Exoplanets · Space Codex
WASP-39 b
A puffy hot Saturn that became JWST's benchmark: the first clear carbon dioxide in an exoplanet atmosphere and the first evidence of photochemistry on another world.
JWST's first exoplanet carbon dioxide
Artist's concept. No telescope has seen WASP-39 b as more than a point of light, and most exoplanets not even that. What we know comes from how a planet dims or tugs its star. Any picture of its surface, colour or sky, anywhere, is an illustration, not an observation.
At a glance
Key points
- First clear CO₂ in an exoplanet atmosphere
- First evidence of exoplanet photochemistry
- Evening side ~200 K hotter than morning side
How it was found
The SuperWASP survey, banks of small wide-field cameras scanning millions of stars, flagged a 4-day dip in 2011. Follow-up spectra weighed the planet at just over a quarter of Jupiter's mass, about Saturn's, yet it is wider than Jupiter. It was one of the least dense planets then known, its atmosphere inflated by heat from a star only about 0.05 AU away.
Why JWST chose it
A light, puffy atmosphere stretches far above the cloud tops, so starlight filtering through it carries large spectral fingerprints. The star is also quiet, with few spots to confuse the signal. In July 2022 the Early Release Science programme observed its transits with four instrument modes spanning about 0.5 to 5 microns, overlapping on purpose so each could check the others. Later MIRI spectra extended the view to longer wavelengths.
A chemistry lab
Starlight splits hydrogen sulfide high in the atmosphere, and the freed sulfur combines with oxygen to make sulfur dioxide. Water, carbon monoxide, sodium and potassium were detected too.
Two terminators
Measured separately, the evening edge is about 200 K hotter than the morning edge, and the morning side is likely cloudier: winds of thousands of km/h carry heat around the planet.
What we still don't know
What its clouds are made of, why they gather on the morning side, and exactly how the planet collected its heavy elements. Its colour in visible light has never been measured.
Missions
- 2021–
Infrared observatory at Sun–Earth L2, about 1.5 million km from Earth.
Related entries
- James Webb Space TelescopeInfrared observatory at L2
- HD 189733 bA cobalt-blue world that smells of rotten eggs
Sources
- Powell et al. 2024, Nature (opens in a new tab)
- STScI: JWST ERS programme 1366 overview (opens in a new tab)
- NASA Exoplanet Archive: WASP-39 overview (opens in a new tab)
- JWST Transiting Exoplanet Community ERS Team 2023, Nature 614:649 (arXiv) (opens in a new tab)
- Tsai et al. 2023, Nature (arXiv) (opens in a new tab)
- UChicago: Webb peers at day–night boundary (opens in a new tab)
- Faedi et al. 2011, A&A (arXiv) (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.