JWST maps weather on WASP-94A b

Johns Hopkins-led astronomers said on May 21 that the James Webb Space Telescope had mapped different weather on opposite sides of the hot Jupiter WASP-94A b, finding cloudy mornings and clearer evenings on the tidally locked world about 700 light-years from Earth. The work, published in Science, used Webb transit data to separate the planet’s leading and trailing edges as it crossed its star. The team said the morning side is cooler and cloudier, while the evening side is hotter and clearer. The result gives researchers a way to measure how clouds alter the atmospheric signals they use to infer what giant exoplanets are made of. ### How did Webb tell morning from evening on a planet this far away? WASP-94A b passes in front of its star from Earth’s point of view, and the team used that transit to isolate the planet’s two limbs, Johns Hopkins said. The leading edge samples air moving from the nightside toward the dayside, which researchers described as morning, while the trailing edge samples air moving from day to night, or evening. (hub.jhu.edu) The Science paper reported a 6-sigma detection of limb asymmetry, with an 11-sigma cloud-covered cooler morning limb and a hotter, clearer evening limb with strong water absorption detected at 10 sigma. The authors said the data show a minimum 280-kelvin temperature difference between the two sides. ### What are those clouds supposed to be made of? Johns Hopkins said the morning clouds are made of magnesium silicate, a mineral common in rocks. The university release described them as sand-like or rock-vapor clouds that condense on the cooler side and then disappear as material circulates into hotter regions. (hub.jhu.edu) David Sing, a Johns Hopkins professor and a co-author on the study, said clouds have long obscured efforts to read hot-Jupiter 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, according to the university release. (arxiv.org) ### Why did this change the planet’s chemistry story? The Science paper said ignoring limb-to-limb cloud differences can “severely” bias inferred chemical abundances from transmission spectra. (hub.jhu.edu) The authors wrote that aerosol cycling between the day and night sides means some earlier atmospheric readings for hot Jupiters may need to be revisited. A separate JWST study of WASP-94A b, accepted by Monthly Notices of the Royal Astronomical Society, had described the planet as relatively cloud-free and estimated a metallicity around twice solar, with carbon-to-oxygen ratios below the host star’s value. (hub.jhu.edu) Coverage of the new cloud-resolved analysis said cloud-aware modeling revises some earlier oxygen and carbon estimates from hundreds of times Jupiter’s levels to about five times Jupiter’s. That comparison is drawn from the newer reporting around the Science result rather than from the MNRAS abstract itself. (arxiv.org) ### Why were earlier readings so far off? Sagnick Mukherjee and co-authors said the problem is that a single blended spectrum can mix signals from a cloudy, cooler limb and a clearer, hotter limb. If that asymmetry is ignored, the model can compensate by inflating the apparent abundance of molecules. Ars Technica and other outlets described the result as evidence that exoplanet observers may need to reassess a decade of Hubble-era atmospheric inferences for some transiting worlds. (arxiv.org) That interpretation tracks with the paper’s statement that limb-resolved spectroscopy is critical for characterizing exoplanet atmospheres from gas giants to terrestrial planets. ### What comes next for this line of research? (arxiv.org) The May 21 Science paper points to limb-resolved spectroscopy as the next step for studying other transiting planets, including smaller worlds, because it can separate cloud effects from chemical signals. The related WASP-94A b composition study remains available through MNRAS, giving researchers a direct comparison between cloud-aware and more conventional retrieval approaches on the same planet. (arxiv.org) (arstechnica.com)