WASP-94A b shows dual atmosphere

Johns Hopkins-led astronomers said on May 21 that the James Webb Space Telescope had detected sharply different conditions on opposite edges of the exoplanet WASP-94A b during a transit. The team reported a cloudier, cooler “morning” limb and a clearer, hotter “evening” limb, using separate measurements of the planet’s leading and trailing edges as it crossed its star. The result gives researchers one of the clearest cases yet of a giant exoplanet showing repeatable cloud cycling rather than a single, uniform atmospheric state. The work was published in *Science*, and a detailed paper describing the measurements is also available as a preprint. ### How did astronomers tell morning from evening on a planet hundreds of light-years away? JWST observed WASP-94A b during a transit, when the planet passed in front of its host star and starlight filtered through the planet’s atmosphere. Johns Hopkins said the telescope measured the leading edge at the start of the transit and the trailing edge at the end, letting researchers compare two different atmospheric slices. On this tidally locked planet, air on the leading edge moves from night to day, making that side the morning limb, while air on the trailing edge moves from day to night, making it the evening limb. (hub.jhu.edu) WASP-94A b is a hot Jupiter orbiting the F8 star WASP-94A in the constellation Microscopium. The planet was previously reported to have a radius of about 1.72 times Jupiter’s, a mass of about 0.452 Jupiter masses and an orbital period of 3.95 days, with a retrograde orbit measured through the Rossiter-McLaughlin effect. ### What exactly was different between the two sides? The arXiv paper reported a 6-sigma detection of limb asymmetry in the transmission spectrum of WASP-94A b. (hub.jhu.edu) The authors said the morning limb was cloud-covered at 11 sigma, while the evening limb showed strong water absorption at 10 sigma and appeared comparatively clear. Johns Hopkins said the morning clouds are made of magnesium silicate, a mineral common in rocks. The university said the data indicate clouds build up on the cooler morning side and then disappear by evening, leaving clearer skies on the hotter side. (arxiv.org) ### Why would one side stay cloudy while the other clears? The preprint said models point to cloud droplets forming near millibar pressures and being lofted to about 0.01 millibar by strong vertical dynamics on the morning limb. (arxiv.org) The authors said those droplets then evaporate after circulation carries them to the hotter evening limb, implying a minimum 280-kelvin temperature difference between the two limbs. David Sing, a co-author and program principal investigator at Johns Hopkins, said clouds on hot Jupiters have long obstructed atmospheric measurements “like trying to look at the planet through a foggy window.” He said the new observations let researchers identify what the clouds are made of and how they condense and evaporate as they move around the planet. (hub.jhu.edu) ### Why does that matter beyond this one planet? (arxiv.org) The preprint said ignoring these cloud differences can severely bias inferred chemical abundances in exoplanet atmospheres. The authors said limb-resolved spectroscopy is therefore important for understanding how transiting exoplanets formed and for reassessing conclusions drawn from earlier Hubble Space Telescope observations. Nature, in a news article published this week, described the result as a view of weather patterns revealed by the way WASP-94A b filters light from its parent star. (hub.jhu.edu) That report put the system at about 690 light-years away, while some secondary reports rounded the distance to roughly 600 or nearly 700 light-years. ### Where can readers find the next layer of detail? The May 21 *Science* publication is the formal journal version of the result, and the arXiv manuscript titled *Cloudy mornings and clear evenings on a giant extrasolar world* lays out the detection significance, cloud model and temperature constraints in full. (arxiv.org) Johns Hopkins and partner institutions also published research summaries on May 21 describing the JWST observing method and the magnesium-silicate cloud interpretation. (nature.com)