is a fundamental process of planetary atmospheres, regulating what they are made of and how stable they stay. It is how ozone exists above our heads. In the atmosphere of another star’s planet, no unambiguous photochemical product had been identified at all.
Now it has, in sulfur dioxide. The James Webb Space Telescope found an at 4.05 micrometres in the atmosphere of WASP-39b, and a suite of photochemical models reproduces it robustly. Starlight breaks up , and the freed sulfur radicals are oxidized step by step into sulfur dioxide.
Photochemistry is a fundamental process of planetary atmospheres that regulates atmospheric composition and stability. Even so, no unambiguous photochemical products had been detected in exoplanet atmospheres, which left a basic piece of planetary chemistry untested away from home.
Observations from the JWST Transiting Exoplanet Community Early Release Science Program found a spectral absorption feature at 4.05 micrometres arising from sulfur dioxide in the atmosphere of WASP-39b. The planet is a gas giant of 1.27 times the radius of Jupiter and about the mass of Saturn, orbiting a Sun-like star.
Its equilibrium temperature of around 1,100 K is hotter than anything orbiting our own Sun. In an atmosphere like that, the most plausible way of generating sulfur dioxide is through photochemical processes, which is why the detection matters beyond the molecule itself.
A suite of photochemical models computes a sulfur dioxide distribution that robustly explains the feature seen with NIRSpec PRISM and, at 4.5 sigma, with G395H. The gas comes from successive oxidation of sulfur radicals freed when hydrogen sulfide is destroyed. Its sensitivity to suggests it can serve as a tracer, with WASP-39b inferred at about 10 times solar.
wat. Starlight tore up hydrogen sulfide and left sulfur dioxide behind.
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The strength of the feature tracks how enriched the atmosphere is in heavy elements, so one molecule becomes a measuring tool.
How do we know?
Tsai and colleagues ran photochemical models against the JWST transmission spectra from NIRSpec, where the feature shows at 4.5 sigma in one mode and more weakly than that in the other.
What is still unknown
The chemistry is inferred from models matched to a spectrum rather than sampled in place.
Learn something?
Where to next
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