HD 209458 b is an extrasolar planet that orbits the Sun-like star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor.
The radius of the planet's orbit is 7 million kilometers, about 0.047 astronomical units, or one eighth the radius of Mercury's orbit. This small radius results in a year that is 3.5 Earth days long and an estimated surface temperature of about 1,000 degrees Celsius or 1,800 degrees Fahrenheit. Its mass is 220 times that of Earth (0.69 Jupiter masses). Combined with the fact that the planet is very large (32% larger than Jupiter), it is known that the planet is a gas giant.
HD 209458 b represents a number of milestones in extraplanetary research. It was the first transiting extrasolar planet discovered, the first extrasolar planet known to have an atmosphere, the first extrasolar planet observed to have an evaporating hydrogen atmosphere, the first extrasolar planet found to have an atmosphere containing oxygen and carbon, and one of the first two extrasolar planets to be directly observed spectroscopically. Based on the application of new, theoretical models, as of April 2007, it is alleged to be the first extrasolar planet found to have water vapor in its atmosphere.
HD 209458 b has been informally referred to as Osiris, though this name was not given by the astronomers credited with its discovery. The use of the Osiris nickname has faded among astronomers and the media. HD 209458 is an 8th magnitude star, visible from Earth with binoculars.