Exoplanets have weather, and for the first time, astronomers can map it. Using the James Webb Space Telescope, researchers have detected a daily cloud cycle on the hot Jupiter WASP-94A b, where sand clouds form every morning and clear by nightfall, and they have found the same pattern on two other worlds. This is the beginning of weather forecasting for planets beyond our solar system.

The Daily Cloud Cycle on WASP-94A b

Sand clouds form every morning but clear up by nightfall on WASP-94A b, a well-studied gas giant nearly 700 light-years away. The research, published in Science in May 2026 by a Johns Hopkins team, used Webb to take separate measurements of the planet's leading edge as it started to cross its star and its trailing edge as it completed the transit. Mornings are riddled with clouds made of magnesium silicate, a common mineral in rocks, while the evening has clear skies.

Why Clear Evenings Matter

Because the evenings are clear of clouds, researchers could look through the trailing edge specifically to see what the atmosphere looked like, something the Hubble telescope could not provide. The clear evening sky showed that WASP-94A b is much more like Jupiter than previously thought, with only five times the amount of oxygen and carbon of our own Jupiter, rather than the much higher enrichment earlier data suggested. Clouds were hiding the real composition, and Webb's ability to separate morning from evening removed the disguise.

The Same Pattern on Other Worlds

Using WASP-94A b as a benchmark, the team looked at eight other hot gas giants and found the same distinctive cloud cycle on two more: WASP-39 b and WASP-17 b. The pattern suggests that daily cloud cycles are common on hot Jupiters, not a quirk of one planet. A large Webb programme will now study cloud cycling across a wide variety of exoplanets, including an eccentric gas giant in the habitable zone, to see how universal the pattern is.

How Webb Sees Clouds

Webb sees clouds by watching starlight filter through a planet's atmosphere during a transit, and by measuring the light from the planet's dayside during an eclipse. Clouds block certain wavelengths and let others through, creating a fingerprint that reveals their composition and altitude. By comparing the morning and evening edges separately, astronomers can map where clouds form and where they clear, building a weather map from light alone.

From Clouds to Climate

Clouds matter because they control a planet's temperature, its chemistry, and what we can see of its surface. On a rocky planet in the habitable zone, clouds could be the difference between a temperate climate and a runaway greenhouse. The techniques proven on hot Jupiters will one day be used on smaller, cooler planets, where the question is whether the climate could support liquid water. Exoplanet weather is no longer a metaphor: it is a measurable science.

The Future of Exoplanet Weather

Exoplanet weather science is still young, and the next decade will bring maps of more planets, with more detail, and perhaps the first weather forecasts for worlds beyond our solar system. The Extremely Large Telescope and the next generation of space telescopes will add ground-based observations, and the combination of space and ground data will reveal weather patterns that Webb alone cannot see. The goal is not just to describe exoplanet weather, but to understand it: why some planets have clouds and others do not, and what that tells us about how planets form and evolve.

Key Takeaways

- Sand clouds form each morning and clear each evening on WASP-94A b - The clouds are made of magnesium silicate, a common rock mineral - The same cloud cycle appears on WASP-39 b and WASP-17 b - Clear evenings let Webb measure the true atmospheric composition

Frequently Asked Questions

Do exoplanets have weather? Yes. Webb has detected daily cloud cycles on hot Jupiters, with sand clouds forming in the morning and clearing by evening, and the same techniques will one day map weather on rocky planets.

Exoplanet weather is now a measurable science. Clouds form, clear, and shape what we see, and the first weather maps are being drawn from starlight.