Which exoplanet is nicknamed Osiris?

HD 209458 b is the exoplanet nicknamed Osiris.

The nickname refers to the planet’s intensely evaporating atmosphere. Its star’s radiation heats the planet so strongly that hydrogen escapes into space, creating a tail-like outflow. Astronomers compared this loss of planetary material with the Egyptian god Osiris, whose body was dismembered in mythology.

HD 209458 b is a hot Jupiter orbiting the Sun-like star HD 209458 in Pegasus, about 157 light-years away. It was discovered in 1999 and soon became a landmark object: observations of its transit provided the first evidence that an exoplanet crossed its star as seen from Earth, and later studies detected atmospheric gases and escaping hydrogen.

“Osiris” is an informal nickname, not the planet’s formal scientific designation. The name HD 209458 b follows the standard convention: HD 209458 identifies the host star, and lowercase b marks the first planet found around it. It is also often confused with other famous evaporating hot Jupiters, including WASP-12b.