JWST maps daily weather on WASP-94A b

The James Webb Space Telescope has given astronomers one of their clearest looks yet at weather on a world outside the solar system. Observations of the hot Jupiter WASP-94A b showed that the planet’s atmosphere changes over the course of its day, with clouds building in the morning and largely clearing by evening. The results were published in Science this week, after researchers used Webb to separate the planet’s leading and trailing edges during transit. The planet is about 690 light-years from Earth in the constellation Microscopium. ### How did Webb see “weather” on a planet it cannot photograph directly? WASP-94A b was studied as it passed in front of its host star, allowing astronomers to read changes in starlight filtering through the planet’s atmosphere. By using transmission spectroscopy during transit, the team measured the planet’s leading edge as it moved onto the star’s disk and its trailing edge as it moved off. Those edges correspond to different local times on the tidally locked planet: the leading limb is the morning side, and the trailing limb is the evening side. (science.org) The James Webb Space Telescope’s infrared sensitivity let the researchers compare those two limbs separately rather than averaging them into a single atmospheric signal. That matters because many earlier exoplanet studies treated the terminator region — the boundary between day and night — as if it were uniform. ### What did the team actually find in the atmosphere? The Science paper said the morning limb is cooler and cloudier, while the evening limb is hotter and shows stronger gaseous water absorption. (hub.jhu.edu) The researchers reported a 6-sigma detection of asymmetry between the two sides, an 11-sigma detection of cloud cover on the morning limb, and a 10-sigma water signal on the evening limb. Johns Hopkins University, describing the findings in a May 21 release, said the clouds are made of magnesium silicate, a rock-forming mineral often described as sand-like. (science.org) The release said those mineral clouds gather in the morning skies and disappear by evening as air circulates into hotter conditions. ### Why would clouds vanish before sunset? The team’s models suggest cloud droplets form deeper in the atmosphere near millibar pressures, then are lofted upward on the cooler morning side. (arxiv.org) As winds carry that material toward the hotter evening side, the droplets evaporate. The paper said that process requires at least a 280-kelvin temperature difference between the two limbs. Nature, in a news report on the study, described the result as clouds “streaming and vanishing” around the planet. (hub.jhu.edu) The underlying interpretation in the paper is that these aerosols are consistent with condensate clouds rather than photochemical haze. ### Why do astronomers care beyond this one planet? Sagnick Mukherjee and co-authors said in the paper that ignoring this kind of limb-to-limb difference can bias inferred chemical abundances in exoplanet atmospheres. (arxiv.org) The paper said the effect is severe enough that some conclusions drawn from a decade of Hubble Space Telescope transmission spectra may need to be revisited. David Sing, a Johns Hopkins professor and principal investigator on the program, said in the university release that cloudiness has long acted like a “foggy window” for exoplanet studies. (nature.com) He said separating cloudy and clearer regions allowed the team to better pin down both what the clouds are made of and how they condense and evaporate around the planet. ### What comes next for this line of research? (arxiv.org) The paper said limb-resolved spectroscopy will be important for characterizing how aerosols form in transiting exoplanets, from gas giants to terrestrial worlds. NASA and ESA have both highlighted Webb’s exoplanet program as a way to move beyond simple detections toward more detailed atmospheric structure and chemistry. Science published the WASP-94A b study under the title “Cloudy mornings and clear evenings on a giant extrasolar world.” The authors include Mukherjee, Sing, Guangwei Fu and Kevin B. (scitechdaily.com) Stevenson, and the data were taken with the NASA/ESA/CSA James Webb Space Telescope. (science.org) (arxiv.org)