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pyspeckitis a toolkit and library for spectroscopic analysis in Python. We describe thepyspeckitpackage and highlight some of its capabilities, such as interactively fitting a model to data, akin to the historically widely-usedsplotfunction inIRAF.pyspeckitemploys the Levenberg–Marquardt optimization method via thempfitandlmfitimplementations, and important assumptions regarding error estimation are described here. Wrappers to usepymcandemceeas optimizers are provided. A parallelized wrapper to fit lines in spectral cubes is included. As part of theastropyaffiliated package ecosystem,pyspeckitis open source and open development, and welcomes input and collaboration from the community.

Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability¹. However, no unambiguous photochemical products have been detected in exoplanet atmospheres so far. Recent observations from the JWST Transiting Exoplanet Community Early Release Science Program^2,3found a spectral absorption feature at 4.05 μm arising from sulfur dioxide (SO₂) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 M_J) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of around 1,100 K (ref. ⁴). The most plausible way of generating SO₂in such an atmosphere is through photochemical processes^5,6. Here we show that the SO₂distribution computed by a suite of photochemical models robustly explains the 4.05-μm spectral feature identified by JWST transmission observations⁷with NIRSpec PRISM (2.7σ)⁸and G395H (4.5σ)⁹. SO₂is produced by successive oxidation of sulfur radicals freed when hydrogen sulfide (H₂S) is destroyed. The sensitivity of the SO₂feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of about 10× solar. We further point out that SO₂also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.