Aeronomy of Extra-Solar Giant Planets

The intense stellar UV radiation field incident upon extra-solar giant planets causes profound changes to their upper atmospheres. Upper atmospheric temperatures can be tens of thousands of kelvins, causing thermal dissociation of H-2 to H. The stellar ionizing flux converts H to H+. The high temperatures also drive large escape rates of H, but for all but the planets with the smallest orbits, this flux is not large enough to affect planet evolution. The escape rate is large enough to drag off heavier atoms such as C and O. For very small orbits, when the hill sphere is inside the atmosphere, escape is unfettered and can affect planet evolution.