James Webb maps WASP-94A b cloud cycle

James Webb Space Telescope data have given astronomers one of their clearest looks yet at weather on a planet outside the solar system. In a study published in *Science* on May 21, researchers reported that the hot Jupiter WASP-94A b shows a repeating cloud cycle, with thick clouds on its morning side and clearer skies on its evening side. The planet lies about 700 light-years from Earth in the constellation Microscopium, according to the research team. (news.ucsc.edu) The observations were made during a transit, when the gas giant passed in front of its star and let Webb sample different edges of its atmosphere separately. ### How did Webb tell morning from evening on a planet this far away? David Sing’s team used the James Webb Space Telescope to measure the planet’s leading and trailing edges at different moments of the transit. The leading edge carries air moving from the nightside to the dayside, which researchers described as the morning limb, while the trailing edge corresponds to the evening limb as air moves back toward night. Sagnick Mukherjee said earlier telescopes effectively blurred those regions together. (news.ucsc.edu) Webb’s spectral measurements let the team separate those two atmospheric slices instead of relying on a single planet-wide average, he said. ### What exactly did the researchers find in those two regions? WASP-94A b’s morning side was found to be cooler and cloud-covered, while its evening side was hotter and comparatively clear. The study reported a 6-sigma detection of limb asymmetry, including an 11-sigma signal for the cloudy morning limb and a 10-sigma water-vapor signal on the clearer evening limb. (hub.jhu.edu) Johns Hopkins and Arizona State University summaries of the paper said the clouds are likely made of magnesium silicate, a mineral found in rocks, rather than water droplets. (news.asu.edu) Sing said the data allowed the team to pin down what the clouds are made of and how they condense and evaporate as they move around the planet. ### Why are these described as “sand clouds”? Magnesium silicate is a rock-forming mineral, so researchers described the clouds as mineral or sand-like clouds rather than water clouds. (arxiv.org) On a hot Jupiter such as WASP-94A b, temperatures are high enough that rocky materials can vaporize, condense and circulate through the atmosphere. The arXiv version of the paper said the pattern requires at least a 280-kelvin temperature difference between the two limbs. The authors said cloud particles likely form deeper in the atmosphere on the cooler side, are lofted upward on the morning limb, and then evaporate after circulating to the hotter evening side. (hub.jhu.edu) ### Why does splitting the planet into regions matter? Older observations with Hubble often produced an averaged transmission spectrum that mixed cloudy and clear regions together. Mukherjee said that made clouds “a thorn” for exoplanet studies because they obscured the underlying chemistry. (hub.jhu.edu) Webb’s limb-by-limb approach gave the team a clearer view of the atmosphere beneath the cloud deck, according to the university summaries. UC Santa Cruz said the clearer evening-side view suggested a composition closer to Jupiter’s than earlier blended observations implied. (arxiv.org) The researchers said that isolating clouds should help them better constrain how hot Jupiters formed and evolved. ### What comes next for this line of research? The May 21 *Science* paper positions WASP-94A b as an early test case for regional exoplanet weather mapping with Webb. Johns Hopkins and UC Santa Cruz said the same method could now be used on other hot Jupiters to compare cloud chemistry, circulation and temperature contrasts across different worlds. (hub.jhu.edu) Sagnick Mukherjee, now a postdoctoral fellow at Arizona State University, said understanding this cloud cycle is important for tracing how such planets formed and evolved. (news.ucsc.edu) Further Webb observations and follow-on modeling are expected to focus on other cloudy exoplanets and on whether similar morning-evening asymmetries appear elsewhere. (hub.jhu.edu)