For the first time, astronomers have peered at the two edges of a distant world and found them profoundly different. One side, the morning limb, is shrouded in high clouds of vaporized rock. The other, the evening limb, is clear — offering an unobscured view of the planet’s true atmospheric composition.
The finding, published in Science by a team led by Sagnick Mukherjee of Arizona State University and David K. Sing of Johns Hopkins University, comes from a clever re-analysis of a single transit observation by the James Webb Space Telescope. Rather than averaging the light from the entire ring of atmosphere visible during a transit — the standard approach — the team extracted separate spectra from the planet’s leading edge (morning) and trailing edge (evening). The difference was stark.
On the exoplanet WASP-94A b, a hot Jupiter about 690 light-years away in the constellation Microscopium, the morning atmosphere is cool and cloudy. Magnesium silicate aerosols — essentially vaporized rock condensed into fine particles — block the view, producing a featureless spectrum that rises toward blue wavelengths. The cloud signal was detected at 11 sigma significance.
The evening atmosphere is hotter by at least 280 Kelvin. Here, the clouds have evaporated, leaving a clear sky with strong water vapor absorption features detected at 10 sigma.
The limb-to-limb asymmetry itself was detected at 6 sigma.
“This is the smoking gun for condensation-driven clouds on hot Jupiters,” Mukherjee said in a statement. “Previous studies that averaged morning and evening spectra together have likely produced systematically incorrect atmospheric compositions.”
WASP-94A b is tidally locked to its star, meaning one hemisphere perpetually faces the F-type host star at a dayside temperature around 1,700 K, while the other faces away into permanent night. This temperature difference drives a global circulation pattern:
1. Magnesium silicate vaporizes on the scorching dayside
2. Atmospheric currents carry the vapor toward the cooler nightside
3. There, it condenses into aerosol droplets — like rain, but made of rock
4. These clouds are carried around to the morning terminator (dawn)
5. As the clouds circulate back toward the dayside, the higher temperatures evaporate the droplets, leaving the evening terminator (dusk) clear
The mechanism requires a minimum limb-to-limb temperature difference of 280 K to sustain the evaporation cycle, which the team confirmed from the spectra.
Beyond a single planet
The team surveyed eight additional hot Jupiters and found the same morning-evening asymmetry signature on two of them: WASP-39 b and WASP-17 b, suggesting the condensation-driven cloud cycle may be common among hot Jupiters.
The finding has immediate methodological implications. Nearly all previous exoplanet atmospheric studies — from Hubble and even earlier JWST observations — have used one-dimensional retrievals that average the entire transit spectrum together. If cloudy mornings and clear evenings are the norm, then those one-dimensional retrievals may have systematically undercounted water vapor and other gases by blending obscured and unobscured views.
“Everyone knew there could be limb asymmetries, but nobody knew they’d be this large,” Sing said. “We need to re-evaluate what we think we know about exoplanet atmospheres.”
Caveats
The result is based on a single transit observation — no repeat measurement has been performed yet. The atmospheric retrievals rely on PICASO and 3D general circulation models that carry their own systematic uncertainties. WASP-94 is also a binary star system, though the companion’s 15-arcsecond separation (about 2,700 AU) minimizes contamination risk.
The cloud composition is also not uniquely determined. Magnesium silicate (MgSiO3) is the best-fit condensate, but other mineral species — enstatite, forsterite, iron — may also contribute.
What’s next
The team plans to observe additional transits of WASP-94A b to confirm the asymmetry and monitor for variability. The method of extracting limb-resolved spectra from single transits can now be applied to the archival data of hundreds of exoplanets observed by JWST, potentially transforming the field’s understanding of hot Jupiter atmospheres.
Source: Mukherjee, S., Sing, D.K., Fu, G., et al. “Cloudy mornings and clear evenings on a gas giant exoplanet.” Science (2026). DOI: 10.1126/science.adx5903. Also available as arXiv:2505.10910.