On WASP-121 b, a gas giant more than 1.5 times the width of Jupiter, the line where day turns to night is not the same temperature all the way around. The edge of the planet where evening is setting runs hotter than the edge where morning is breaking, and on the hotter side the air is fierce enough to tear water molecules into their separate atoms.
A team of astronomers reported the difference in Nature Astronomy on June 10, and the way they found it is almost as striking as the finding itself. They did not photograph the planet or fly anything near it. They watched WASP-121 b cross the face of its star with the James Webb Space Telescope and noticed that the planet’s shadow changed shape as it slowly rotated during the few hours of the crossing. That change is the first time anyone has measured a planet’s rotation during a single transit and read its weather from it.
How you watch a planet turn from light-years away
WASP-121 b is what astronomers call an ultrahot Jupiter, a gas giant with an equilibrium temperature above roughly 2,000 kelvin, which is hotter than some stars are cool. It hugs its star so tightly that a full year there lasts about 30 hours. At that range the planet is tidally locked, meaning one face is held toward the star permanently, the way the same side of the Moon always faces Earth. One hemisphere bakes in endless daylight; the other never sees the star at all.
The usual way to study such an atmosphere is transmission spectroscopy. When the planet passes in front of its star, a sliver of starlight filters through the ring of atmosphere around the planet’s edge, and different gases absorb different colors of that light. Read the colors that go missing and you can tell what the air is made of around the planet’s rim.
The new twist is timing. Because WASP-121 b is so close to its star, it turns far enough during a single transit that the slice of atmosphere backlit at the start of the crossing no longer overlaps the slice backlit at the end. So as the planet rotates, fresh longitudes swing into view, and the amount of starlight the planet blocks shifts from one moment to the next. The team, led by Cyril Gapp, built a light-curve model that allowed the planet’s apparent size to change during the transit, and tested it against the older assumption that a planet’s silhouette stays fixed. Across two separate Webb observations, the changing-size model won by a wide statistical margin.
The two stares were years apart and used different instruments: one in October 2022 with Webb’s near-infrared spectrograph, another in October 2023 with a separate slitless imager. That spread mattered, because WASP-121 b is an awkward target. It circles a hot, fast-spinning star on a tilted orbit badly misaligned with the star’s spin, close enough that the star’s gravity is slowly pulling the planet apart, and the star itself is brighter at its poles than at its equator. Each of those quirks can smear a transit on its own, so the team had to model them out before trusting that the leftover asymmetry belonged to the planet’s weather rather than to its star.
Why the evening edge runs hotter than the morning edge
The shape of the change told the team which way the temperatures ran. As WASP-121 b rotated, the side rotating into view absorbed more and more carbon monoxide while its water signal stayed flat or dipped slightly. The carbon monoxide feature grew by about 200 parts per million relative to the rest of the spectrum.
That pattern points back to heat. Water survives in cooler air but breaks apart on a scorching dayside, a process called thermal dissociation, while carbon monoxide holds together even when it is roasting. So a stretch of atmosphere rich in carbon monoxide but poor in water is a stretch that is extremely hot. The team found that signature strengthening across the evening terminator, the dividing line where the dayside rotates around toward night.
The reading lines up with a picture other instruments have been sketching for years: WASP-121 b’s dayside is lopsided, with the eastern half hotter than the western half. On planets like this a deep circulation carries heat from the permanently lit dayside toward the night side, and a strong equatorial jet drags that flow eastward, piling heat onto the evening side. Webb has now caught that lopsidedness not as a static map but in motion, by letting the planet’s own spin carry the hot air across the telescope’s line of sight.
What this does and does not prove
The detection is statistically strong, and the team did the unglamorous work of ruling out the boring explanations. They showed the asymmetry was not an instrument artifact, and not just an illusion cast by the host star’s uneven brightness, which can mimic a lopsided planet on its own.
Even so, the result comes with honest caveats, and the paper does not hide them. One of Webb’s two detectors recorded a symmetric transit while the other recorded the asymmetric one; the signal lives at the longer infrared wavelengths, which is consistent with the physics the team proposes but is a reminder that the effect is subtle. The interpretation also leans on a three-dimensional climate simulation of the planet, and that simulation does not reproduce the observed asymmetry unless its morning side is made colder by hand. In other words, the model and the data agree on the direction of the temperature difference but not yet on its size.
There is also a number worth not over-reading. The simulation puts WASP-121 b’s dayside at around 2,800 kelvin, but that is a modeled value, not a thermometer reading, and the study deliberately stops short of quoting a precise temperature gap between dawn and dusk. The honest claim is about which side is hotter and why, not by exactly how many degrees. A faint hint in the data even suggests the planet may be slightly squashed by its star’s gravity, but the team declined to make that measurement, judging the evidence too shaky to stand on.
A new way to read distant weather
What makes the work matter beyond one planet is the method. Until now, the difference between a planet’s morning and evening had to be teased out either from the static shape of its silhouette or from ground-based instruments sensitive enough to catch the Doppler shift of moving air. Using the planet’s rotation during a single transit gives astronomers a third handle, one that works at the lower resolution that space telescopes deliver.
The team points to other planets where the same trick should work, fast-rotating ultrahot worlds such as WASP-33 b and KELT-9 b, though those orbit rapidly spinning stars whose own distortions will have to be untangled first. For now, WASP-121 b stands as the test case: a world where the sky on the evening side and the sky on the morning side are different climates, close enough in the data that a telescope more than a million kilometres from Earth could tell them apart by watching the planet quietly turn.
