JWST maps dawn-dusk atmospheric split on ultra-hot exoplanet WASP-121 b, dayside near 2,770 K and night near 1,000 K
James Webb has mapped a planet where dawn and dusk run on entirely different chemistry. On WASP-121 b the dayside reaches about 2,770 Kelvin while the night falls near 1,000 Kelvin, with water torn apart in the hottest regions. Led from Max Planck Heidelberg, published in Nature
The James Webb Space Telescope has produced one of the most detailed looks yet at how conditions differ between the two edges of a single exoplanet. The target is WASP-121 b, an ultra-hot gas giant far beyond our solar system, and the new measurements show its morning and evening skies run on strikingly different chemistry. On the side permanently facing its star, average temperatures sit near 2,770 Kelvin. The night side is far cooler, closer to 1,000 Kelvin. Webb detected stronger carbon monoxide signals later in the planet's transit and noticeably less water vapour in the hottest regions, which researchers read as evidence that the extreme heat is splitting water molecules apart. The evening edge of the planet absorbs more starlight than the morning edge, consistent with powerful eastward winds that haul heat from the scorching dayside around toward the night. One proposed explanation for the contrast involves mineral clouds, likely made of silicates, that may form more readily on the cooler morning side and block infrared radiation rising from deeper layers. The work matters because it moves exoplanet study past treating these worlds as single points of light and toward mapping weather and chemistry across them. The research was led by Cyril Gapp at the Max Planck Institute for Astronomy in Heidelberg and published in Nature Astronomy.