James Webb Space Telescope forecasts extreme weather on exoplanet that rains rubies and sapphires

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Imagine a world where the weather forecast calls for winds blowing at 11,000 miles per hour (18,000 kilometers per hour) and nighttime showers of liquid metal, rubies and sapphires.

This is the chaotic reality astronomers have pieced together for WASP-121b, an "ultra-hot Jupiter" that ranks among the most extreme planets known beyond the solar system.

The gas giant orbits its host star at such a punishingly close distance that a single "year" there lasts just 30.5 hours. At that proximity — so close that if it got any closer, stellar gravity would start ripping it apart — the host star's immense tidal forces have warped the planet from a sphere into a football-like shape. Temperatures on its dayside climb high enough to vaporize metals, while previous studies have suggested that iron may condense and fall as rain on the cooler nightside. Now, astronomers using the James Webb Space Telescope (JWST) have added another piece to the world's meteorological portrait.

By tracking subtle changes in starlight passing through WASP-121 b's atmosphere as the planet crossed in front of its star, researchers detected differences between atmospheric conditions at dawn and dusk, according to the study.

"With its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date," study lead author Cyril Gapp of the Max Planck Institute for Astronomy in Germany, said in a statement.

"By measuring how star light absorption changes as WASP-121 b rotates, we probe its atmosphere longitude by longitude," Gapp said.

The observations suggest that the planet's evening terminator — the region rotating out of daylight — is hotter than its morning counterpart. The finding is consistent with powerful winds transporting heat from the planet's intensely hot dayside toward its cooler nightside, researchers say.

Because WASP-121 b is tidally locked to its star, one hemisphere permanently faces the star while the other remains in darkness. Yet, during a transit, the planet rotates just enough from JWST's vantage point for different regions of its atmosphere to come into view.

By examining how the atmospheric signal changed over time, Gapp and his team found that the evening side absorbed slightly more starlight than the morning side, the study reports. The researchers also detected changes in signals associated with water vapor and carbon monoxide, which they interpret as evidence of temperature differences across the atmosphere.

The hotter evening side appears warm enough to break apart water molecules in the upper atmosphere, the study notes. The cooler morning side, meanwhile, may be partially obscured by clouds made of silicate minerals, although the study notes more sophisticated models will be needed to determine whether such clouds are indeed present.

The findings add to a growing body of research of turbulent weather on WASP-121 b, including recent data from the Very Large Telescope in Chile that revealed complex, layered and violent wind patterns and jet streams spanning half the world.

Previous observations with the Hubble Space Telescope also found evidence that magnesium and iron were escaping from the planet's atmosphere, likely driven by intense ultraviolet radiation from its host star.

The team's new technique could eventually be applied to other ultra-hot planets, allowing astronomers to compare atmospheric conditions across a broader sample of distant worlds, the study notes.

The study was published Wednesday (June 10) in the journal Nature Astronomy.

Sharmila Kuthunur is an independent space journalist based in Bengaluru, India. Her work has also appeared in Scientific American, Science, Astronomy and Live Science, among other publications. She holds a master's degree in journalism from Northeastern University in Boston.

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