The James Webb Space Telescope has produced something close to a daily weather report for a planet 690 light-years away, and the forecast is strange: cloudy mornings of vaporized rock, clear evenings, and winds racing fast enough to carry sand-like particles around the world before they evaporate in the heat of perpetual day.
The planet is WASP-94A b, a tidally locked hot gas giant orbiting one star in a binary system. Using a technique called limb-resolved spectroscopy, a team led by Sagnick Mukherjee of Johns Hopkins University managed to separate the morning side of the planet from the evening side as it transited its star. The findings appear in the May 21 issue of Science.
A planet with two atmospheres at once
WASP-94A b 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. The difference is large enough that the chemistry and cloud cover on each side behave like two different planets stitched together.
On the morning side, the JWST data revealed thick clouds made not of water but of vaporized magnesium silicate — essentially sand. By the time the same parcels of atmosphere rotated around to the evening side, the skies had cleared.
How weather travels around a tidally locked world
The driver is equatorial super-rotation. Powerful winds at the day-night terminator lift magnesium silicate high into the atmosphere over the cold night side, where it condenses. Those clouds then ride the wind toward the dayside, descending and evaporating in the heat before reaching the evening limb.
According to the research team, the goal was to understand the atmospheres of such planets — whether they are static or dynamic, and whether they have winds and clouds.
The answer, at least for this world, is that the weather is in constant motion. The cycle resembles a sandstorm in the sky, with vaporized rock circulating between hemispheres in a closed loop.
The bias hiding in older exoplanet data
The bigger implication is methodological. For more than a decade, astronomers have inferred the chemical composition of exoplanet atmospheres by averaging light from across the entire planet during transit. That approach treated each world as a single homogenous sphere.
For WASP-94A b, that assumption produced a striking error. Averaged transmission spectroscopy suggested the planet’s oxygen content was about 100 times higher than the Sun’s. Once the limbs were resolved separately, the actual enrichment came in at only three to five times solar — a value consistent with what planet-formation models predict for a gas giant.
The corrected enrichment values significantly changed the understanding of the planet’s composition, Mukherjee noted.
The discrepancy matters because oxygen and carbon ratios are how astronomers try to reconstruct where and how a gas giant formed within its parent disk. A hundred-fold overestimate would push a planet into an exotic category. A five-fold enrichment makes it ordinary.
Why this changes the catalog
Tidally locked worlds are common among the exoplanets accessible to current instruments. They tend to be close to their stars, which makes them easier to detect and easier to study during transit. If the averaging bias seen on WASP-94A b applies broadly, then many published composition estimates for hot Jupiters may need revision.
The team emphasized that the field needs to develop better methods to account for this observational bias.
The team has already extended the work. Follow-up JWST observations of eight other hot Jupiters turned up a similar morning-evening cloud cycle on at least two of them, WASP-17b and WASP-39b. Both had previously been studied with Hubble, but Hubble could not resolve the limbs separately.
What limb-resolved spectroscopy actually does
During a transit, the planet’s atmosphere appears as a thin ring around its silhouette. The leading edge of that ring corresponds to the morning terminator; the trailing edge corresponds to the evening terminator. As the planet moves across the star, the two edges enter and exit transit at slightly different times.
JWST’s sensitivity is high enough to pull apart the spectra from those two phases. Earlier instruments could see the average. JWST can see the difference.
That capability is the central technical advance behind the Science paper. It turns transmission spectroscopy from a snapshot of a planet into something closer to a stereo image, with two channels feeding back information about circulation, chemistry, and cloud physics.
The broader picture for Webb
JWST was designed in part to characterize exoplanet atmospheres, and the WASP-94A b result fits a pattern of the telescope reshaping conclusions drawn from earlier missions. Recent Webb observations have also mapped bare rock on a distant super-Earth, and Webb imagery has forced reassessments of early-universe galaxy formation.
The common thread is resolution. Older instruments produced averages. Webb produces detail, and the detail keeps disagreeing with the averages in ways that matter.
What comes next
Mukherjee’s team plans to apply the limb-resolved approach to a wider set of worlds, including a gas giant on a highly eccentric orbit that swings from its star’s habitable zone to a much closer pass. The temperature swings on that planet could drive weather systems unlike anything in the current catalog.
For now, WASP-94A b is the test case. It is a world where the morning skies are full of sand, the evenings are clear, and the winds blow fast enough to redraw the chemistry of an entire atmosphere in a few hours. It is also a reminder that an exoplanet seen as a single dot of filtered starlight may, on closer inspection, be two very different planets at once.
