Stockholm university

James Webb telescope detects water vapor, sulphur dioxide and sand clouds around a nearby exoplanet

A team of European astronomers recently utilized observations made with the James Webb Space Telescope to study the atmosphere of the nearby exoplanet WASP-107b. By peering deep into WASP-107b's fluffy atmosphere, they discovered not only water vapor and sulfur dioxide but even clouds of silicates (i.e., sand). The results of the study were published today in the prestigious journal Nature.

Astronomers worldwide are currently leveraging the James Webb Space Telescope (JWST) and the capabilities of the Mid-Infrared Instrument (MIRI) to conduct groundbreaking observations of exoplanets—planets orbiting stars other than our Sun. One of these captivating worlds is WASP-107b, a unique gas giant planet orbiting a star slightly cooler and less massive than our own Sun. Although the planet's mass is comparable to Neptune, its size is much larger, almost approaching that of Jupiter. This characteristic renders the atmosphere of WASP-107b rather 'fluffy' compared to the gas giants in our solar system, enabling astronomers to explore its atmosphere approximately 50 times deeper than a Jupiter-like planet.

Glowing planet closely circling its host star
Artist impression of the planetsystem WASP 107. Image: LUCA School of Arts, Belgium/ Klaas Verpoest (visuals), Johan Van Looveren (typography)

The observations unveil the complex chemical composition of the atmosphere. The key to their success lies in the fact that signals, or spectral features, are more prominent in a less dense atmosphere compared to a more compact one. The study, now published in Nature, reveals the presence of water vapor, sulfur dioxide (SO2), and silicate clouds, notably with no traces of the greenhouse gas methane (CH4).

These discoveries provide crucial insights into the atmosphere and chemistry of this captivating exoplanet. Firstly, the absence of methane suggests a potentially warm interior, offering a tantalizing glimpse into the movement of heat energy in the planet's atmosphere. Secondly, the discovery of sulfur dioxide (known for the smell of burnt matches) was a surprise. New models now show that the fluffiness of WASP-107b enables the formation of sulfur dioxide in its atmosphere. Despite the host star emitting very little UV radiation due to its cooler nature, photons can reach deep into the planet's atmosphere, thanks to its fluffy nature, facilitating the chemical reactions required to produce sulfur dioxide.

However, spectral signatures of sulfur dioxide and water vapor are weaker than they would be in a cloudless scenario. High-altitude clouds partially obscure water vapor and sulfur dioxide in the atmosphere. While cloud presence has been suggested on other exoplanets, this marks the first case where the chemical composition of such clouds has been definitively determined. In this instance, the clouds consist of small silicate particles, a familiar substance for humans, constituting the primary component of sand.

"JWST is revolutionizing the characterization of exoplanets, providing unprecedented insights at remarkable speed," says lead author Achrene Dyrek at CEA in France. "The discovery of clouds of sand, water, and sulfur dioxide on this exoplanet by JWST's MIRI instrument is a milestone. It reshapes our understanding of planetary formation and evolution, shedding new light on our own Solar System."

In contrast to Earth's atmosphere, where water freezes at low temperatures, silicate particles in giant planets can reach temperatures of around 1000 degrees Celsius and freeze out to form clouds. However, in the case of WASP-107b, with a temperature of around 500 degrees Celsius in the outer atmosphere, traditional models predicted that these silicate clouds should form deeper within the atmosphere, where temperatures are significantly higher. Additionally, sand clouds rain out high up in the atmosphere. How, then, can these sand clouds persist?

This is very similar to the water vapor and cloud cycle on our own Earth but here with droplets made of sand

"The fact that we see these sand clouds high up in the atmosphere must mean that sand droplets evaporate in deeper, very hot layers, and the resulting silicate vapor is efficiently moved back up, where it condenses and forms clouds. This is very similar to the water vapor and cloud cycle on our own Earth but here with droplets made of sand," says Göran Östlin, a professor at Stockholm University and one of the researchers behind the study.

MIRI has been constructed by a consortium of European (from Belgium, Denmark, France, Ireland, the Netherlands, Switzerland, Spain, the United Kingdom, Sweden, Germany, and Austria) and American research institutes. From Sweden, researchers from Stockholm University and Chalmers, have been involved, with the Swedish contribution funded by the Swedish National Space Agency and the Knut and Alice Wallenberg Foundation.