Journal
NATURE
Volume 450, Issue 7171, Pages 845-848Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/nature06378
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Recent observations of the planet HD209458b indicate that it is surrounded by an expanded atmosphere of atomic hydrogen that is escaping hydrodynamically(1-3). Theoretically, it has been shown that such escape is possible at least inside an orbit of 0.1 AU ( refs 4 and 5), and also that H-3(+) ions play a crucial role in cooling the upper atmosphere(5,6). Jupiter's atmosphere is stable(7), so somewhere between 5 and 0.1 AU there must be a crossover between stability and instability. Here we show that there is a sharp breakdown in atmospheric stability between 0.14 and 0.16 AU for a Jupiter- like planet orbiting a solar- type star. These results are in contrast to earlier modelling(4,8) that implied much higher thermospheric temperatures and more significant evaporation farther from the star. ( We use a three- dimensional, time- dependent coupled thermosphere - ionosphere model(6) and properly include cooling by H-3(+) ions, allowing us to model globally the redistribution of heat and changes in molecular composition.) Between 0.2 and 0.16 AU cooling by H-3(+) ions balances heating by the star, but inside 0.16 AU molecular hydrogen dissociates thermally, suppressing the formation of H-3(+) and effectively shutting down that mode of cooling.
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