James Webb Space Telescope (JWST) data are allowing researchers to resolve a gas giant’s cloud cover, providing a new level of detail that significantly alters our understanding of exoplanet conditions.
A tidally locked hot Jupiter, WASP-94A b, was the focus of the JWST for these observations, which captured a much more dynamic atmosphere than earlier simplified assumptions. In a new paper published in Science, an American team of astronomers reveals how the boundary between the planet’s permanent day and night sides creates an extreme temperature difference that dramatically affects the atmospheric chemistry.
A Hot Jupiter
The exoplanet WASP-94b is located about 700 light-years from Earth, in the constellation Microscopium. Hot Jupiters such as this intrigue scientists because their tight orbits produce incredibly intense surface temperatures, making them excellent natural laboratories for studying planetary surface dynamics.
As atmospheric aerosols travel across WASP-94A b, when they encounter the planet’s dramatic temperature shifts, they form into clouds to circulate over the exoplanet before eventually evaporating. Researchers were aware that these processes shape the appearance, chemistry, and temperature of exoplanet atmospheres, but had a limited understanding of how the exoplanet aerosol particles at their core behave.
The nature of hot Jupiter clouds was unclear, with researchers suggesting either photochemical hazes produced by stellar radiation or mineral clouds generated by condensation.
Adding to the difficulty is the tendency of these clouds to distort or obscure the spectral signals used to observe exoplanets. However, in the new work, JWST data provided strong confirmation of one of these hypotheses.
JWST Atmospheric Imaging
The JWST’s Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument allowed the researchers to separately analyze WASP-94A b’s permanent morning and night atmospheric horizons. To do this, the team waited until the exoplanet passed in front of its home star, providing an opportunity to measure the light passing through the atmosphere at its edges. Their findings revealed that the two sides of the planet held extremely different atmospheres.
The cooler morning side contains high-mineral clouds that offer dense cover, while the morning side is clearer and shows high levels of water vapor absorption. These findings are consistent with condensation rather than the photochemical hypothesis.
The team also employed a 3D circulation model to study the dynamic cloud cycle, discovering that the massive 450 Kelvin temperature disparity between the hemispheres was a driving factor, with clouds forming not on the night side but evaporating before reaching the intense temperatures of the day side.
The work reveals that the common assumption that exoplanet atmospheres are relatively uniform can lead to a gross misunderstanding of what is occurring on these distant bodies.
James Webb Space Telescope Breaks Through
“I’ve been looking at exoplanets for 20 years, and general cloudiness has been a thorn in our side,” said co-author and program PI, David Sing, a Bloomberg Distinguished Professor of Earth and Planetary Sciences at Johns Hopkins. “We’ve known for quite a while that clouds are pervasive on Hot Jupiter planets, which is annoying because it’s like trying to look at the planet through a foggy window.”
“Not only have we been able to clear the view, but 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,” Sing added.
The researchers offered two possible explanations for the cloud movement: either strong winds are pushing clouds up on the night side, causing them to descend when they reach the day side, or the phenomenon could be similar to morning fog, but on a planetary scale, evaporating the day-side heat.
“It was a huge surprise. People have expected some differences, like it’s cooler in the morning than the evening—that’s something natural that we experience here on Earth,” Sing said. “But what we saw was a real dichotomy between the weather on both sides of the planet, and huge differences in cloud coverage, and that changes our whole picture of the planet.”
JWST Advances Continue
“With the Hubble telescope, when we used to do this type of observation, we got an average view of the whole planet with data from the clouds and the atmosphere squished together and indistinguishable,” said first author Sagnick Mukherjee, a postdoctoral fellow at Arizona State University who was a student at Johns Hopkins and UC Santa Cruz at the time of the research. “This approach with the JWST lets us localize our observations, which helped us see the cloud cycle.”
The team was surprised to find that the night side atmosphere resembled our local Jupiter’s sky much more than was initially expected, based on earlier data. When previously working with a simplified average planetary cloud cover, researchers arrived at figures indicating that the planet contained hundreds of times as much oxygen and carbon as Jupiter. Now, with a clearer understanding of the day/night differential, it was revealed that the planet actually contained only five times as much oxygen and carbon.
Comparing their findings with those for eight other gas giants, the team found two cognates of the same cloud-cycling mechanism: WASP-39 b and WASP-17 b. Following this work, the team is currently studying cloud cycling on multiple other types of exoplanets.
The paper, “Cloudy Mornings and Clear Evenings on a Gas Giant Exoplanet,” appeared in Science on April 27, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.