Exoplanets come in many forms, from dense, rocky planets like Earth and Mars to gas giants like Jupiter and Saturn. But some planets discovered outside our solar system are even less dense than gas giants and are a type known informally as super-puff or cotton candy planets. One of the least dense exoplanets known, WASP-107b, was recently investigated using the James Webb Space Telescope (JWST) and the planet’s weather seems to be as strange as its puffiness.

The planet is more atmosphere than core, with a fluffy atmosphere in which Webb spotted water vapor and sulfur dioxide. Strangest of all, Webb also saw silicate sand clouds, suggesting that it would rain sand between the upper and lower layers of the atmosphere. The planet is almost as big as Jupiter but has a tiny mass similar to that of Neptune.

“JWST is revolutionizing exoplanet characterization, providing unprecedented insights at remarkable speed,“ says lead author of the study, Leen Decin of KU Leuven, in a statement. “The discovery of clouds of sand, water, and sulfur dioxide on this fluffy exoplanet by JWST’s MIRI instrument is a pivotal milestone. It reshapes our understanding of planetary formation and evolution, shedding new light on our own solar system.”

Understanding the planet’s formation and evolution is important because it seems impossible that it could have formed in its current location. It is thought to have formed further out in the star system and migrated inward over time. That could allow for its extremely low density. Its close orbit to its star means it has a very high temperature, with its outer atmosphere reaching 500 degrees Celsius. But those temperatures are not normally hot enough to form clouds of silicate, which would be expected to form in lower layers where the temperatures are higher.

The researchers theorize that the sand rain is evaporating in the lower, hotter layers and the silicate vapor moves upwards in the atmosphere before recondensing to form clouds and falling as rain, similar to the water cycle on Earth.

“The value of JWST cannot be overstated: wherever we look with this telescope, we always see something new and unexpected,” said fellow researcher Paul Mollière from the Max Planck Institute of Astronomy. “This latest result is no exception.”

The research will be published in the journal Nature.

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