WASP-39 b

Planet· orbits the Sun

WASP-39 b — a 'hot Saturn' (~0.28 Jupiter masses but 1.28 Jupiter radii) skimming its star every 4 days. Because it transits AND its mass is measured, its density is real — extraordinarily puffy, less dense than cork. In 2022 JWST read its air in detail, finding the first unambiguous carbon dioxide at any exoplanet, plus water, SO₂ and clouds. Its cloud tops have never been imaged.

Open in 3D viewSee WASP-39 b in the live, interactive 3D map

Key facts

Mass

5.33 × 10²⁶ kg

89.3 M⊕

Radius

91,360 km

14.3× wider than Earth

Surface gravity

4.27 m/s²

43% of Earth's

Year

4.06 Earth days

Distance from WASP-39

7,222,585 km

Science

Physical

Mass: 5.33 × 10²⁶ kg · 89.3 M⊕
Radius: 91,360 km · 14.3× wider than Earth
Density: 167 kg/m³ · 0.17× water
Surface gravity: 4.27 m/s² · 43% of Earth's
Escape velocity: 27.9 km/s · 2.5× Earth's

Orbital

Semi-major axis: 7,222,585 km
Eccentricity: 0
Inclination: 0.00 °
Orbital period: 4.1 days
Perihelion: 7,222,585 km
Aphelion: 7,222,585 km
Mean orbital velocity: 130 km/s

Atmosphere

Composition: Hydrogen-dominated, and the best-read exoplanet atmosphere of its kind: in 2022 JWST's first exoplanet observations delivered the first unambiguous carbon dioxide detected at any exoplanet, alongside water, sulphur dioxide (evidence of photochemistry) and patchy clouds — a landmark opening the era of detailed exoplanet atmospheric chemistry.

Composition

Bulk composition: A 'hot Saturn' of ~0.28 Jupiter masses but 1.28 Jupiter radii — extraordinarily puffy, with a real, measured density less than that of cork, because it both transits and has a measured mass.
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: 308 S⊕ · 308× the sunlight Earth receives
Equilibrium temperature: 1,166 K (893 °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 found (the star is the unwritten 'a'); no IAU proper name has been assigned.

Discovered transiting by the WASP survey in 2011; in 2022 it became the target of JWST's landmark first exoplanet atmosphere results — the debut clear CO₂ detection.

Images

Artist's impression of the puffy hot Saturn WASP-39 b and its star — an artist's concept, not a photograph. — NASA / ESA / CSA / STScI, M. Zamani (CC BY 4.0) · Wikimedia Commons

JWST transmission spectrum of WASP-39 b — the first unambiguous carbon-dioxide detection at any exoplanet (a data graphic, 2022). — NASA / ESA / CSA / STScI, J. Olmsted (CC BY 4.0) · Wikimedia Commons

Images from open archives · credit shown per image