JWST forecasts extreme weather on exoplanet WASP-121b： 11,000-mph winds and nighttime rains of rubies and sapphires
Summary: On June 16, 2026, JWST mapped the chaotic atmosphere of hot Jupiter WASP-121b, revealing 11,000-mph winds and nighttime rains of liquid gems formed by condensing metal oxides.
The phrase rains of rubies and sapphires is more than a metaphor — it captures a real phase-change process. Under the extreme ultraviolet radiation from the host star, aluminum oxide (Al₂O₃, also known as corundum) in the upper atmosphere of WASP-121b is broken apart molecule by molecule. Strong winds sweep these fragments across the terminator, where they recondense on the cooler night side into tiny corundum grains. Trace amounts of chromium in the crystal lattice produce a red hue, the same coloring that defines rubies on Earth; titanium and iron impurities produce blue, the signature of sapphires. Because WASP-121b is tidally locked, one hemisphere is permanently scorched while the night side is cool enough for metal condensates to fall as rain — giving the planet a self-sustaining day-to-night gemstone cycle.
The wind measurement comes from phase-resolved spectroscopy with JWST, in which spectra are taken at successive orbital phases and then combined. By mapping how spectral features shift with orbital phase and at different atmospheric depths, the team reconstructed a global wind field covering latitude and altitude. The result is a planet-wide jet stream that drives hot gas from the day side to the night side at roughly 11,000 mph (about 17,700 km/h) — more than 14 times the speed of sound at sea level. Such extreme circulation is the natural atmospheric response to the enormous temperature contrast between the hemispheres of a hot Jupiter.
This is the first time astronomers have obtained a true three-dimensional weather map of an exoplanet, with temperature, composition, and wind speed measured at every latitude and altitude. The team used the JWST observations to benchmark global climate models and found the real atmosphere far more turbulent than most predictions. Beyond the gemstone rain, the spectra also revealed clear signatures of metals and metal compounds — iron, titanium, vanadium, chromium — suggesting that hot Jupiter chemistry is far richer than previously thought. The dataset is expected to anchor a growing body of work on hot Jupiters, brown dwarfs, and other ultrahot worlds.