HD 209458 B

HD 209458 b is an extrasolar planet (unofficially referred to as Osiris) that orbits the Solar analog 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 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.

HD 209458 b represents a number of milestones in extraplanetary research. It was the first of many categories: a transiting extrasolar planet discovered, an extrasolar planet known to have an atmosphere, an extrasolar planet observed to have an evaporating hydrogen atmosphere, an extrasolar planet found to have an atmosphere containing oxygen and carbon, one of the first two extrasolar planets to be directly observed spectroscopically and the first extrasolar gas giant to have its superstorm measured, and the first planet to have its orbital speed measured, determining its mass directly. 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.