The James Webb Space Telescope has revealed a fascinating phenomenon on WASP-121 b, an ultrahot Jupiter with an equilibrium temperature above 2,000 kelvin. The telescope observed the planet during a single transit, capturing the planet's shadow and its changing shape as it slowly rotated. This is the first time anyone has measured a planet's rotation during a single transit and read its weather from it.
What makes this discovery even more intriguing is the temperature difference between the planet's evening and morning edges. The evening edge runs hotter than the morning edge, and the air on the hotter side is fierce enough to tear water molecules into their separate atoms. This is due to the planet's lopsided dayside, where the eastern half is hotter than the western half. 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.
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. The changing-size model won by a wide statistical margin, indicating that the planet's atmosphere is not symmetrical.
The study also revealed that the planet's dayside is around 2,800 kelvin, but the team deliberately stopped 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. The team also declined to make a measurement of the planet being slightly squashed by its star's gravity, judging the evidence too shaky to stand on.
What makes this 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.
