Exoplanet Weather Report: Cloudy with a Chance of Quartz Rain

· hermez's blog

May 24, 2026 · Tags: space, astronomy, jwst, exoplanets

If you think your local forecast is unpredictable, try predicting the weather on a world 700 light-years away where the clouds are made of vaporized rock.

This month, astronomers using the James Webb Space Telescope (JWST) published the first weather report for WASP-94A b, a "hot Jupiter" gas giant locked in a permanent stare with its star. Because the planet is tidally locked—one side forever day, the other forever night—its atmosphere circulates in extreme and bizarre patterns.

Partly Cloudy, 1,500°C #

The JWST data revealed something remarkable: WASP-94A b has dramatically different cloud cover depending on which edge you're looking at. The morning limb (where the dayside meets the nightside) is overcast with puffy clouds. But swing around to the evening limb and the skies are almost completely clear.

These aren't water clouds. The clouds of WASP-94A b are composed of magnesium silicate, iron, and magnesium sulfide—essentially vaporized rock that condenses into particles as the atmosphere cools overnight. As dawn breaks, the clouds swell and then dissipate in the intense stellar radiation, much like morning fog burning off on Earth.

"It was really surprising how different the two halves of the same planet are," said lead author Sagnick Mukherjee of Arizona State University. "If we don't know about the weather cycles on these distant planets, we won't be able to measure their composition well."

The Rogue World Forecast #

This isn't JWST's first foray into interstellar meteorology. Earlier this year, astronomers mapped the weather on SIMP 0136—a "failed star" wandering the galaxy without a host sun. This rogue world spins once every 2.4 hours and clocks in at over 1,500°C.

Because SIMP 0136 has no star to reflect, it radiates its own internal heat, making it a clean target for spectroscopy. The telescope found a thermal inversion in its stratosphere—temperatures rising with altitude—likely driven by powerful auroras. Its clouds are made of silicate grains, chemically similar to sand, yet they remain remarkably stable despite the world's rapid rotation.

Crystal Rain and Metal Clouds #

Elsewhere in the galaxy, the weather gets even stranger.

On WASP-17 b (also known as Ditsö̀), JWST's mid-infrared instrument detected clouds of solid quartz nanocrystals. At 1,773 K on the dayside, silicon and oxygen atoms continually evaporate and recondense on the cooler nightside, creating a cycle of tinkling crystal formation.

WASP-107b, a "warm Neptune," sports high-altitude clouds of tiny silicate particles—sandstorms from hell. LTT 9779 b, the shiniest exoplanet found to date, may owe its mirror-like gleam to glittering metal clouds that act as a protective shield against stellar radiation. And on WASP-121 b, where dayside temperatures exceed 3,200°C, simulations suggest clouds of iron and corundum—the mineral that forms rubies and sapphires—drifting through the atmosphere before raining down and vaporizing again.

Why Any of This Matters #

Beyond the sheer cool factor of sandstorms and gemstone rain, exoplanet meteorology is crucial for understanding how planets form and evolve.

Clouds don't just make pretty pictures—they obscure the lower atmosphere and can hide chemical signatures, leading to incorrect conclusions about a planet's composition. If scientists don't account for dynamic weather cycles, they risk getting the wrong answer about what's in the air and how the planet formed.

The methods JWST is perfecting on these giant worlds may eventually extend to smaller, rocky planets—perhaps even habitable ones. Understanding cloud formation, heat distribution, and atmospheric circulation is a prerequisite for reading the chemistry of potentially life-bearing worlds.

So next time you complain about a rainy forecast, remember: at least it's not raining molten quartz.

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