James Webb detects mineral clouds on WASP-94A b

NASA’s James Webb Space Telescope has given astronomers one of their clearest looks yet at weather on a world beyond the solar system. In a paper published May 21 in Science, researchers reported that the hot Jupiter WASP-94A b shows sharply different conditions at opposite edges of its atmosphere during transit, with cloudier “mornings” and clearer “evenings.” The planet orbits the star WASP-94A about 690 light-years from Earth in the constellation Microscopium, according to Johns Hopkins University and the underlying research. The result offers a case study in how exoplanet atmospheres can vary from one side of a planet to the other, rather than behaving like a single uniform shell. ### How did Webb see different weather on the same planet? The team used JWST’s Near Infrared Imager and Slitless Spectrograph, or NIRISS, to watch WASP-94A b pass in front of its host star and to separate the signal from the planet’s leading and trailing edges. Johns Hopkins said the leading edge corresponds to the planet’s “morning” limb, where air is moving from night side to day side, while the trailing edge corresponds to its “evening” limb, where air flows from day side to night side. (science.org) Science reported that the transmission spectrum showed atmospheric asymmetry: the morning limb was cooler and cloudy, while the evening limb was hotter and displayed gaseous water absorption. The paper described this as a measurement of the tidally locked gas giant’s atmosphere, with one side of the terminator obscuring spectral features more strongly than the other. ### What are the clouds thought to be made of? (hub.jhu.edu) Johns Hopkins said the morning side is “riddled with clouds made of magnesium silicate,” a mineral commonly found in rocks. The Science summary and the arXiv version of the paper describe the aerosols as condensation-driven clouds rather than photochemical hazes, meaning the particles are consistent with minerals condensing out of hot gas and then cycling through the atmosphere. (science.org) David Sing, a co-author and principal investigator on the program at Johns Hopkins, said clouds have long limited efforts to read exoplanet atmospheres. “We can finally pin down what the clouds are made out of and how they’re condensing and evaporating as they move around the planet,” Sing said in the university release. ### Why do the clouds appear in the morning and fade by evening? The paper reported a 6-sigma detection of limb asymmetry, with an 11-sigma cloud-covered cooler morning limb and a 10-sigma water signal on the clearer evening limb. (hub.jhu.edu) The authors said models suggest cloud droplets form near millibar pressures, are lofted upward on the morning side, and then evaporate after circulating to the hotter evening side. EurekAlert’s summary of the Science paper said a 3D circulation model pointed to temperature contrasts of roughly 450 kelvin between the two hemispheres. (hub.jhu.edu) The arXiv abstract said the observations require at least a 280-kelvin limb-to-limb temperature difference. Those figures describe related but not identical model outputs, and both point to a large thermal contrast driving the cloud cycle. ### Why does this matter for exoplanet studies? (arxiv.org) The authors said treating an exoplanet atmosphere as uniform can bias measurements of chemical abundances and physical properties. In the Science summary, they said the asymmetry could “severely bias” abundance estimates unless limb-resolved spectroscopy is available. Johns Hopkins said isolating the clouds lets researchers measure the atmosphere more accurately and gives a clearer picture of the planet’s composition. (e3.eurekalert.org) The broader implication, according to the paper and the university release, is that similar cloud cycles may need to be considered when interpreting other hot Jupiters and, eventually, smaller transiting planets. The Science paper was published on May 21, 2026, under the title “Cloudy mornings and clear evenings on a gas giant exoplanet,” led by Sagnick Mukherjee with David Sing and other co-authors. (science.org)