The Method Is Called Limb-Resolved Spectroscopy. The Planet Is WASP-94A b. The Morning Skies Are Full Of Rock Clouds. The Evenings Are Clear. And The Data That Previously Suggested This World Had Hundreds Of Times Jupiter’s Oxygen And Carbon Turns Out To Have Been Wrong.
According to Phys.org and the journal Science, sand clouds form every morning but clear up by nightfall on WASP-94A b, a well-studied gas giant located nearly 700 light-years from Earth. Research published on May 21, 2026 in Science is among the first to detect cloud cycles on a Hot Jupiter exoplanet, and by isolating those clouds, researchers have provided one of the clearest pictures to date of the planet’s actual composition.
The paper is authored by Sagnick Mukherjee, a postdoctoral fellow at Arizona State University who conducted the research while a student at Johns Hopkins and UC Santa Cruz, alongside co-author David Sing, a Bloomberg Distinguished Professor of Earth and Planetary Sciences at Johns Hopkins and the program’s Principal Investigator.
To understand why this finding changes planetary science, you first need to understand what has been making exoplanet atmosphere measurements unreliable for years and why clouds, of all things, are the culprit.
WASP-94A b is a tidally locked hot gas giant orbiting one star in a binary system. It has a mass slightly below half of Jupiter’s but a diameter more than 70 percent wider. Its average temperature exceeds 1,500 Kelvin. Because it is tidally locked, one side bakes under permanent daylight while the other remains in permanent night.
That arrangement produces a sharp asymmetry. The evening limb of the planet is roughly 450 Kelvin hotter than the morning limb; a difference large enough that the chemistry and cloud cover on each side behave like two different planets stitched together.
By analyzing the light passing through WASP-94A b’s limbs as it crossed its star, the team found that the “morning” side is completely covered in thick clouds, while the “evening” side is much hotter and clear enough to show signs of water vapor. The technique used to achieve this separation is called limb-resolved spectroscopy and it is what makes this study methodologically transformative.
When a planet transits its star, the starlight that passes through the planet’s atmosphere at the edges; the limbs carries chemical fingerprints.
Standard transmission spectroscopy captures both limbs simultaneously and produces a blended average. “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 Mukherjee. “This approach with the JWST lets us localize our observations, which helped us see the cloud cycle.”
That localisation is the key innovation. By separating the morning limb; cloudy from the evening limb; clear, the team could analyse the cloud-free atmosphere directly, without the clouds obscuring the chemical signal. The results produced a correction that overturned a decade of accepted data.
Previously, when the clouds were averaged in, the data suggested the planet was made of hundreds of times more oxygen and carbon than Jupiter; a finding that baffled researchers given it couldn’t be explained by planet formation theory. The new data, however, shows WASP-94A b has only five times the amount of oxygen and carbon.
The corrected measurement is not simply more accurate. It is physically sensible in a way the previous measurement was not. Five times Jupiter’s heavy element abundance places WASP-94A b comfortably within the range predicted by current models of how giant planets form and migrate.
Hundreds of times was not just puzzling; it was theoretically impossible to explain. The old measurement was not observational reality. It was a measurement error caused by averaging clouds into a composition estimate.
The clouds themselves are made of magnesium silicate; a rock-forming mineral. They build as air flows over the dark side of the planet, reaching a large swell by daybreak. The clouds dissipate on the dayside, leaving clear skies in the early evening. The mechanism is driven by temperature and circulation.
On the night side, temperatures are low enough for silicate vapour to condense into solid particles; essentially sand grains forming in an atmosphere. Those particles are then carried by circulation to the dayside, where the higher temperatures vaporise them again. The cycle repeats with every four-day orbit, producing a daily cloud weather system on a world 700 light-years away.
Using WASP-94A b as a benchmark, the team looked at eight other hot gas giants and discovered the same distinctive cloud cycle on two other worlds: WASP-39 b and WASP-17 b.
That finding; that limb asymmetry and cyclical cloud cover are not unique to one planet but appear across a class of objects transforms this from an individual discovery into a systematic problem.
If the composition measurements of WASP-94A b were wrong by a factor of hundreds due to cloud contamination, the same error may be present in measurements of every Hot Jupiter where cloud cycles were not isolated. “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,” said Mukherjee.
Professor Nathan Mayne, from Exeter’s Department of Physics and Astronomy, added: “This exciting project shows the power of combining the exquisite observations from JWST with cutting-edge theoretical and numerical modelling of planetary atmospheres.
We have been able to determine what the clouds are made of in the atmosphere of a planet 700 light years from Earth, which is crazy. This work also helps us to test, develop and improve our modelling approaches leading to improvements in Earth weather and climate prediction.”
That connection to Earth climate modelling is not incidental. The 3D atmospheric models used to simulate the winds, chemistry and cloud physics on WASP-94A b were developed in partnership with the UK Met Office; the same modelling framework used for terrestrial weather prediction. Validating those models against JWST observations of exoplanet weather systems provides an extreme test environment that no terrestrial weather system can replicate.
Next, Sing and his team will be using data from a new large JWST program to study cloud cycling across a wide variety of exoplanets, including an eccentric gas giant planet in the habitable zone. An eccentric orbit means the planet swings through a wide range of distances from its star during each orbit; passing through the habitable zone at closest approach and retreating far beyond it at maximum distance.
The temperature swings that produces could generate weather systems unlike anything currently in the exoplanet catalog, and the limb-resolved technique developed for WASP-94A b may be the only tool capable of reading them.
“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,” said Sing.
For twenty years, clouds on exoplanets were noise in the data; an obstruction that blurred every compositional measurement and inflated the apparent abundance of heavy elements.
JWST has now demonstrated that those clouds can be isolated, characterised and corrected for. The first measurement was not the atmosphere. It was the fog. The real atmosphere; five times Jupiter’s heavy element abundance, clear evenings, sandy mornings has been there all along.
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