JWST Maps Day-Night Weather on Hot Jupiter WASP-94Ab

JWST’s NIRISS was used by a team led by Sagnick Mukherjee to perform limb-resolved transmission spectroscopy on WASP-94Ab, a tidally locked hot Jupiter orbiting one star of a binary system 690 light-years from Earth. The planet is about 1.7 times Jupiter’s radius and half its mass, giving a low density that inflates its atmosphere. By separating the spectra of the planet’s leading (morning) and trailing (evening) limbs during transit, the study obtained distinct atmospheric signatures for each side—something earlier Hubble observations could not achieve.

Ars Technica

The morning limb shows clouds of vaporized magnesium silicate, while the evening limb is cloud-free, revealing a hydrogen-rich atmosphere. A ~450 K temperature contrast drives equatorial winds that lift condensates on the night side, transport them eastward, and cause them to evaporate before reaching evening terminator. They propose either wind-driven vertical transport or rapid daytime evaporation as the mechanism for cloud clearing.

JWST just mapped the morning weather on a planet 690 light-years away, and the forecast of sand-like clouds exposed a 100-fold bias in how exoplanet atmospheres have been read for more than a decade

Earlier Hubble-based spectra suggested oxygen and carbon abundances up to 100 times solar, a value inconsistent with planet-formation models. Limb-resolved JWST data reduced the enrichment to 3–5 times solar, aligning WASP-94Ab’s composition with that of Jupiter and with theoretical expectations.

JWST just mapped the morning weather on a planet 690 light-years away, and the forecast of sand-like clouds exposed a 100-fold bias in how exoplanet atmospheres have been read for more than a decade

David Sing, Bloomberg Distinguished Professor at Johns Hopkins, called cloudiness a long-standing obstacle and said the new observations “clear the view” and identify the cloud material. Mukherjee emphasized that prior analyses treated each planet as a homogeneous sphere, whereas the new technique reveals dynamic, region-specific weather.

The Debrief

The discrepancy between Hubble-derived and JWST-derived elemental abundances highlights a bias in exoplanet spectroscopy. The limb-resolved technique applied to eight hot Jupiters revealed similar morning-evening cloud cycles on WASP-17b and WASP-39b. The exact process that removes clouds—whether wind-driven uplift and descent or daytime evaporation—remains unresolved, and surveys are needed to determine how common this day-night dichotomy is among hot Jupiters.

Space.com

Demonstrating that a single hot Jupiter can host dramatically different atmospheres on its day and night sides calls for a reassessment of composition estimates for many known exoplanets. The limb-resolved approach will guide future JWST programs targeting a wider variety of worlds, including eccentric gas giants with extreme temperature swings.

Space.com

• “I've been looking at exoplanets for 20 years and general cloudiness has been a thorn in our side,” — David Sing, Bloomberg Distinguished Professor, Johns Hopkins University

Space.com

• “We wanted to understand the atmospheres of such planets,” — Sagnick Mukherjee, Johns Hopkins University

Ars Technica

• “The problem with this approach is that the light filtering through the entire circumference of the planet’s silhouette was averaged out, as though its atmosphere was one homogenous ball of gas.” — Sagnick Mukherjee, Johns Hopkins University

Ars Technica

• “The answer, at least for this world, is that the weather is in constant motion.” — Sagnick Mukherjee, Johns Hopkins University

JWST just mapped the morning weather on a planet 690 light-years away, and the forecast of sand-like clouds exposed a 100-fold bias in how exoplanet atmospheres have been read for more than a decade