HD 209458 b (sometimes unofficially called
Osiris) is an
exoplanet that orbits the
solar analog HD 209458 in the constellation
Pegasus, some 150
light-years from the
Solar System. The radius of the planet's orbit is 7 million kilometres, 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 °C (about 1,800 °F). Its mass is 220 times that of Earth (0.69
Jupiter masses) and its volume is some 2.5 times greater than that of Jupiter. The high mass and great volume of HD 209458 b indicate that it is a
gas giant.