1,775°C separates dawn from dusk on this alien world — JWST finally mapped why

NASA’s James Webb Space Telescope has read the morning sky and evening sky of the same alien planet separately — and found them 1,775°C apart.

The planet is WASP-121 b, an ultra-hot gas giant orbiting its star every 30 hours. It is tidally locked: one hemisphere permanently faces the star, baking at around 2,500°C, while the other remains in perpetual night at roughly 725°C. Where those two hemispheres meet, there are two boundaries — a morning terminator at dawn, and an evening terminator at dusk. A study published June 11 in Nature Astronomy has mapped both at the same time, revealing them as chemically distinct environments separated by nearly two thousand degrees.

How Webb read one transit as two separate skies

A transit occurs when a planet crosses in front of its star. Astronomers analyze starlight filtered through the planet’s rim to detect chemical fingerprints. Normally, the morning and evening edges blend into a single spectrum — averaged, unresolved.

What changed here is scale and timing. WASP-121 b is large and orbits so close to its star that it rotates roughly 30 degrees during a single transit. That rotation sweeps first the morning rim, then the evening rim, across the telescope’s line of sight. Using Webb’s NIRSpec spectrograph alongside the NIRISS instrument, the researchers recorded how the light signal shifted continuously as the planet turned — effectively sampling each edge in sequence.

“With its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date,” said lead author Cyril Gapp of the Max Planck Institute for Astronomy in Heidelberg.

A morning sky still building its clouds

The morning terminator arrives in Webb’s view first, and it absorbs less starlight than the evening side.

The team’s favored explanation is silicate clouds — not water droplets, but mineral particles formed when rock-forming compounds condense at high altitude. Because the morning atmosphere is fed by air arriving from the colder nightside, it briefly reaches temperatures low enough for these silicates to solidify and scatter incoming radiation. That scattering makes the morning sky appear muted in the spectrum, absorbing less than a hotter, clearer atmosphere would.

Carbon monoxide levels at this edge are relatively stable. Water molecules — heavily dissociated under the extreme conditions — still register more strongly at the morning limb than at the evening one.

An evening too hot for water

By the end of transit, the evening terminator has swept into view and the signal has shifted measurably. Carbon monoxide absorption grows stronger — a sign that the eastern limb is hotter. Water becomes less abundant, not because the planet has less of it, but because upper-atmosphere temperatures are extreme enough to split H₂O molecules into hydrogen and oxygen atoms before they can absorb light in detectable quantities.

The evening edge is also physically larger. Heat expands the upper atmosphere upward, increasing the depth of gas that starlight must pass through. The evening side intercepts more radiation than the morning side at the same orbital position — both because it is hotter and because it extends higher.

Winds that write the 1,775°C gap

Both terminators sit at the boundary between a permanent dayside furnace and a permanent nightside cold. But they are not mirror images of each other.

WASP-121 b sustains fast eastward jet streams that carry superheated air from the dayside across the evening terminator before it can cool. The morning terminator, by contrast, receives air that has already shed much of its heat during its passage across the nightside. The result is a gap — 1,775°C — that measures directly how much energy the atmospheric circulation transfers before it reaches dusk.

This matches predictions from circulation models for tidally locked planets, but prior measurements — including Hubble observations — could only detect the combined signal from both terminators, not resolve them individually. Thomas Evans-Soma of the Max Planck Institute, who designed the JWST observing program, and Johns Hopkins astronomer David Sing were among the co-authors.

What this unlocks for planets we care about

WASP-121 b will not host life. But the question it raises reaches further. Rocky planets in habitable zones around cool stars are also expected to be tidally locked, with two distinct terminator edges. If those edges carry different chemical signatures — one cloud-covered, one clear; one showing water, the other not — telescopes searching for signs of life could reach different conclusions depending on which limb they happen to sample.

The WASP-121 b result is a worked example at the extreme end of the spectrum. Knowing that terminator asymmetries exist, and what drives them, is the first step toward reading them correctly.

Common questions about WASP-121 b

Q: What does it mean for a planet to be tidally locked?

Tidal locking occurs when a star’s gravity gradually slows a planet’s rotation until one face permanently points toward the star and the other permanently faces away. WASP-121 b has a permanent dayside at around 2,500°C and a permanent nightside at roughly 725°C, with no seasons and no day-night cycle.

Q: Why do mineral clouds form at dawn but not at dusk?

The morning terminator receives air from the colder nightside. That cooler air can drop to temperatures where silicate compounds solidify into particles and form clouds. By the time it reaches the evening side, it has been reheated by the jet streams crossing the dayside and is too warm for clouds.

Q: Has WASP-121 b been studied before?

Extensively. Previous observations with Hubble and Spitzer gave broad atmospheric data but could not resolve the two terminators separately. This is the first study to read morning and evening edges as distinct environments within a single transit.

Q: Does this affect the search for life on other planets?

Not directly — WASP-121 b is too hot and too massive to be habitable. But the technique matters: tidally locked rocky planets in habitable zones may also have distinct terminator edges, and misreading one could give a false picture of the planet’s habitability.

Cyril Gapp et al., “Atmospheric asymmetries in WASP-121 b revealed by rotational transits detected with JWST,” Nature Astronomy, June 11, 2026. DOI: 10.1038/s41550-026-02887-6

Tags: astronomy, exoplanet, JWST, Cyril Gapp, NIRSpec, terminator