Gas envelopes of exoplanets—hot Jupiters

We consider the physical characteristics and dynamics of the gaseous envelopes of hot Jupiters (HJs)—gas giants with a mass comparable to Jupiter's and a semi-major axis of less than 0.1 a.u. Although HJs were discovered almost a quarter of a century ago, many issues about their origin remain open. There are two reasons for the scientific interest in HJs. The first’is the absence of such planets in the Solar System, which is’challenging in all cosmogonical theories. The second is that the exoplanet atmospheres' characteristics can now be derived primarily for transit HJs by examining their absorption spectra. Thanks to their large size, such planets can be readily observed, as opposed to others, and their transits can be observed at much higher orbital inclinations. Comparatively recently, in at least some HJs, extended gas envelopes far exceeding their Roche lobes have been found. The paper focuses on the results of theoretical investigations and numerical modeling of the dynamics of HJ envelopes. We also discuss experimental testing of the obtained results and predictions using the planned Russian Spectrum-UV (international name: WSO-UV) and Millimetron space telescopes.