A stability limit for the atmospheres of giant extrasolar planets

The extrasolar 'hot Jupiter' HD209458b is orbiting close to a solar-type star and is subject to intense heating as a result. It is surrounded by an expanded atmosphere of atomic hydrogen that is escaping from the planet. Such escape is theoretically possible at least inside an orbit of 0.1 AU (one AU is the distance between Earth and the Sun). But at 5 AU from the Sun, Jupiter has a stable atmosphere. So somewhere between those extremes there must be a crossover between stability and instability. Numerical modelling now suggests that crossover occurs between 0.14 and 0.16 AU for a Jupiter-like planet.