JWST sees daily weather on WASP-94Ab

On May 21, researchers using NASA’s James Webb Space Telescope reported one of the clearest detections yet of day-to-day weather structure on a planet outside the solar system. The target was WASP-94A b, a gas giant nearly 700 light-years from Earth, where the team found mornings shrouded in mineral clouds and evenings comparatively clear. The study was published in *Science* and led by a team including scientists at Johns Hopkins University. The observations give astronomers a rare time-resolved look at how clouds form and disappear on a so-called hot Jupiter. ### How did Webb tell morning from evening on a planet that far away? JWST observed WASP-94A b as it passed in front of its star, a transit geometry that let researchers separate the planet’s leading edge from its trailing edge. The leading edge corresponds to air moving from the night side to the day side — effectively morning — while the trailing edge samples air moving from day to night, or evening, according to Johns Hopkins. (hub.jhu.edu) WASP-94A b orbits an F-type star every four days at about 0.055 astronomical units, according to NASA’s exoplanet catalog. The planet was discovered in 2014 and has about half Jupiter’s mass, making it a standard example of a hot Jupiter: a gas giant orbiting extremely close to its star. ### What did the telescope actually see in the atmosphere? The May 21 study found that the morning side was covered by clouds made of magnesium silicate, a rock-forming mineral sometimes described as “sand” clouds. (hub.jhu.edu) The evening side, by contrast, appeared much clearer and showed stronger water-vapor signatures, according to the university release and reporting based on the paper. (science.nasa.gov) The arXiv version of the paper described the result as a 6-sigma detection of limb asymmetry, with an 11-sigma cloud-covered cooler morning limb and a hotter, clearer evening limb with 10-sigma water absorption. Those figures quantify how strongly the data distinguish the two sides of the planet during transit. ### Why would one side be cloudy and the other clearer? David Sing, a co-author and principal investigator on the program at Johns Hopkins, said clouds have long obscured efforts to read exoplanet atmospheres. “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,” he said in the university release. (hub.jhu.edu) (arxiv.org) Researchers said two mechanisms could explain the pattern. One is that strong winds loft clouds on the cooler side and then drive them downward into deeper layers on the hotter dayside. The other is that the process resembles fog burning off after sunrise, but under far more extreme temperatures. That explanation was attributed by Johns Hopkins and partner institutions to the team’s interpretation of the Webb data. (hub.jhu.edu) ### How far away is WASP-94A b, and what kind of world is it? Forbes, citing the new study and NASA background, put the distance at 689 light-years, while Johns Hopkins described the planet as nearly 700 light-years away in the constellation Microscopium. Both accounts describe WASP-94A b as a tidally locked gas giant, meaning one hemisphere permanently faces its star while the other remains in darkness. (hub.jhu.edu) That geometry helps create large atmospheric contrasts. Forbes reported scientists estimate a temperature contrast of about 350 degrees Fahrenheit, or 177 degrees Celsius, across the hemispheres. ### Why does this matter for exoplanet research? The Johns Hopkins team said isolating the clouds lets researchers measure the rest of the atmosphere more accurately. (forbes.com) Sing said general cloudiness has been “a thorn in our side” for exoplanet work because it acts like a fogged window over the data. The paper published on May 21 in *Science* gives astronomers a template for studying cloud cycles on other hot Jupiters and, potentially, more temperate worlds. (forbes.com) The next step is further atmospheric characterization using Webb transit data and related analyses by the Johns Hopkins-led team and other exoplanet groups. (hub.jhu.edu)