Webb Observes a Hot Jupiter's Daily Weather Cycle for the First Time, Revealing Rock Clouds That Form Every Morning and Dissolve by Nightfall
JWST has tracked the first complete daily cloud cycle on an exoplanet, finding that WASP-94Ab grows magnesium silicate clouds each morning and clears them each evening, while correcting a decade of skewed atmospheric measurements from Hubble.
Overview
For the first time, astronomers have tracked a complete daily weather cycle on a planet outside the solar system. Using the James Webb Space Telescope, a team led by Sagnick Mukherjee — a postdoctoral fellow at Arizona State University who began the work as a PhD student at UC Santa Cruz — observed that the hot Jupiter WASP-94Ab grows thick clouds of vaporized rock on its morning side every day and clears them entirely by evening. The findings, published in Science on May 21, 2026, also revealed that a decade of atmospheric measurements from the Hubble Space Telescope had been distorted by those very clouds, producing estimates of the planet’s oxygen and carbon abundance that were hundreds of times too high.
What We Know
WASP-94Ab is a gas giant located roughly 690 light-years from Earth, according to TechTimes. It is about 1.7 times larger than Jupiter but carries only about 45 percent of Jupiter’s mass, making it one of the puffier known hot Jupiters. The planet orbits its host star once every 3.95 days at a distance of roughly 5.1 million miles — so close that it is tidally locked, with one hemisphere permanently facing the star and the other in constant darkness, as TechTimes reports. Surface temperatures exceed 2,200°F (1,200°C), and the temperature difference between the planet’s two limbs is approximately 450 Kelvin.
The cloud cycle is a direct consequence of this tidal locking. Strong circulation winds carry gas from the cooler nightside toward the dayside, and as the atmosphere rotates through the planet’s morning limb — the boundary between night and day — the drop in temperature allows magnesium silicate, the mineral found in sand, to condense into clouds, according to EarthSky. By the time that gas reaches the evening limb, the star’s heat has evaporated the clouds entirely, leaving a clear window into the hydrogen-dominated lower atmosphere. The cycle then repeats.
The JWST team used transit spectroscopy with the NIRISS instrument, measuring the planet’s atmosphere as it passed in front of its host star, according to TechTimes. Crucially, JWST’s infrared sensitivity was sufficient to observe the planet’s morning and evening limbs separately during ingress and egress of the transit — something Hubble could not do. Earlier Hubble observations could only average the signal from both sides of the planet together.
“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,” David Sing, Bloomberg Distinguished Professor of Earth and Planetary Sciences at Johns Hopkins University and program principal investigator, told The Daily Galaxy. “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.”
Mukherjee said the asymmetry between the planet’s two sides came as a genuine surprise. “It was really surprising how different the two halves of the same planet are,” he told The Daily Galaxy. “What this tells us is: if we don’t know about the weather cycles on these distant planets, we won’t be able to measure their composition well.”
Correcting a Decade of Measurements
The cloud cycle has consequences beyond meteorology. Because Hubble blended the cloudy morning side with the clearer evening side in every spectrum it took, the resulting data carried a systematic bias. According to TechTimes, previous Hubble-era estimates placed the oxygen and carbon abundance of WASP-94Ab at hundreds of times higher than Jupiter’s levels. JWST’s limb-separated measurements revised that figure down to approximately five times Jupiter’s levels — a correction that brings the planet in line with theoretical models of hot Jupiter formation.
According to UC Santa Cruz News, the team used the same approach to study two additional hot Jupiters — WASP-39b and WASP-17b — and found similar daily cloud patterns on both worlds, suggesting that the morning-cloud, clear-evening cycle may be a common feature of tidally locked gas giants at this temperature range.
“This approach with the JWST lets us localize our observations, which helped us see the cloud cycle,” Mukherjee said, as quoted by UC Santa Cruz News.
What We Don’t Know
The study focuses on gas giants with extreme conditions — temperatures well above 1,000 degrees Celsius, orbital periods of only a few days. Whether similar cloud cycles operate on cooler or smaller planets, including rocky worlds in the habitable zone, remains an open question. The measurement technique also depends on favorable orbital geometry: planets must transit their host star to yield limb-separated spectra, which limits the sample of targets to which this approach can be applied.
It is also unclear how stable the cloud cycle is over longer timescales, or whether storm-like disruptions can shift the cloud boundary significantly. Future observations across multiple transits of the same planets could help resolve this.
Analysis
The finding demonstrates that JWST is not simply providing better versions of what Hubble could already do — it is qualitatively changing the kinds of measurements that are possible in exoplanet science. The ability to separate a planet’s morning and evening spectra during a single transit is a capability gap that could, over time, allow researchers to build three-dimensional weather maps of distant atmospheres rather than globally averaged chemical snapshots. For the longer-term goal of characterizing rocky, potentially habitable worlds, understanding how cloud coverage varies across a planet — and how that variation biases chemical abundance measurements — will be essential.