JWST maps weather on WASP-94Ab

The James Webb Space Telescope has given astronomers something close to a weather map for a planet nearly 700 light-years away. In observations of the hot Jupiter WASP-94A b, researchers found that the planet’s “morning” side is veiled by mineral clouds while its “evening” side is much clearer and shows stronger water vapor signatures. The result was published on May 21 in Science by Sagnick Mukherjee and colleagues. The measurements add a day-night weather cycle to a class of planets that had often been treated as having more uniform atmospheres. ### How can Webb tell morning from evening on a planet it cannot directly image? WASP-94A b is a tidally locked gas giant, which means the same side always faces its star and the planet has permanent day and night hemispheres. During a transit, when the planet crosses in front of its star, astronomers can sample starlight filtering through different edges of the atmosphere. The leading edge corresponds to air moving from night into day — effectively morning — while the trailing edge corresponds to air moving from day into night, or evening. (eurekalert.org) The team used JWST’s Near Infrared Imager and Slitless Spectrograph, or NIRISS, to separate those two atmospheric limbs rather than averaging them together. Mukherjee said in an Arizona State University release that older Hubble observations gave “an average view of the whole planet,” with clouds and atmospheric signals blended together. Webb’s sharper data let the team localize the cloud cycle instead. (eurekalert.org) ### What did the telescope actually see in the atmosphere? The morning limb of WASP-94A b appeared cooler and much cloudier, while the evening limb appeared hotter and clearer. The Science abstract says the evening side also showed gaseous water absorption features that were muted on the cloudy morning side. In university releases summarizing the work, researchers identified the clouds as magnesium silicate — a rock-forming mineral often described as sand-like. (eurekalert.org) David Sing of Johns Hopkins said the observations let researchers pin down what the clouds are made of and how they condense and evaporate as they move around the planet. Johns Hopkins said mornings on WASP-94A b are “riddled” with magnesium silicate clouds, while evenings have clear skies. ### Why do the clouds build in one place and vanish in another? (science.org) The Science summary says the planet’s two limbs differ because cloud droplets likely form near the cooler morning limb and evaporate as circulation carries them toward the hotter evening limb. A EurekAlert summary of the paper said a 3D general circulation model indicated a temperature contrast of about 450 kelvin across the two hemispheres, enough to drive cloud formation, transport and evaporation. (hub.jhu.edu) Arizona State University and Johns Hopkins described two possible mechanisms. In one, strong winds loft clouds on the cooler side and then push them downward into deeper layers on the hotter side. In the other, the clouds form in darkness and then vaporize as they drift into dayside temperatures above 1,000 degrees. ### Why does this matter beyond one strange planet? (science.org) The paper argues that treating exoplanet atmospheres as uniform can bias measurements of their chemistry and physical properties. That matters because clouds can hide or distort the spectral fingerprints astronomers use to infer what an atmosphere contains. The clearer evening limb on WASP-94A b gave the team a less cloud-obscured view of gases including water, while the cloudier morning limb showed how much a single averaged measurement can miss. (hub.jhu.edu) Nature described the result as the first evidence of clouds changing across the day-night transition on this kind of planet. Johns Hopkins said isolating the clouds gave researchers one of the clearest views yet of the planet’s composition. ### What comes next for this line of research? The next step is likely more limb-by-limb and phase-resolved observations of hot Jupiters with JWST and follow-up modeling by teams studying cloud microphysics and atmospheric circulation. (eurekalert.org) The May 21 Science paper, “Cloudy mornings and clear evenings on a gas giant exoplanet,” provides the benchmark result, and Mukherjee, Sing and their co-authors say the broader point is that asymmetric weather systems need to be built into future exoplanet atmosphere studies. (nature.com)