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Stellar irradiation and particles forcing strongly affect the immediateenvironment of extrasolar giant planets orbiting near their parent stars. Here,we use far-ultraviolet emission spectra from HD209458 in the wavelength range(1180-1710)A to bring new insight to the composition and energetic processes inplay in the gas nebula around the transiting planetary companion. In thatframe, we consider up-to-date atmospheric models of the giant exoplanet wherewe implement non-thermal line broadening to simulate the impact on the transitabsorption of superthermal atoms (HI, OI, and CII) populating the upper layersof the nebula. Our sensitivity study shows that for all existing models, asignificant line broadening is required for OI and probably for CII lines inorder to fit the observed transit absorptions. In that frame, we show that OIand CII are preferentially heated compared to the background gas with effectivetemperatures as large as T_{OI}/T_B~10 for OI and T_{CII}/T_B~5 for CII. Bycontrast, the situation is much less clear for HI because several models couldfit the Lyman-a observations including either thermal HI in an atmosphere thathas a dayside vertical column [HI]~1.05x10^{21} cm^{-2}, or a less extendedthermal atmosphere but with hot HI atoms populating the upper layers of thenebula. If the energetic HI atoms are either of stellar origin or populationslost from the planet and energized in the outer layers of the nebula, ourfinding is that most models should converge toward one hot population that hasan HI vertical column in the range [HI]_{hot}(2-4)x10^{13} cm^{-2} and aneffective temperature in the range T_{HI}(1-1.3)x10^6 K, but with a bulkvelocity that should be rather slow.

On the existence of energetic atoms in the upper atmosphere of exoplanet HD209458b