Sulfur chemistry on a warm giant

Astronomers used the James Webb Space Telescope to detect carbon disulfide, or CS2, in the atmosphere of the warm giant exoplanet WASP-80 b. (arxiv.org) The team measured the planet as it crossed in front of its star, letting starlight filter through the atmosphere like light through tinted glass. NASA says this method, called transmission spectroscopy, lets Webb identify gases from the wavelengths they absorb. (science.nasa.gov) In the new spectrum, the researchers report evidence for water, methane, carbon dioxide, ammonia, and CS2, and they place upper limits on carbon monoxide and sulfur dioxide. They observed WASP-80 b in three transits with Webb’s Near-Infrared Camera and Mid-Infrared Instrument across wavelengths from 2.4 to 10 microns. (arxiv.org) WASP-80 b is a gas giant about 0.538 times Jupiter’s mass and 0.999 times Jupiter’s radius, orbiting a K-type star every 3.1 days at 0.0344 astronomical units. NASA lists it as a 2012 discovery, making it one of the cooler giant planets now being studied with Webb rather than an ultra-hot Jupiter. (science.nasa.gov) Sulfur has been one of the least constrained ingredients in giant exoplanet atmospheres. The paper says sulfur matters because a planet’s sulfur budget can trace refractory material — the rock-forming solids available during planet formation. (arxiv.org) The reported CS2 abundance is log10X = -2.25 with an uncertainty of about 0.33 dex, which corresponds to a mixing ratio of roughly 0.6%. The authors say that is much higher than older sulfur-chemistry models predicted for a hydrogen-rich atmosphere at WASP-80 b’s temperature. (arxiv.org) Instead, the measurement lines up with newer chemical networks that let carbon and sulfur react together efficiently through CH2S, or thioformaldehyde. In plain terms, the result supports a model in which sulfur chemistry in these atmospheres cannot be treated as a side branch separate from carbon chemistry. (arxiv.org; arxiv.org) Webb had already shown sulfur dioxide in the hotter planet WASP-39 b, which NASA described as evidence of light-driven chemistry in a gas giant atmosphere. The WASP-80 b result extends sulfur detections into a cooler regime where methane is also present and different reaction pathways can dominate. (science.nasa.gov; arxiv.org) WASP-80 b had already become a key target because Webb previously found strong methane in both transmission and emission spectra at about 825 kelvin. That made it a useful test case for chemistries that are hard to see in hotter planets, where methane is often depleted. (arxiv.org; arxiv.org) The paper was posted to arXiv on April 14, 2026, and the NASA Exoplanet Archive added new Webb spectra for WASP-80 b this week. For exoplanet researchers, that means sulfur chemistry on warm giants is moving from theory papers into measured atmospheres. (arxiv.org; exoplanetarchive.ipac.caltech.edu)