James Webb maps weather on WASP-94A b

On May 21, 2026, astronomers reported one of the clearest weather measurements yet on a planet outside the solar system: a repeating cycle of cloudy mornings and clearer evenings on WASP-94A b. The observations came from the James Webb Space Telescope and were published in Science by a team led by Sagnick Mukherjee. The planet is a hot Jupiter — a gas giant orbiting very close to its star — about 690 light-years from Earth in the constellation Microscopium. The result gives researchers a way to separate cloud effects from the underlying atmospheric chemistry, a problem that has complicated exoplanet studies for years. ### How did Webb tell morning from evening on a planet this far away? WASP-94A b is tidally locked, meaning one side permanently faces its star and the opposite side remains in darkness. That geometry creates two different “limbs” at the planet’s edge during a transit: a morning limb, where gas rotates from night into day, and an evening limb, where gas moves from day into night. By measuring starlight filtering through those regions as the planet crossed in front of its star, the team was able to compare the two sides separately. (eurekalert.org) The James Webb Space Telescope data showed that those two limbs do not look the same. Science said the morning limb is cooler and cloudy, while the evening limb is hotter and shows gaseous water absorption features more clearly. The paper reported a 6-sigma detection of atmospheric asymmetry. ### What are the clouds likely made of? Johns Hopkins University said the clouds are consistent with magnesium silicate particles, a mineral component found in common rocks. (science.org) Space and other coverage described that as sand-like material in vaporized-rock clouds. On the planet’s cooler morning side, those clouds build up; on the hotter evening side, they dissipate. The arXiv version of the paper said models suggest cloud droplets form near millibar pressures and are lofted higher in the atmosphere by strong vertical dynamics. As that material circulates to the hotter evening limb, it evaporates. The researchers said the pattern requires at least a 280-kelvin temperature difference between the two limbs. ### Why does that matter for exoplanet chemistry? (hub.jhu.edu) Science said aerosols can mute or distort the spectral fingerprints researchers use to infer atmospheric composition. In WASP-94A b, the cloudy morning limb suppresses some of those signals, while the clearer evening limb reveals stronger water features. That means a planet can look chemically different depending on which side is contributing more to the measurement. (arxiv.org) Johns Hopkins said isolating the clouds allowed researchers to get a cleaner view of the planet’s makeup. Mukherjee said in the university release that “general cloudiness has been a thorn in our side” in exoplanet work, and that the new method helps researchers distinguish cloud structure from abundance measurements. ### Where does the “100-fold bias” claim come from? (science.org) The peer-reviewed paper said ignoring this kind of limb-to-limb asymmetry can “severely bias” inferred chemical abundances and indicates a need to reassess conclusions drawn from about a decade of Hubble Space Telescope observations. Ars Technica, citing the study, reported that the effect could throw off how scientists study planetary atmospheres more broadly. (hub.jhu.edu) That “100-fold” figure appears in secondary coverage summarizing the scale of the possible error, not in the wording surfaced from the Science abstract or the Johns Hopkins release reviewed here. What the primary sources do say is that unresolved clouds can materially skew abundance estimates unless limb-resolved spectroscopy is available. ### What comes next for this method? (arxiv.org) The Science paper said limb-resolved spectroscopy is critical for characterizing transiting exoplanets, from gas giants to terrestrial worlds. Johns Hopkins and AAAS both framed the WASP-94A b result as a demonstration that cloud cycles can be detected and separated from chemistry with Webb-quality data. Future follow-up will depend on applying the same approach to other transiting planets with complex atmospheres. (science.org)