In June 2026, the James Webb telescope read the dawn and dusk skies of WASP-121b separately and found one side hotter and stripped of water, the other cooler and possibly cloudy, on a world where it can rain liquid rock

The James Webb Space Telescope has tracked how the atmosphere of a distant gas giant changes from longitude to longitude during transit, revealing pronounced differences between its morning and evening regions.

On the ultra-hot Jupiter WASP-121b, the evening region appears hotter and more expanded, with some of its water molecules broken apart by the heat. The cooler morning region may contain silicate clouds formed as vaporised rocky material condenses, although those clouds have not been directly detected.

The finding is the first detection of this particular rotational-transit effect, in which changing atmospheric longitudes alter a planet’s transmission spectrum during a single transit.

What the June 2026 data showed

The results, led by Cyril Gapp of the Max Planck Institute for Astronomy, were published in Nature Astronomy on 10 June 2026. The technique relied on the planet’s own rotation. As WASP-121b crossed in front of its star, it rotated by roughly thirty degrees, allowing different atmospheric longitudes to move into view. Webb measured how the starlight filtering through the atmosphere changed during the transit.

Gapp described the method plainly: “By measuring how star light absorption changes as WASP-121 b rotates, we probe its atmosphere longitude by longitude.”

That changing view allowed the team to distinguish atmospheric regions nearer the planet’s morning and evening edges. The atmosphere blocked more starlight later in the transit, when more of the evening-side region had rotated into view. The stronger absorption points to a hotter and more expanded atmosphere there.

That is consistent with atmospheric circulation shifting the planet’s hottest region eastward from the point directly beneath the star. Heat transported towards the evening side causes the atmosphere to expand. At the highest temperatures, water molecules begin to dissociate, weakening the water signal Webb would otherwise detect.

The extremes are difficult to overstate. Earlier phase-curve modelling by members of the team placed the average dayside temperature near 2,770 Kelvin and the nightside near 1,000 Kelvin. Co-author Tom Evans-Soma of the University of Newcastle in Australia put it this way: “WASP-121b is particularly extreme, with average temperatures on the dayside hemisphere being around 2770 Kelvin, while those on the nightside are closer to about 1000 Kelvin.”

The planet is tidally locked, so one face remains turned towards its star while the other faces away. It circles an F-type star approximately every 1.3 days. At such temperatures, rocky and metallic compounds can vaporise in hotter regions and potentially condense again in cooler parts of the atmosphere.

Why resolving the limbs matters

Conventional transmission spectroscopy combines the planet’s morning and evening limbs into one atmospheric spectrum. If one limb is cloudy and the other is clear, the two signals blur together. The resulting average can resemble neither side particularly well.

Separating the limbs changes not only a planet’s inferred weather map but also the chemical abundances astronomers can derive from its spectrum. That matters because those abundances are used to reason about how a planet formed and where in its parent disk it took shape.

The broader lesson is that these planets are not uniform globes with a single atmospheric profile. They have permanent daysides and nightsides, distinct morning and evening regions, and winds that move heat and material between them. Treating the atmosphere as one averaged column was a limitation of earlier observations, not a property of the planets themselves.

The question the data leaves open

For WASP-121b, the temperature and chemistry asymmetry is now on solid footing. What remains uncertain is whether mineral clouds genuinely account for the cooler morning-side signal.

The observed contrast was stronger than cloud-free models predicted, and adding an approximate layer of silicate cloud improved the fit. But the clouds themselves have not been directly identified in the spectrum.

Until follow-up observations and more detailed modelling can confirm them, the cloudy morning side of WASP-121b remains a plausible interpretation rather than a settled picture.