^{A team of European astronomers, including Prof Tom Ray from DIAS Astronomy & Astrophysics, used recent observations made with the James Webb Space Telescope 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 sulphur dioxide, but even silicate sand clouds. These particles reside within a dynamic atmosphere that exhibits vigorous transport of material. The results of the study appeared in the prestigious journal Nature.}

Astronomers worldwide are harnessing the advanced capabilities of the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST) to conduct groundbreaking observations of exoplanets—planets orbiting stars other than our own Sun. One of these fascinating worlds is WASP-107b, 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 the remarkable fluffiness of this exoplanet, enabling them to look deep into its atmosphere. This opportunity opened a window into unravelling the complex chemical composition of its atmosphere. The reason behind this is quite straightforward: the signals, or spectral features, are far more prominent in a less dense atmosphere compared to a more compact one. Their recent study, now published in Nature, reveals the presence of water vapour, sulphur dioxide (SO₂), and silicate clouds, but notably, there is no trace of the greenhouse gas methane (CH₄). 

Commenting on the significance of the findings, Prof. Tom Ray, said: “These detections provide crucial insights into the dynamics and chemistry of this captivating exoplanet. The absence of methane in its upper layers hints at a potentially warm interior. This is because a hot enough interior drives enormous convection currents which, if fast enough, can destroy the chemical equilibrium of molecules like methane. The discovery of sulphur dioxide – known as the odour of burnt matches – was also a major surprise. Previous models had predicted its absence, but new climate models of WASP-107b’s atmosphere now show that the fluffiness of the exoplanet accommodates the formation of sulphur 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 sulphur dioxide to occur.

But that’s not all they’ve observed. Both the spectral features of sulphur 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 sulphur 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 sulphur 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.”

This pioneering research not only sheds light on the exotic world of WASP-107b but also pushes the boundaries of our understanding of exoplanetary atmospheres. It marks a significant milestone in exoplanetary exploration, revealing the intricate interplay of chemicals and climatic conditions on these distant worlds.

This study combines the results of several independent analyses of the JWST observations, and represents the years of work invested not only in building the MIRI instrument but also in the calibration and analysis tools for the observational data acquired with MIRI. Prof Ray is a Co-PI of MIRI and played a key role in the development of the instrument.

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These observations were taken as part of the Guaranteed Time Observation program 1280. This finding has been published in the journal Nature (DOI: 10.1038/s41586-023-06849-0).

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

The European consortium team consists of 46 astronomers from 29 research institutions across 12 countries. Prof Tom Ray leads the team in the DIAS Astronomy & Astrophysics Section, with support from the European Research Council EASY Project (Grant No. 743029).