submitted to a lower radiation pressure and are accelerated on longer timescales. The atmosphere below the exobase is uniformely filled with high-density neutral hydrogen gas (striped disk centered on the planet in deep blue).