JWST detects mineral clouds in WASP-94Ab's atmosphere

Sagnick Mukherjee and colleagues reported in *Science* on May 21 that NASA’s James Webb Space Telescope separated the morning and evening sides of the hot Jupiter WASP-94A b and found a repeating cloud cycle in its atmosphere. The team said the planet’s cooler morning side is covered by mineral clouds, while the hotter evening side is relatively clear and shows stronger water vapor signatures. The work used transit observations to measure the two limbs of the planet separately rather than averaging the whole atmosphere together. That approach, the researchers said, changes how astronomers read cloud-covered exoplanet atmospheres. ### How did Webb see different weather on opposite sides of one planet? The James Webb Space Telescope observed WASP-94A b as it crossed in front of its star, allowing researchers to isolate the planet’s leading and trailing edges during transit. Johns Hopkins University said the leading edge corresponds to morning conditions, where air moves from the night side to the day side, while the trailing edge corresponds to evening conditions as air moves back toward night. (science.org) The *Science* paper said the transmission spectrum showed atmospheric asymmetry at 6 sigma, with an 11-sigma detection of a cloudy, cooler morning limb and 10-sigma water absorption on the hotter evening limb. The authors said that difference indicates the two sides cannot be treated as a single averaged atmosphere. ### What are the clouds made of? Johns Hopkins University said observations pointed to clouds made of magnesium silicate, a rock-forming mineral, on the morning side of WASP-94A b. (hub.jhu.edu) UC Santa Cruz described them as mineral or “sand” clouds that form under cooler conditions and then disappear later in the planetary day. David Sing, a co-author and principal investigator on the program at Johns Hopkins, said the new data let researchers move beyond treating hot-Jupiter clouds as a generic fog. “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. (science.org) ### Why do the clouds vanish by evening? The *Science* paper said cloud droplets likely form near the morning limb and evaporate as atmospheric circulation carries them to the hotter evening limb. (hub.jhu.edu) The authors said that process requires at least a 280-kelvin temperature difference between the two limbs. UC Santa Cruz said the clouds condense from mineral vapors on the cooler night side and are then lofted upward before daybreak. (hub.jhu.edu) By the time that material reaches the hotter side, the condensates evaporate, leaving clearer skies and a less obscured view of atmospheric gases. ### Why does this change earlier Hubble-era readings? The researchers said older observations often blended cloudy and cloud-free regions together, which could skew inferred chemical abundances. (science.org) Mukherjee said understanding the cloud cycle is necessary for determining how these planets formed and evolved and for comparing them with gas giants in the solar system. The arXiv version of the study said ignoring limb-to-limb differences can severely bias chemical inferences and means a decade of Hubble Space Telescope interpretations may need to be reassessed for some transiting planets. (news.ucsc.edu) That is the authors’ conclusion from the Webb measurements and modeling. ### What comes next for this line of research? The paper in *Science*, titled “Cloudy mornings and clear evenings on a gas giant exoplanet,” names Mukherjee, Sing and 26 other authors. (news.ucsc.edu) Johns Hopkins and UC Santa Cruz said the method could be applied to other transiting exoplanets to separate cloud effects from atmospheric chemistry more cleanly. WASP-94A b is about 700 light-years from Earth, according to the university releases, and the team said limb-resolved spectroscopy will be needed as Webb continues to characterize atmospheres from giant planets to smaller rocky worlds. (arxiv.org) (news.ucsc.edu) (science.org)