Fluffy exoplanet blasted by its sun has clouds that rain sand

Huge clouds made of sand soar in the skies of fluffy Jupiter-sized planet WASP-107b, according to data from the James Webb Space Telescope.

In 2017, astronomers discovered this unique planet, about 200 light years away from Earth in the constellation Virgo. With a similar mass to Neptune, but a radius much bigger, closer to that of Jupiter, WASP-107b is much less dense than other giant gas planets, about as dense as cotton candy. This is what makes it look fluffy, says Leen Decin at KU Leuven in Belgium.

“In fact, this fluffy planet has one of the lowest densities we’ve ever seen,” she says. “That allows us to really look very deeply into the atmosphere of that planet.”

By using the James Webb Space Telescope’s Mid-Infrared Instrument, Decin and her colleagues have now peered into WASP-107b.

They have found that two of the key components of its atmosphere are sulphur dioxide and water vapour. Sulphur dioxide has previously been detected on hot gas giants with an average temperature of 1200 kelvin (927°C), says Decin, but it was surprising to see it on WASP-107b, which is more like 700K (427°C), thought to be too cold for large amounts of sulphur dioxide to form.

One possible explanation for its presence may be that more ultraviolet radiation from the host star, WASP-107, can penetrate the planet due to its relatively low density, triggering chemical reactions that form the compound.

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Perhaps more strangely, in the planet’s upper atmosphere, Decin and her colleagues found clouds made of tiny silicate particles – the matter that forms sand. The researchers think that gaseous silicate deeper in the planet’s atmosphere, where it is hotter, must rise up to where it is cooler, condense to form the clouds, and then rain back down, much like what happens on Earth with water.

“This is the first time we’ve identified the composition of exoplanetary clouds,” says Decin.

The findings could improve models of planetary formation and evolution. “We understand things based on our own experience here on Earth, but that’s a very limited view,” she says. “We can really enhance our view on the universe by understanding the dynamics and chemistry of exoplanets.”