JWST detects water vapour, sulfur dioxide and sand clouds on nearby exoplanet
15 Nov 2023



MIRI offers insights into atmosphere of WASP-107b.

Artists impression of what exoplanet Wasp 107b might look like according to the scientific data.

A team of European astronomers, including researchers at the UK ATC, used recent observations made with the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST) to study the atmosphere of the nearby exoplanet WASP-107b. Peering deep into the fluffy atmosphere of WASP-107b they discovered not only water vapour and sulfur dioxide, but even silicate sand clouds.

WASP-107b is a unique gaseous exoplanet that orbits a star slightly cooler and less massive than our Sun. The mass of the planet is similar to that of Neptune but its size is much larger than that of Neptune, almost approaching the size of Jupiter.

This characteristic renders WASP-107b rather 'fluffy' when compared to the gas giant planets within our solar system. The fluffiness of this exoplanet enables astronomers to look roughly 50 times deeper into its atmosphere compared to the depth of exploration achieved for a solar-system giant like Jupiter. 

The team of European astronomers took full advantage of this, looking deep into its atmosphere. Their recent study, now published in Nature, reveals the presence of water vapour, sulfur dioxide (SO2), and silicate clouds, but notably, there is no trace of the greenhouse gas methane (CH4). 

These detections provide crucial insights into the dynamics and chemistry of this captivating exoplanet. First, the absence of methane hints at a potentially warm interior, offering a tantalising glimpse into the movement of heat energy in the planet's atmosphere. Secondly, the discovery of sulfur dioxide (known for the odour of burnt matches), was a major surprise.

Previous models had predicted its absence, but novel climate models of WASP-107b's atmosphere now show that the very fluffiness of WASP-107b accommodates the formation of sulfur dioxide in its atmosphere. Even though its host star emits a relatively small fraction of high-energy photons due to its cooler nature, these photons can reach deep into the planet's atmosphere thanks to its fluffy nature. This enables the chemical reactions required to produce sulfur dioxide to occur.

But that's not all they've observed. Both the spectral features of sulfur dioxide and water vapour are significantly diminished compared to what they would be in a cloudless scenario.

High-altitude clouds partially obscure the water vapour and sulfur dioxide in the atmosphere. While clouds have been inferred on other exoplanets, this marks the first instance where astronomers can definitively identify the chemical composition of these clouds. In this case, the clouds consist of small silicate particles, a familiar substance for humans found in many parts of the world as the primary constituent of sand.

"JWST is revolutionising exoplanet characterisation, providing unprecedented insights at remarkable speed," says lead author Prof. Leen Decin of KU Leuven. "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." 

According to lead author Dr Michiel Min:​ "The fact that we see these sand clouds high up in the atmosphere must mean that the sand rain droplets evaporate in deeper, very hot layers and the resulting silicate vapour is efficiently moved back up, where they recondense to form silicate clouds once more. This is very similar to the water vapour and cloud cycle on our own Earth but with droplets made of sand."

“This data exemplifies the information that we can now gather via MIRI," added Professor Gillian Wright, Director at UK ATC, Principal Investigator on MIRI and co-author on the paper. “This is a new era in terms of being able to explore and study exoplanet atmospheres in this level of detail."

​Find out more.

Read the full paper at Nature.