HD 209458 b (unofficially named Osiris) is an extrasolar planet that orbits the solar analogue 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 (the distance from Earth to the Sun is 1 astronomical unit [150,000,000 km] or AU), 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. 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 extra planetary research. It was the first of many categories:
- a transiting extrasolar planet
- the first planet detected through more than one method
- 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
- the first extrasolar gas giant to have its super storm measured
- 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 vapour in its atmosphere.
In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter’s.