A Planet's Rotation Revealed Its Fiery Secrets express
The planet WASP-121 b is a gas giant, heated to 2500°C. It always faces its star with one side, like a globe frozen under a lamp. But as it passes in front of the star's disk, its rotation reveals different facets to us — as if that globe were slowly turned, letting us peer into every corner.
Using the James Webb Space Telescope, scientists applied the splitting of light into colors and noticed something odd: during transit, the light absorbed by water vapor weakens, while that by carbon monoxide strengthens. The secret is that the planet's eastern side is heated more fiercely and literally tears water molecules apart. Carbon monoxide, on the other hand, endures this inferno.
This approach turns a transit into a thermal scanner: previously, we only saw the day-night boundary, but now — temperature swings across the entire surface. This is a new way to read weather on distant worlds.
Original abstract
as written by the authors arXiv:2606.19487
Close-in exoplanets are tidally locked to their host star and thus exhibit extreme atmospheric temperature gradients. It has been theorized that the fraction of star light absorbed by such planets during transit changes as a function of orbital phase as progressively hotter or colder atmospheric gas rotates into view, but this effect has not been observed so far. Here, we show that two transits of the ultra-hot Jupiter WASP-121 b, acquired with JWST/NIRSpec and NIRISS, exhibit asymmetric transit light curves caused by the planet's rotation during transit. We observe increasing CO absorption and slightly decreasing H$$_2$$O absorption in the transmission spectrum, as the planet rotates. These results are indicative of a stronger longitudinal temperature gradient across the evening than across the morning terminator, consistent with higher temperatures in the eastern half than in the western half of the dayside. The observed changes of the transmission spectrum with orbital phase are in line with the temperature increase causing thermal dissociation of H$$_2$$O, while CO remains abundant. The observation of longitudinal gradients of atmospheric temperature and chemistry from the planet's rotational transit provides a new probe for constraining atmospheric heterogeneity using JWST beyond differences between morning and evening terminators from limb asymmetries.
Strong winds carry heat from the hottest point eastward, so the eastern side is always hotter than the western — as if the planet is constantly blow-dried by a giant hairdryer.