A stability limit for the atmospheres of giant extrasolar planets

• Published in: Nature Vol. 450; no. 7171; pp. 845 - 848
• Main Authors: Koskinen, Tommi T., Aylward, Alan D., Miller, Steve
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 06.12.2007; Nature Publishing; Nature Publishing Group
• Subjects: Astronomy; Atmosphere; Cooling; Earth, ocean, space; Evaporation; Exact sciences and technology; Extrasolar planetary systems; Extrasolar planets; Heating; Humanities and Social Sciences; Hydrogen; Ionosphere; Ions; letter; multidisciplinary; Planets; Science; Science (multidisciplinary); Stars; Substellar companions ; planets; Temperature; Theory; Upper atmosphere; Planetary atmospheres; Extrasolar planets; Giant planet; Orbits; Evaporation; Molecular ions; Ionosphere thermosphere interaction; Jupiter planet; Atmospheric stability; Ion heating; Three dimensional model; Hydrogen ions; Instability; Hydrogen molecules; Solar type star
• ISSN: 0028-0836; 1476-4687; 1476-4687; 1476-4679
• DOI: 10.1038/nature06378

Close companions 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. Recent observations of the planet HD209458b indicate that it is surrounded by an expanded atmosphere of atomic hydrogen that is escaping hydrodynamically 1 , 2 , 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.