WASP-121b
WASP-121b is a tidally locked, ultra-hot hot-jupiter exoplanet, notable for the first clear detection of a dawn–dusk atmospheric asymmetry resolved across its terminators.
Properties
- Tidally locked ultra-hot Jupiter; dayside ~2,770 K (~2,500 °C), nightside ~1,000 K (~725 °C).
- Rotates ~30° during a full transit, enabling limb-resolved transmission-spectroscopy via rotational transits.
Dawn–dusk asymmetry (2026)
Using jwst/NIRSpec rotational transits, cyril-gapp and tom-evans-soma (Gapp et al., Nature Astronomy 2026, DOI 10.1038/s41550-026-02887-6) found:
- The evening (trailing) terminator absorbs more starlight than the morning side, consistent with hot eastward winds carrying dayside heat and expanding the evening region.
- A rising CO signal interpreted as a temperature effect; a real H₂O drop attributed to thermal dissociation in the heated evening limb.
- Observed amplitude exceeded model predictions, attributed to unmodeled silicate clouds cooling the morning terminator — a challenge for exoplanet-atmosphere-models.
The result is a direct sibling to the wasp-94ab daily-weather-cycle finding (src-jwst-wasp-94ab-weather-cycle-2026-05); together they show limb-resolved JWST observations reveal longitudinal structure invisible to blended-disc methods — a caveat relevant to biosignatures retrieval.