JWST maps WASP-94A b weather 700 light-years

Johns Hopkins University researchers used the James Webb Space Telescope to do something exoplanet studies usually cannot: split one planet’s atmosphere into a morning side and an evening side. On WASP-94A b, a hot Jupiter nearly 700 light-years from Earth, the team found a cooler, cloudier morning limb and a hotter, clearer evening limb. The observations were published on May 21 in Science. The work points to a practical problem for astronomers, because many earlier measurements treated exoplanet atmospheres as if conditions were uniform around the planet. ### How did Webb separate “morning” from “evening” on one distant planet? WASP-94A b is tidally locked, meaning one side permanently faces its star and the other remains in darkness. As the planet crossed in front of its star, the team used JWST’s Near Infrared Imager and Slitless Spectrograph, or NIRISS, to measure the leading edge at the start of transit and the trailing edge at the end. That let them sample air moving from night to day on the “morning” side and from day to night on the “evening” side. (hub.jhu.edu) Sagnick Mukherjee and colleagues reported a 6-sigma detection of atmospheric asymmetry. Their paper said the morning limb was cloud-covered, while the evening limb was clearer and showed strong water-vapor absorption. ### What kind of weather did the telescope find? The Johns Hopkins team said mornings on WASP-94A b are “riddled with clouds” while evenings are comparatively clear. The cloud material is consistent with magnesium silicate, a rock-forming mineral, rather than a haze made mainly by photochemistry. (hub.jhu.edu) AAAS said the two limbs differ by roughly 450 kelvin in the circulation-model analysis used to interpret the data. (arxiv.org) The researchers said clouds likely form on the cooler night side, move toward the morning limb, and then evaporate as they circulate into the intensely heated day side. ### Why does that matter for previous exoplanet measurements? Science’s summary of the paper said averaged spectra can be biased when one side is cloudy and the other is clear. (hub.jhu.edu) The arXiv version of the study said ignoring these effects “severely biases” inferred chemical abundances and called for a reassessment of a decade of Hubble Space Telescope-era inferences. David Sing, a co-author and Johns Hopkins professor, said clouds have long acted like “a foggy window” for exoplanet studies. (eurekalert.org) He said the new observations let researchers pin down what the clouds are made of and how they condense and evaporate as they move around the planet. ### Is the “100-fold bias” claim actually about this paper? The Science and Johns Hopkins materials do not use that exact phrase. (science.org) They do say that assuming a uniform atmosphere can significantly distort inferred chemistry and physical properties, and outside coverage has described the effect as reaching roughly two orders of magnitude in some abundance estimates. That is an inference from the paper’s broader warning about asymmetric atmospheres, not a standalone headline number in the primary release. (hub.jhu.edu) ### What comes next for this method? The May 21 paper presents limb-resolved spectroscopy as a tool that can be applied beyond this one planet. Mukherjee and colleagues wrote that the method is important for characterizing transiting exoplanets “from gas giants to terrestrial exoplanets,” suggesting future JWST studies can test whether similar day-night cloud cycles appear on other worlds. (arxiv.org) (eurekalert.org)