HD 209458 is one of the benchmark objects in the study of hot Jupiter atmospheres and their evaporation through planetary winds .
The expansion of the planetary atmosphere is thought to be driven by high-energy EUV and X-ray irradiation .
We obtained new Chandra HRC-I data , which unequivocally show that HD 209458 is an X-ray source .
Combining these data with archival XMM- Newton observations , we find that the corona of HD 209458 is characterized by a temperature of about 1 MK and an emission measure of 7 \times 10 ^ { 49 } cm ^ { -3 } , yielding an X-ray luminosity of 1.6 \times 10 ^ { 27 } erg s ^ { -1 } in the 0.124 - 2.48 keV band .
HD 209458 is an inactive star with a coronal temperature comparable to that of the inactive Sun but a larger emission measure .
At this level of activity , the planetary high-energy emission is sufficient to support mass-loss at a rate of a few times 10 ^ { 10 } g s ^ { -1 } .