Sandy mornings. Clear skies by sunset. It sounds like an ordinary weather forecast — except this one is for a world 690 light-years away, where the temperature never drops below 2,200 degrees Fahrenheit and the clouds are made of vaporized rock.
For the first time, the James Webb Space Telescope has tracked a complete daily weather cycle on an exoplanet. The target: WASP-94Ab, a gas giant one and a half times larger than Jupiter, locked in a scorching four-day orbit around its star. Astronomers have long suspected that hot Jupiters cycle through shifting cloud patterns — but until now, no telescope had actually watched it happen.
A weather forecast 690 light-years away
WASP-94Ab isn’t just any gas giant. Measuring 1.7 times the diameter of Jupiter, it completes a full orbit around its host star every four days — at a distance of just 5.1 million miles. That proximity keeps surface temperatures permanently above 2,200°F (1,200°C), earning it the classification of “hot Jupiter.”
Hot Jupiters sit extremely close to their stars, placing them among the most extreme environments known to science. Their inflated atmospheres and punishing heat make atmospheric signals easier to detect than on smaller, cooler worlds. WASP-94Ab orbits one star in a wide binary system, with a second star at a far greater distance. That tight, blazing orbit drives the weather dynamics astronomers are now beginning to map — and with JWST’s latest observations, it became the first hot Jupiter to have a complete daily weather cycle documented.
Clouds made of sand: the long-standing obstacle
Understanding what’s actually inside a hot Jupiter’s atmosphere has never been straightforward. The central problem is cloudiness — pervasive, persistent, analytically frustrating.
“I’ve been looking at exoplanets for 20 years and general cloudiness has been a thorn in our side,” said David Sing of Johns Hopkins University. “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.”
These aren’t clouds like the ones drifting over Earth. Hot Jupiter clouds are composed of vaporized metals and rock — essentially giant flying sandstorms. In WASP-94Ab’s case, they consist of magnesium silicate, the same mineral family that makes up much of Earth’s mantle. At temperatures exceeding 2,200°F, that material vaporizes, rises, and condenses into a drifting silicate haze. For two decades, that haze blocked clean atmospheric readings.
How JWST read the planet’s two faces
The key technique is transit spectroscopy. As WASP-94Ab crossed in front of its host star, starlight filtered through the planet’s atmosphere on both sides, and different gases absorbed light at specific wavelengths — revealing their chemical identities like a fingerprint.
What made this observation decisive was an asymmetry. On the leading edge — the “morning” side — JWST detected thick magnesium silicate clouds. On the trailing “evening” side, those clouds had cleared entirely, exposing a hydrogen-dominated atmosphere in full detail. This was possible because WASP-94Ab is tidally locked. Like our Moon relative to Earth, it always shows the same face to its star, creating a permanent dayside and nightside that drives strong atmospheric circulation. Previous Hubble observations couldn’t separate the absorption signatures of the two limbs, producing a blended, cloud-contaminated reading and skewed data.
Correcting a decade of flawed measurements
That blended Hubble data had produced a striking — and implausible — conclusion: WASP-94Ab contained hundreds of times more oxygen and carbon than Jupiter. Gas giant formation models don’t easily accommodate numbers that extreme, and astronomers suspected something was off.
JWST’s cloud-free view of the evening limb resolved the discrepancy. With the silicate interference removed, the actual enrichment came out to roughly five times Jupiter’s oxygen and carbon levels — consistent with established models of how gas giants form and evolve. “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 said.
The revision carries weight beyond this single planet. It suggests that cloud contamination has likely skewed atmospheric readings across multiple exoplanets, and that separating morning and evening limb signals may be essential to accurate measurements going forward.
Two other worlds, and what comes next
WASP-94Ab wasn’t alone in the study. Sing’s team also observed WASP-17b — an unusually large but low-density world that orbits its star in retrograde — and WASP-39b, a low-density planet with an atmosphere rich in water vapor, carbon, and sulfur dioxide. Both showed similar cloud cycles.
Two mechanisms may explain why the clouds clear. One involves wind-driven circulation: silicate particles blow high into the atmosphere, form clouds over the nightside, then sink as they rotate into dayside heat before recirculating. The other is more familiar — the clouds simply burn off, much like morning fog dissipating on Earth, just at temperatures high enough to melt rock. Researchers plan to expand the search to a wider variety of exoplanets, including a gas giant on a highly eccentric orbit that swings between near the habitable zone and close stellar proximity — a heating cycle that could generate powerful, JWST-detectable weather systems.
The findings for WASP-94Ab, WASP-17b, and WASP-39b were published May 21 in the journal Science. With three confirmed weather cycles already documented, exoplanet meteorology is just getting started.