WASP-121 b

Planet· orbits the Sun

WASP-121 b — an ultra-hot Jupiter (~1.16 Jupiter masses, 1.75 Jupiter radii) orbiting in just 30 hours, its dayside seething above 2,400 K. Because it transits with a measured mass, its density is real — a vastly inflated world. Telescopes have seen glowing water, a hot stratosphere, and iron and magnesium streaming off into space. Its glowing air has never been imaged directly.

Open in 3D viewSee WASP-121 b in the live, interactive 3D map

Key facts

Mass

2.22 × 10²⁷ kg

1.17 M♃ (372 M⊕)

Radius

124,400 km

19.5× wider than Earth

Surface gravity

9.58 m/s²

1 g (defines the unit)

Year

1.27 Earth days

Distance from WASP-121

3,846,161 km

Science

Physical

Mass: 2.22 × 10²⁷ kg · 1.17 M♃ (372 M⊕)
Radius: 124,400 km · 19.5× wider than Earth
Density: 275 kg/m³ · 0.28× water
Surface gravity: 9.58 m/s² · 1 g (defines the unit)
Escape velocity: 48.8 km/s · 4.36× Earth's

Orbital

Semi-major axis: 3,846,161 km
Eccentricity: 0
Inclination: 0.00 °
Orbital period: 30.60 hours
Perihelion: 3,846,161 km
Aphelion: 3,846,161 km
Mean orbital velocity: 219 km/s

Composition

Bulk composition: An ultra-hot Jupiter of ~1.16 Jupiter masses and 1.75 Jupiter radii; because it transits with a measured mass, its very low density is real — a grossly inflated world. Telescopes have seen glowing water, a hot stratosphere, and iron and magnesium streaming off the planet. Its glowing air has never been imaged directly.
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: 5,615 S⊕ · 5,615× the sunlight Earth receives
Equilibrium temperature: 2,409 K (2136 °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.

Discovered transiting by the WASP survey in 2015. Its extreme heat and puffed-up size made it a premier target: Hubble and later JWST mapped water changing state across it and caught heavy metals escaping the planet — a world being boiled away.

Images

NASA/JPL computer-simulation views of the ultra-hot Jupiter WASP-121 b, whose atmosphere is glowing and escaping — a simulation, not a photograph. — NASA/JPL-Caltech/Aix-Marseille University/V. Parmentier · Wikimedia Commons

Images from open archives · credit shown per image