Overview
- In June 2026, James Webb Space Telescope transit data showed, for the first time, distinct morning and evening atmospheres on ultrahot Jupiters rather than a single averaged limb.
- A rotational‑transit study of WASP‑121 b detected the planet turning during one transit and found the evening terminator hotter with rising carbon monoxide and signs that water is thermally dissociated.
- Limb‑resolved spectra of WASP‑94A b revealed a strong cloud cycle with very cloudy mornings and clear evenings, which lowered prior oxygen and carbon abundance estimates from extreme values to roughly a few times solar.
- The detections are statistically significant but include caveats: one JWST detector gave a symmetric result for WASP‑121 b, the asymmetry is strongest at longer infrared wavelengths, and current 3D climate models reproduce the direction but not the full size of the temperature difference.
- Teams are scaling the methods through an expanded JWST survey totaling more than 180 hours to test other ultrahot Jupiters and to force improvements in 3D climate and cloud microphysics needed to interpret weather and formation clues.