JWST maps weather on WASP-94A b

The James Webb Space Telescope has given astronomers something close to a weather map for a planet nearly 700 light-years away. On WASP-94A b, a hot Jupiter in the constellation Microscopium, the data show a cloudier, cooler morning side and a hotter, clearer evening side. The work was published in Science on May 22 and described by the authors as one of the first detections of a repeating cloud cycle on a hot Jupiter. The observations also let researchers separate cloud effects from the underlying atmosphere, a longstanding problem in exoplanet studies. ### How did JWST tell morning from evening on a planet this far away? JWST observed WASP-94A b during transit, when the planet passed in front of its star. By measuring the leading edge as the planet began crossing and the trailing edge as it finished, the team effectively sampled two different limbs of the atmosphere. Because air on the leading edge is moving from night to day, the researchers treated that side as morning; air on the trailing edge is moving from day to night, making it evening. (phys.org) Sagnick Mukherjee, the study’s first author, said that older Hubble observations tended to compress clouds and atmospheric chemistry into a single averaged view. JWST’s limb-resolved measurements, he said, let the team localize the signal and identify the cloud cycle instead of blending it into a planet-wide average. ### What weather did the telescope actually find? (phys.org) The morning limb of WASP-94A b appears cloud-covered, while the evening limb appears comparatively clear. The arXiv version of the paper reports a 6-sigma detection of limb asymmetry, with an 11-sigma detection for a cloud-covered cooler morning limb and a 10-sigma detection of strong water absorption on the clearer, hotter evening limb. Johns Hopkins and Exeter summaries said those clouds are made of magnesium silicate, a mineral common in rocks. (news.asu.edu) Several write-ups described them as “sand” clouds because the condensates are silicate-based rather than water droplets like clouds on Earth. The picture that emerges is of clouds building on the cooler side before daybreak and thinning or disappearing by early evening. ### Why do the clouds show up in the morning and not later? (arxiv.org) The team outlined two main possibilities. In one, strong winds loft clouds upward on the cooler side and then drive them downward on the hotter dayside, pushing the particles deep enough that they drop out of view. In the other, the clouds form on the nightside and then evaporate as they rotate into the hotter daylight hemisphere, in an extreme version of fog burning off after sunrise. (phys.org) The arXiv paper says the observations require at least a 280-kelvin temperature difference between the two limbs. The authors’ modeling suggests cloud droplets form near millibar pressures, are lofted to lower-pressure altitudes on the morning limb, and then evaporate once they circulate to the hotter evening side. ### Why does this matter beyond one strange gas giant? David Sing, a co-author and principal investigator at Johns Hopkins, said clouds have long acted like a “foggy window” for exoplanet studies, obscuring what astronomers are trying to measure. (phys.org) Isolating the clouds lets researchers get a cleaner read on atmospheric composition, including molecules such as water. The preprint also argues that ignoring day-night cloud differences can bias inferred chemical abundances. (arxiv.org) That matters because many earlier transmission-spectrum studies, especially from Hubble, treated each planet’s limb as a single blended atmosphere. The WASP-94A b result suggests some of those interpretations may need to be revisited with limb-resolved methods where possible. ### What comes next for this kind of exoplanet weather work? (phys.org) Science published the WASP-94A b study on May 22, and the paper’s authors include researchers from Arizona State University, Johns Hopkins and other institutions. The next step is likely more JWST transit spectroscopy on other hot Jupiters and, where feasible, reanalysis of older datasets to test whether similar morning-evening asymmetries were hidden in averaged observations. That expectation is an inference from the paper’s stated warning that limb asymmetry can skew abundance measurements and from the authors’ emphasis on limb-resolved spectroscopy. (arxiv.org)