HD 209458 b

Planet· orbits the Sun

HD 209458 b ('Osiris') — a transiting hot Jupiter (~0.73 Jupiter masses, 1.39 Jupiter radii) skimming its star every 3.5 days. As the FIRST transiting exoplanet, it gave the first measured size and density of any world beyond the Sun; Hubble later found sodium in its air (the first exoatmosphere) and a huge hydrogen tail boiling into space. Its cloud tops have never been imaged.

Open in 3D viewSee HD 209458 b in the live, interactive 3D map

Key facts

Mass

1.39 × 10²⁷ kg

0.73 M♃ (232 M⊕)

Radius

99,263 km

15.6× wider than Earth

Surface gravity

9.39 m/s²

96% of Earth's

Year

3.52 Earth days

Distance from HD 209458

7,041,572 km

Science

Physical

Mass: 1.39 × 10²⁷ kg · 0.73 M♃ (232 M⊕)
Radius: 99,263 km · 15.6× wider than Earth
Density: 338 kg/m³ · 0.34× water
Surface gravity: 9.39 m/s² · 96% of Earth's
Escape velocity: 43.2 km/s · 3.86× Earth's

Orbital

Semi-major axis: 7,041,572 km
Eccentricity: 0
Inclination: 0.00 °
Orbital period: 3.5 days
Perihelion: 7,041,572 km
Aphelion: 7,041,572 km
Mean orbital velocity: 145 km/s

Composition

Bulk composition: A hot Jupiter of ~0.73 Jupiter masses and 1.39 Jupiter radii; because it transits with a measured mass, its low density is real — a puffed-up gas giant. Hubble detected sodium in its air (the first exoatmosphere, 2001) and later a comet-like tail of escaping hydrogen. Its cloud tops have never been imaged.
Cloud appearance: Appearance based on NASA/JPL-Caltech's artist's concept. No spacecraft has imaged this world, so its real appearance is unknown — The banded clouds shown are a representation of its class, shown only in Vision mode; Realism shows a plain spectral colour.

Radiation & temperature

Insolation: 793 S⊕ · 793× the sunlight Earth receives
Equilibrium temperature: 1,459 K (1186 °C)

Discovery & history

Method: Transit detection
Name: Designated 'b' under the IAU convention that letters a star's planets 'b', 'c', 'd'... in the order they are found (the star is the unwritten 'a'); no IAU proper name has been assigned.

Found by radial velocity in 1999 and caught transiting the same year — the first exoplanet transit ever seen. In 2001 it became the first exoplanet with a detected atmosphere (sodium, Hubble), and in 2003–2010 the first seen to be evaporating, trailing a comet-like tail of escaping hydrogen.

Images

Artist's impression of the gas giant HD 209458 b ('Osiris') orbiting its Sun-like star — an artist's concept, not a photograph. — NASA/ESA/STScI, G. Bacon (public domain) · Wikimedia Commons

HD 209458 b emerging from behind its star — the first exoplanet seen transiting and the first found to have an atmosphere. Artist's concept. — NASA (public domain) · Wikimedia Commons

Images from open archives · credit shown per image