HD 209458b was the first extrasolar planet discovered that transits in front of its parent star. The size of the planet can be measured using the total optical obscuration of the stellar disk during an observed transit, and the structure and composition of the planetary atmosphere can be studied using additional planetary absorption signatures in the stellar spectrum. HD 209458 is a seventh- magnitude star about 150 light-years away in the constellation Pegasus. The star, which has physical characteristics similar to those of the Sun, was shown in late 1999 to have a planet, HD 209458b, by detection of the planet’s gravitational effects on the star’s motion. In the case of the planet HD 209458b, this energy deposition results in a hydrodynamic state in the upper atmosphere, allowing for sizeable expansion and … Extrasolar Planet HD 209458 b (Osiris) The exoplanet HD 209458 b is one of only a handful that has a nickname (Osiris).It was one of the first transiting exoplanets to be discovered, and is one of the best-studied hot Jupiters.There are many results in the literature concerning the physical properties of HD 209458 b and the system in which it resides.