HD 209458 b is an exoplanet located about 159 light-years away in the constellation Pegasus. It was discovered in 1999 and is classified as a "hot Jupiter" due to its large size and close orbit around its host star, HD 209458. This planet has a mass similar to that of Jupiter but orbits its star in just 3.5 days. HD 209458 b is notable for its atmosphere, which has been studied extensively, revealing the presence of water vapor and other elements. Its unique characteristics make it a key subject in the study of exoplanets.