JWST Finds Morning Clouds and Evening Clear Skies on Tidally Locked Exoplanet WASP-94A b

*TL;DR: JWST observations show that the hot, tidally locked gas giant WASP-94A b is cloudy in the morning and clear in the evening, proving its atmosphere is far from static.

Context WASP-94A b orbits one star of a binary pair about 690 light‑years from Earth. The planet is tidally locked, meaning one side always faces its star while the opposite side stays in perpetual night. Its mass is just under half that of Jupiter, yet its diameter exceeds Jupiter’s by more than 70 %, giving it a low‑density, puffed‑up atmosphere that is easier to study.

Key Facts A team led by astrophysicist Sagnick Mukherjee at Johns Hopkins University used the James Webb Space Telescope (JWST) to record the planet’s spectrum as it transited its star. The method—transmission spectroscopy—splits the starlight that passes through the planet’s atmospheric limb into its component colors. By comparing spectra taken at different orbital phases, the researchers isolated light from the morning limb and the evening limb separately. The study, published in *Science*, found a distinct increase in scattering particles on the morning side, consistent with thick clouds, while the evening side showed a clear, cloud‑free spectrum.

The planet’s low density means its atmosphere extends far into space, amplifying the spectral signal. Previous surveys treated the whole limb as a single, uniform ring, averaging out any regional differences. Mukherjee’s team deliberately timed observations to capture the planet at sunrise and sunset, revealing that the atmosphere is dynamic rather than static.

What It Means The discovery overturns the assumption that tidally locked hot Jupiters have homogeneous atmospheres. Cloud formation appears tied to the planet’s rotation, with condensates forming on the cooler morning side and dissipating as winds carry them to the hotter evening side. This pattern suggests strong east‑west winds that transport material around the globe. Models of exoplanet chemistry that relied on averaged spectra may need revision, as cloud coverage can mask or mimic certain molecular signatures.

Future JWST campaigns will target additional tidally locked worlds to test whether morning‑cloud/evening‑clear cycles are common. Tracking changes over multiple orbits could reveal how atmospheric dynamics evolve, sharpening our understanding of weather on worlds far beyond the Solar System.