Views of GJ 436 b exosphere (blue dots) from the perpendicular to the orbital plane. The planet is the small disk at the intersection of the star/Earth LOS (dashed black line) and the planetary orbit (green line). All system properties are similar in the three panels, except for radiation pressure. Upper panel: without radiation pressure, stellar gravity shears the extended cloud of hydrogen, and gas at shorter orbital distance than the planet falls toward the star. Middle panel: with even a moderate radiation pressure (~60% of stellar gravity at maximum), all the escaping gas decelerates and moves to larger orbits, forming a longer comet-like tail trailing behind the planet. This is the real radiation pressure that corresponds to the observed Lyman-α flux. Lower panel: with a high radiation pressure (~400% of stellar gravity at maximum), the escaping gas is swiftly blown away. The planet is surrounded by a much smaller coma but trailed by a narrower, more radial, cometary-tail.