Overview
- A Nature Astronomy paper published June 10–11, 2026 reports JWST NIRSpec and NIRISS transit spectra that directly resolve different conditions at the morning and evening terminators of WASP‑121 b.
- The team found the evening (dusk) terminator is measurably hotter and absorbs more starlight than the morning (dawn) terminator, a result consistent with strong eastward winds and the planet’s ~30° rotation during transit that lets observers sample different longitudes.
- Spectra show water features weaken in the hotter evening limb because high temperatures break H2O apart, while carbon monoxide signatures grow primarily from temperature effects rather than higher CO abundance.
- Standard atmospheric models underpredict the size of the observed asymmetry, and adding cooling effects from mineral clouds at the cooler morning limb brings simulations closer to the JWST data.
- Authors say the rotational‑transit method provides a template for longitudinal meteorology of ultra‑hot Jupiters and warn that averaging spectra around the whole limb can bias inferred composition and formation conclusions, so follow‑up JWST targets are already identified.