JWST spots mineral clouds on hot Jupiter

On May 21, a team using NASA’s James Webb Space Telescope reported one of the clearest signs yet of weather on a world outside the solar system: mineral clouds that build on the morning side of the hot Jupiter WASP-94A b and fade by evening. The findings, published in *Science*, came from transit observations that let researchers separate the planet’s leading and trailing edges as it crossed in front of its star. (news.ucsc.edu) That split showed a cooler, cloud-covered morning limb and a hotter, clearer evening limb with water absorption visible in the spectrum. (arxiv.org) The planet lies about 690 to 700 light-years away in the constellation Microscopium. ### How did JWST tell morning from evening on a planet it cannot image directly? JWST measured starlight filtering through different edges of WASP-94A b during transit, the Johns Hopkins and UC Santa Cruz teams said. Because the planet is tidally locked, the leading edge corresponds to air rotating from night into day — effectively morning — while the trailing edge samples air moving from day toward night, or evening. The researchers said older observations usually blended those two limbs together. By separating them, the team could compare two different local times on the same planet instead of averaging them into a single, blurrier atmospheric reading. (news.ucsc.edu) ### What exactly did the team find in the atmosphere? The *Science* paper reported a 6-sigma detection of limb asymmetry in the transmission spectrum of WASP-94A b. The morning limb was cooler and cloudy at 11-sigma significance, while the evening limb was hotter and showed strong gaseous water absorption at 10-sigma significance, according to the paper summary. (hub.jhu.edu) Johns Hopkins said the morning clouds are made of magnesium silicate, a rock-forming mineral. The team interpreted the contrast as cloud droplets forming on the cooler side of the atmosphere and evaporating as circulation carries them into hotter regions. (news.ucsc.edu) The paper said the observations require at least a 280-kelvin temperature difference between the two limbs. ### Why do “cloudy mornings” matter for exoplanet chemistry? Sagnick Mukherjee, the study’s lead author, said understanding the planet-wide cloud cycle is necessary for determining how such exoplanets formed and evolved. (arxiv.org) David Sing, a co-author at Johns Hopkins, said clouds have long obscured efforts to read hot Jupiter atmospheres, comparing the problem to looking through a foggy window. The *Science* paper said ignoring the asymmetry can severely bias chemical abundance measurements. UC Santa Cruz said that once the team isolated the cloudier and clearer regions, the atmosphere of WASP-94A b looked much closer in composition to Jupiter than earlier blended measurements had suggested. (hub.jhu.edu) ### Are these clouds haze, dust, or something else? The researchers said the dominant aerosols are likely clouds cycling between the day and night sides, not a permanent photochemical haze. That distinction matters because both mechanisms can mute spectral features, but they imply different atmospheric processes and different histories for the planet. (news.ucsc.edu) Nature said the observations showed clouds “streaming and vanishing” across the planet, while Johns Hopkins described two possible mechanisms: winds may loft clouds upward on the cooler side and then drive them downward on the hotter dayside, or the particles may simply evaporate as temperatures rise. (science.org) ### What comes next after this result? The May 21 *Science* paper said limb-resolved spectroscopy will be important for characterizing transiting exoplanets more broadly, from gas giants to smaller worlds. The authors said the result also points to the need to revisit some atmospheric inferences drawn from earlier Hubble observations that averaged cloudy and clear regions together. (science.org) Mukherjee is now a postdoctoral fellow at Arizona State University, according to UC Santa Cruz, and the named collaborators include David Sing, Guangwei Fu and Kevin Stevenson. Future JWST observations of other transiting planets will test whether morning-evening cloud contrasts are common or unusual among hot Jupiters. (nature.com) (news.ucsc.edu) (science.org)