Physical and chemical aeronomy of HD 209458b

We report on the physical and chemical aeronomy of the hot Jupiter HD 209458b, a prominent case in the growing sample of known extrasolar planets. Our work is motivated by the recent detections of hydrogen, carbon and oxygen atoms obscuring about one tenth of the disk of the host star at the detection wavelengths and which have been interpreted as evidence for an escaping atmosphere. We model the escape and composition of the irradiated atmosphere by solving the equations of mass, momentum and energy conservation. At an orbital distance a similar to 0.05 AU, intense Extreme Ultraviolet stellar irradiation may lead to the massive escape of its atmosphere. It is shown that for a planet of the characteristics of HD 209458b at small enough orbital distances, tidal forces may enhance the escape rate over the 1/a(2) law inferred from simple energetic arguments, shortening the lifetime of the planet to a few Gigayears. This conclusion is contingent upon the premise of supersonic escape, on which we have based our calculations. It is expected that the atmosphere of HD 209458b contains hydrogen, helium and trace amounts of heavier elements such as carbon, oxygen and nitrogen. Indeed, the observations indicate that some of the heavier species reach as far above the surface of the planet as the lighter hydrogen atoms. We evaluate the abundances of the likely species forming from these elements and from the deuterium isotope throughout the upper atmosphere. Beyond a few planetary radii, all elements are strongly ionized, the atoms of carbon, helium and nitrogen being the first to do so. Our model, in the scenario of solar abundance for heavy constituents appears to be consistent with the observation depths of the three detected atoms. We have implemented a mass-consistent treatment of molecular and ambipolar diffusion suitable for multi-temperature multi-component gases that can be readily implemented in the modelling of planetary atmospheres. (c) 2007 Elsevier Ltd. All rights reserved.