James Webb discovers sand clouds on 'cotton candy planet' WASP-107b

An international team of astronomers, co-led by Michiel Min (SRON), has discovered a silicate-based weather system on a fluffy gas planet around the star WASP-107. It is the first time that scientists find clouds and rain made of sand. They also conclude that the temperature deeper in the atmosphere is rising surprisingly rapid. Publication in Nature.

Credit: Klaas Verpoest.
Credit: Klaas Verpoest.

Exoplanet WASP-107b orbits a star that is slightly cooler and lighter than our Sun. The planet is about the same mass as Neptune, but much larger, almost the size of Jupiter. That makes its density much lower than what we are used to in our Solar System, hence the nickname cotton candy planet. This fluffiness allows astronomers to look fifty times deeper into its atmosphere compared to Jupiter. It enabled them to spot the presence of water vapor, sulfur dioxide, silicate clouds, and the absence of methane.

The astronomers use the spectra of water vapor and sulfur dioxide to compare them with those in their models for planets with and without clouds. The spectra show few sharp details, just like in the models with clouds. ‘Clouds high in the atmosphere largely block the water vapor and sulfur dioxide from view,’ says Rens Waters (Radboud University), one of the authors. '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 developed in part by NOVA in The Netherlands, we can measure the composition of sand clouds very accurately for the first time.’

Whereas on earth water evaporates on a warm day, weather systems follow a different script on gas planets with temperatures around 1000 °C. In that case, silicate particles are the ones that evaporate when it gets warm. However on WASP-107b the outer atmospheric layer is 'just' 500 °C. So when a cloud rains out, the sand rain wouldn’t evaporate until it reaches deep into the interior—too deep to drift all the way back to the top as a cloud. Yet the clouds must be forming over and over again, otherwise we wouldn’t see them now. ‘It seems that it gets very hot surprisingly close beneath the top layer,’ says co-lead 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 says. 'With low temperatures it is expected that methane is an important component of the atmosphere, comparable to Jupiter where methane is common. A high internal temperature destroys methane deep in the atmosphere.’

Nature article