Summary
jwst, using its NIRISS instrument and transmission-spectroscopy, resolved the first confirmed repeating daily weather cycle on an exoplanet: the hot-jupiter wasp-94ab, ~690 light-years away. The planet’s permanent morning limb is shrouded in thick magnesium-silicate clouds while its evening limb is clear, a contrast detected at 9σ (morning cloud) and 10σ (evening water vapor). Published in Science on May 21, 2026, the work was led by sagnick-mukherjee with senior author david-sing. Crucially, separating the limbs corrected a decade of cloud-contaminated, full-disc measurements: WASP-94Ab’s oxygen and carbon abundances drop from “hundreds of times” Jupiter to ~5× Jupiter, a result consistent with standard gas-giant formation. The same morning-cloud / evening-clear pattern recurs on WASP-39b and WASP-17b, implying prior pre-JWST composition estimates for hot Jupiters may be systematically biased — a caveat that extends to biosignature searches on smaller worlds.
Key Claims
- JWST/NIRISS resolved a repeating daily weather cycle on WASP-94Ab — the first confirmed for any exoplanet (Mukherjee et al., Science, May 21 2026).
- Morning (leading) limb: thick magnesium-silicate (vaporized rock) clouds, cloud absorption at 9σ; evening (trailing) limb: clear, water-vapor absorption at 10σ.
- WASP-94Ab is a tidally locked hot Jupiter: 1.72 R_Jup, ~0.45 M_Jup, 3.95-day orbit at 5.1M miles, surface >2,200°F (1,200°C); the evening limb runs ~450 K hotter than the morning limb.
- Proposed mechanism: Mg-silicate clouds condense on the cool nightside, ride equatorial super-rotation winds to the morning terminator, then sink or evaporate crossing into the dayside; evening skies clear (attributed to the paper’s authors).
- Limb-resolved data revised WASP-94Ab’s O and C abundance from “hundreds of times Jupiter” (blended) to ~5× Jupiter (limb-resolved); the two estimates differ at >4σ.
- The same cloud-cycle pattern was found on WASP-39b and WASP-17b among 8 additional hot Jupiters surveyed (T_eq ~800–1,700 K); Hubble could not resolve the limbs separately.
- The authors warn that failing to account for limb asymmetry can skew inferred metallicity and cloud properties for any planet class, including habitable-zone biosignature targets.
Notable Quotes
“It was really surprising how different the two halves of the same planet are.” — Sagnick Mukherjee (lead author, ASU / formerly UC Santa Cruz)
Clouds are “a thorn in our side” … “like trying to look at the planet through a foggy window.” — David Sing (Johns Hopkins, senior author)
“With the Hubble telescope, when we used to do this type of observation, we got an average view of the whole planet with data from the clouds and the atmosphere squished together and indistinguishable. This approach with JWST lets us localize our observations, which helped us see the cloud cycle.” — Sagnick Mukherjee