Hydrogen-dominated upper atmosphere of an exoplanet: Heating by stellar radiation from soft X-rays to extreme ultraviolet

Abstract

A study is presented of how the upper atmosphere of a planet is heated by extreme radiation from the parent star, depending on the distribution of the radiation flux in the soft X-ray and extreme ultraviolet (EUV) ranges. Calculations are performed to find the efficiency of heating by stellar X-ray to EUV radiation in a hydrogen-dominated upper atmosphere for the extrasolar gas giant HD 209458b. It is shown that heating efficiency by extreme stellar UV radiation in a hydrogen-dominated upper atmosphere does not exceed 20–25% at the main thermospheric heights given that the calculation takes into account the photoelectron impact. It is found that an increase in the X-ray flux by several orders of magnitude leads to a slight decrease in the heating efficiency.