Atmospheric escape is the loss of gas from a planet to space. In Jeans escape, particles in the high-speed tail of a velocity distribution leave a nearly collisionless upper atmosphere if they exceed the local escape velocity. Hydrodynamic escape instead treats a collisional bulk outflow. Photochemical escape supplies another route by creating energetic reaction products.
Which description applies depends on gravity, heating, composition and the collision rate, rather than only on the planetary equilibrium temperature. Absorption of energetic stellar radiation can heat high atmospheric layers far above that temperature. Removing a light envelope changes the measured radius and future exoplanet atmosphere, connecting escape to the interpretation of a planetary mass-radius relation.
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