JWST Maps a Dawn-Dusk Divide on Ultra-Hot Jupiter WASP-121 b, Where the Evening Sky Runs Hotter and Water Falls Apart

Overview

Astronomers using the James Webb Space Telescope have measured a sharp difference between the morning and evening edges of an ultra-hot exoplanet, finding that the world’s evening sky is hotter and more puffed-up than its dawn. The planet, WASP-121 b, is a gas giant whose dayside reaches around 2,770 Kelvin while its nightside sits closer to about 1,000 Kelvin, according to Phys.org. The study, led by Cyril Gapp, a Ph.D. student at the Max Planck Institute for Astronomy, was published in Nature Astronomy on June 10, 2026, the Max Planck-affiliated announcement on EurekAlert reports.

What We Know

WASP-121 b orbits extraordinarily close to its host star — only 1.9 stellar diameters away, according to Phys.org. The planet’s rotation is synchronized to its orbit, with both taking about 30 hours to complete, The Brighter Side of News reports. That tidal locking leaves one hemisphere permanently facing the star at roughly 2,770 Kelvin — almost 2,500 degrees Celsius — while the perpetually dark side cools to around 725 degrees Celsius, the EurekAlert release states. Phys.org describes the night side as about 1,775 degrees Celsius cooler than the day.

To separate the planet’s morning and evening edges, the team used a technique it calls rotational transits. By measuring how starlight absorption changes as WASP-121 b rotates, the researchers probed its atmosphere longitude by longitude using JWST’s NIRSpec (Near-Infrared Spectrograph) instrument, according to Phys.org. The planet turns through about 30 degrees during a full transit, the EurekAlert release notes, enough rotation to bring different regions of the atmosphere into view as the planet crosses its star.

The central finding is an asymmetry between the two terminators — the twilight boundaries between day and night. The evening terminator absorbs more light than the morning terminator, ScienceDaily reports. Because a larger atmosphere presents a bigger cross-section to incoming starlight, that extra absorption points to an evening side that is hotter and more expanded, The Brighter Side of News reports. The cause, according to ScienceDaily, is powerful atmospheric winds that transport heat from the intensely hot dayside toward the cooler nightside; because those winds move eastward in the direction of the planet’s rotation, they heat the evening region more strongly.

The observations also captured the chemistry of an atmosphere under extreme stress. The amount of water in the atmosphere appears to drop because temperatures in the upper atmosphere are high enough to break water molecules apart into their constituent elements, ScienceDaily reports. The data showed a stronger carbon monoxide signal toward the end of the transit, but the team concluded the change was caused by temperature effects rather than an actual increase in carbon monoxide abundance, ScienceDaily notes. The researchers also found that their models improved when they accounted for clouds composed not of water droplets but of minerals such as silicates, according to Phys.org.

“With its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date,” Gapp said. “By measuring how star light absorption changes as WASP-121 b rotates, we probe its atmosphere longitude by longitude,” the EurekAlert release quotes him as saying.

What We Don’t Know

The silicate clouds remain a model-dependent inference rather than a direct detection: the data are consistent with mineral clouds in the cooler regions, but ScienceDaily frames their presence as a possibility the models accommodate rather than a confirmed feature. The publicly available coverage also does not quantify how the wind speeds or the precise temperature gap between the two terminators were derived, leaving the magnitude of the dawn-dusk contrast described qualitatively in the announcements.

Analysis

The result matters less for WASP-121 b itself than for the method. Treating a transit as a slow rotation — and reading the atmosphere longitude by longitude as the planet turns — lets a single telescope resolve weather-scale structure on a world hundreds of light-years away, rather than averaging the entire globe into one blurred spectrum. The work was carried out by a team including Thomas M. Evans-Soma of the University of Newcastle in Australia and Eva-Maria Ahrer of the Max Planck Institute for Astronomy, according to the EurekAlert release. As JWST accumulates more rotational-transit observations of ultra-hot Jupiters, the technique could turn each transit into a partial map, sharpening the still-coarse picture of how heat and chemistry circulate across these extreme atmospheres.