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James Webb Space Telescope sees sand clouds on 'cotton candy planet' WASP-107b

A team of European astronomers has found a silicate-based weather system on a cloudy gas planet around the star WASP-107. It is the first time astronomers have found silicate clouds and rain. They also conclude that temperatures deeper in the atmosphere are rising rapidly. 'The presence of clouds has already been demonstrated on other planets, but this is the first time we see what they are made of,' says Leiden astronomer Nicolas Crouzet.

Exoplanet WASP-107b orbits a star slightly cooler and lighter than our sun. The planet is about as heavy as Neptune, but a lot bigger, with almost the size of Jupiter. That makes its density much lower than what we are used to in our solar system, earning it the nickname cotton candy planet. That woolliness allows astronomers to look 50 times deeper into its atmosphere than Jupiter's. This gave them insight into the presence of water vapour, sulphur dioxide, silicate clouds, and the absence of methane.

The astronomers use the spectra of water vapour and sulphur dioxide to compare them with those in their models for planets with and without clouds. The spectra show little sharp detail, as in the models with clouds. 'Clouds high in the atmosphere largely hide the water vapour and sulphur dioxide,' says co-author Rens Waters (Radboud University). 'The presence of clouds has already been demonstrated on other planets, but this is the first time we see what they are made of. In this case, the answer is silicate, so basically sand.' Co-author Nicolas Crouzet (Leiden University): 'Thanks to the MIRI instrument on the James Webb Space Telecope, co-developed by NOVA in the Netherlands, we can measure the composition of sand clouds very accurately for the first time.'

Artistic impression of WASP-107b and its parent star © Klaas Verpoest

'Only' 500 °C in outer atmosphere layer

While on Earth we see water evaporating at high temperatures, weather systems look different on gas planets with temperatures around 1000 °C. There, it is silicate particles that evaporate when it gets hot. But on WASP-107b, it is 'only' 500 °C in the outer atmosphere layer. When raining out a cloud, the sand rain would then only evaporate again deep inside - too deep to float all the way back up as a cloud. Still, the clouds must keep reappearing, otherwise we wouldn't see them now. 'It seems that it gets very hot below the upper layer surprisingly quickly,' says co-principal author Michiel Min (SRON). 'There, the sand rain evaporates to form clouds again and float upwards. Basically just like the water system on Earth.'

'The absence of methane is a second indication of rapidly rising temperatures,' Min adds. 'At low temperatures, methane is expected to be an important component of the atmosphere, similar to Jupiter where methane is abundant. A high internal temperature destroys methane deep in the atmosphere.'

Scientific article

SO2, silicate clouds, but no CH4 detected in a warm Neptune’, Achrene Dyrek, Michiel Min, Leen Decin, et al., Nature

This article appeared as a press release on the wesbite of de Nederlandse Onderzoekschool voor Astronomie (NOVA)

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