Full Breakdown
JWST Uncovers Asymmetric Weather on Ultra-Hot Jupiters
6/17/2026, 11:30:17 AM
JWST Reveals Asymmetric Weather on Ultra-Hot Jupiters
JWST observed transits of ultra-hot Jupiters WASP-121b and WASP-94A b, finding distinct morning-evening differences. WASP-121b’s evening terminator absorbed more starlight, indicating temperatures high enough to dissociate water, while its morning side showed silicate-cloud signatures. WASP-94A b displayed strong water absorption on the morning limb and a clear evening limb, implying a wind-driven cloud cycle that evaporates clouds as they move toward the hotter dayside.
From Hubble to JWST
Earlier Hubble spectra averaged the whole planet, yielding uncertain elemental abundances. Recent Very Large Telescope data revealed complex winds on WASP-121b, but JWST’s higher resolution now enables longitudinal probing, allowing separate analysis of morning and evening limbs.
Key Atmospheric Findings
JWST found WASP-121b’s evening side hotter, with water-vapor and carbon-monoxide signals varying by longitude and temperatures sufficient to break water molecules; the morning side likely hosts silicate clouds. WASP-94A b’s morning limb shows water absorption, while its evening limb is clear, indicating cloud evaporation as particles are advected to the dayside. Both planets are tidally locked, producing permanent day and night hemispheres that drive strong east-west winds.
Significance for Exoplanet Science
Separating morning and evening spectra reduces compositional uncertainty. Revised measurements for WASP-94A b show oxygen and carbon about five times solar, correcting earlier estimates of hundreds-fold enrichment. The results demonstrate how clouds can mask or reveal chemical signatures, affecting models of planetary formation and atmospheric evolution.
Official Statements & Responses
Cyril Gapp of the Max Planck Institute for Astronomy highlighted JWST’s unprecedented observational quality and its ability to probe atmospheres longitude by longitude. Sagnick Mukherjee of Johns Hopkins University emphasized the shift from detecting exoplanet atmospheres to mapping three-dimensional weather, chemistry, and cloud structures. The teams said the technique can be applied to a broader sample of ultra-hot planets.
Conflicting Reports & Gaps
WASP-121b shows metal-vapor weather while WASP-94A b exhibits mineral cloud cycles, underscoring the need for more sophisticated models to confirm cloud composition and quantify wind speeds across diverse hot Jupiters.
Verbatim Quotes
- “With its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date,” — Cyril Gapp, lead author, Max Planck Institute for Astronomy
- “By measuring how star light absorption changes as WASP-121 b rotates, we probe its atmosphere longitude by longitude,” — Cyril Gapp
- “This finding baffled us because this result could not be explained by planet formation theories.” — Sagnick Mukherjee
- “As these cloud particles are dragged farther toward the hotter dayside, the clouds evaporate or sink deeper, leaving the evening side much clearer,” — Sagnick Mukherjee
What’s Next
The Johns Hopkins team, co-led by Mukherjee, David Sing, and Guangwei Fu, will receive over 180 hours of JWST observations to study weather and cloud coverage across a range of exoplanet temperatures and gravities. The approach demonstrated on WASP-121b and WASP-94A b will be applied to additional hot gas giants, including WASP-39 b and WASP-17 b, expanding comparative atmospheric studies.
