HD 209458 b is an extrasolar planet that orbits the Sunlike star HD 209458 in the constellation Pegasus,The radius of the planet's orbit is one eighth the radius of Mercury's orbit. Combined with the fact that the planet is very large, it is yet another 'hot Jupiter'.It is the first transiting extrasolar planet discovered, It is also the first extrasolar planet known to have an atmosphere, and the first extrasolar planet found to have an atmosphere containing oxygen and carbon.