JWST maps WASP-94Ab weather

Scientists using NASA’s James Webb Space Telescope have produced a weather map for WASP-94Ab, a hot Jupiter about 690 to 700 light-years from Earth, finding cloudy mornings and clearer evenings on the giant planet. The observations were reported in coverage published on May 22 and in a Science paper released on May 21. The team said the pattern points to mineral clouds that form on the cooler side of the planet and dissipate as they move into hotter regions. The result gives astronomers one of their clearest looks yet at how clouds can skew measurements of exoplanet atmospheres. ### How did Webb tell morning from evening on a planet so far away? Researchers led by Sagnick Mukherjee used JWST’s Near Infrared Imager and Slitless Spectrograph, or NIRISS, to observe WASP-94Ab as it crossed in front of its star. By separating the light from the planet’s leading edge and trailing edge during the transit, they sampled what amounts to the planet’s “morning” and “evening” limbs. (scientificamerican.com) Johns Hopkins University said the leading edge corresponds to air moving from the night side to the day side, making it the morning side, while the trailing edge corresponds to air moving from day to night, making it the evening side. That geometry let the team compare two different atmospheric regions on a world that is tidally locked to its star. (eurekalert.org) ### What did the telescope actually see in the atmosphere? The Science summary said the cooler morning side appeared heavily shrouded in high mineral clouds that obscured gaseous signatures, while the hotter evening side was comparatively clear and showed stronger water-vapor absorption. Johns Hopkins said the morning clouds are made of magnesium silicate, a mineral common in rocks, which is why some coverage described the planet as having “sandy skies.” (hub.jhu.edu) Scientific American described the result as partly cloudy skies on a distant giant exoplanet, with clouds building on the cooler side and thinning later in the local day. Ars Technica and Space both framed the finding as a direct look at alien weather, including a clearer sunset-side sky. ### Why are the clouds different on the two sides? (eurekalert.org) The AAAS summary said a 3D general circulation model pointed to a cloud cycle driven by an extreme temperature contrast of about 450 degrees Kelvin between the two hemispheres. In that picture, clouds form on the cooler night side, circulate toward the morning side, and evaporate as they move into the intensely heated day side. (scientificamerican.com) David Sing, a co-author at Johns Hopkins, said clouds have long been “a thorn in our side” for exoplanet studies because they act like a foggy window. He said the new observations let researchers pin down what the clouds are made of and how they condense and evaporate as they move around the planet. ### Why does this matter for exoplanet research beyond one planet? (eurekalert.org) The Science summary said the findings suggest WASP-94Ab’s aerosols are dominated by condensation-driven clouds rather than photochemical hazes generated by stellar radiation. That distinction matters because aerosols can obscure or distort spectral signals, making it harder to infer a planet’s chemistry. (hub.jhu.edu) Mukherjee and colleagues also warned that treating an exoplanet atmosphere as uniform — a common simplifying assumption — can bias estimates of chemistry and physical properties. Ars Technica said the asymmetry seen on WASP-94Ab could be throwing off how scientists interpret other planetary atmospheres as well. ### What comes next from this result? The paper, “Cloudy mornings and clear evenings on a gas giant exoplanet,” was published in Science on May 21 with DOI 10.1126/science.adx5903. (eurekalert.org) The next step for researchers will be to apply similar transit-based comparisons to other hot Jupiters and test whether asymmetric cloud cycles are common rather than exceptional. That last point is an inference from the paper’s method and stated warning about one-dimensional atmospheric assumptions.