JWST finds 'dual atmospheres' on a single exoplanet about 600 light-years away

On May 21, researchers reported in *Science* that the James Webb Space Telescope had separated two very different atmospheric views on the same exoplanet during a transit observation. The planet, WASP-94A b, is a hot Jupiter about 690 to 700 light-years away in the constellation Microscopium, according to Johns Hopkins University and UC Santa Cruz. The team said the planet’s “morning” limb appears heavily clouded while its “evening” limb is much clearer, producing what some coverage described as “dual atmospheres,” though the underlying result is a split atmospheric structure rather than two fully separate atmospheres. JWST made that distinction by observing the planet as it crossed in front of its star, a method known as transmission spectroscopy. Instead of averaging the whole thin ring of atmosphere around the planet, the researchers extracted separate signals from the leading and trailing edges of the transiting world. That let them compare air flowing from night to day — the morning side — with air flowing from day to night — the evening side. (hub.jhu.edu) ### So what did JWST actually see? WASP-94A b’s leading edge was “completely covered in thick clouds,” while the trailing edge was hot and clear enough to show water vapor, UC Santa Cruz said in its summary of the study. Arizona State University said the morning side is blanketed in magnesium silicate clouds, a mineral also found in common rocks, while the evening side is under clearer skies. That contrast is the core of the finding. (hub.jhu.edu) Nature’s news coverage described the result as clouds “streaming and vanishing” around the planet, based on how the atmosphere filters starlight during transit. Johns Hopkins said the work is among the first detections of repeating cloud cycles on a hot Jupiter exoplanet. ### Why are people calling it “dual atmospheres”? The phrase comes from the fact that the two limbs look chemically and physically different when seen in transmission. (news.ucsc.edu) ZME Science described them as “different atmospheres — or at least atmospheres that look very different when starlight passes through them.” The primary institutional descriptions are more precise: they refer to a daily cloud cycle and to distinct cloudy and clear regions on the same planet. (nature.com) Sagnick Mukherjee, the study’s first author, said JWST allowed the team to localize the observations instead of blending cloud and atmospheric signals together, as earlier Hubble-era measurements did. David Sing, a Johns Hopkins professor and program principal investigator, said clouds on hot Jupiters had long been “a thorn in our side” because they obscured atmospheric measurements. (zmescience.com) ### Where do the 3D climate models come in? The May 22 ZME Science report said researchers used three-dimensional climate models to help confirm the interpretation. The university releases describe two candidate mechanisms consistent with the observations: winds could loft clouds on the cooler side and drive them downward on the hotter dayside, or the mineral grains could evaporate as temperatures rise after sunrise. Both explanations point to a planet-wide cloud cycle rather than a static haze. (news.asu.edu) UC Santa Cruz said understanding that cycle is important for reconstructing how such planets formed and evolved. Johns Hopkins said isolating the clouds gave researchers one of the clearest looks yet at the planet’s composition. ### Why does this matter for exoplanet studies? NASA says JWST is designed to probe exoplanet atmospheres with far greater sensitivity than earlier observatories, and this result shows the payoff of that precision. (zmescience.com) Instead of treating a hot Jupiter as one blurred atmospheric average, researchers can now start mapping how conditions change from one region to another. In this case, separating cloudy mornings from clearer evenings also made the planet’s composition appear closer to Jupiter’s than earlier blended measurements had suggested, according to UC Santa Cruz. (news.ucsc.edu) On May 21, Johns Hopkins said the team also used WASP-94A b as a benchmark to examine other hot Jupiters. A related release said the same cloud-cycle signature turned up on two more worlds, WASP-39 b and WASP-17 b, pointing to follow-up work on how common these day-night cloud patterns may be in JWST exoplanet data. (eurekalert.org) (science.nasa.gov)