Summary
jwst, using its NIRSpec instrument and rotational-transit transmission-spectroscopy, detected a dawn–dusk atmospheric asymmetry between the morning and evening terminators of the ultra-hot hot-jupiter wasp-121b. Led by cyril-gapp (a PhD student at the max-planck-institute-for-astronomy) with co-author tom-evans-soma (University of Newcastle / MPIA), the study was published in Nature Astronomy (DOI 10.1038/s41550-026-02887-6). The evening (trailing) terminator absorbs more starlight than the morning (leading) side, consistent with hot eastward winds that carry heat from the dayside, expanding the evening region and increasing its cross-section. A rising CO signal toward egress is interpreted as a temperature effect, while a real drop in H₂O is read as thermal dissociation of water in the heated evening limb. Observed signal amplitude exceeded model predictions, which the team attributes to unmodeled silicate clouds cooling the morning terminator. The finding closely parallels the wasp-94ab daily-weather-cycle result (src-jwst-wasp-94ab-weather-cycle-2026-05), reinforcing that limb-resolved JWST observations reveal longitudinal structure invisible to blended-disc methods.
Key Claims
- JWST/NIRSpec rotational transits detected a dawn–dusk asymmetry in infrared absorption on WASP-121 b — limb-resolved by measuring how starlight absorption changes as the planet rotates ~30° during transit (Gapp et al., Nature Astronomy 2026).
- The evening (trailing) terminator absorbs more light than the morning (dawn) side, consistent with hot eastward winds transporting dayside heat and expanding the evening region.
- A rising CO signal is a temperature effect (not more CO molecules); a real H₂O decrease reflects thermal dissociation of water in the hot upper atmosphere (Gapp et al.).
- WASP-121 b is tidally locked, with dayside temperatures ~2,770 K and nightside ~1,000 K (Tom Evans-Soma).
- Observed signal amplitude was larger than atmospheric models predicted; the team suspects unmodeled cooling at the morning terminator, likely from silicate (mineral) clouds that shield deeper IR emission (Gapp et al.).
- The method is a “blueprint” for surveying other ultra-hot gas giants to map their longitudinal structure (the authors).
Notable Quotes
“With its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date: By measuring how starlight absorption changes as WASP-121 b rotates, we probe its atmosphere longitude by longitude.” — Cyril Gapp (MPIA)
“WASP-121b is particularly extreme, with average temperatures on the dayside hemisphere being around 2,770 Kelvin, while those on the nightside are closer to about 1,000 Kelvin.” — Tom Evans-Soma (University of Newcastle / MPIA)