The fluffy atmosphere of the exoplanet WASP-107b

Data of the James Webb Space Telescope contain signatures of water vapour, sulfur dioxide and sand clouds

November 29, 2023

Thousands of planets orbiting stars in our Milky Way have already been discovered. However, they are too far away to be able to photograph the nature and appearance of their surface. Even the best telescopes have too low a resolution. The new James Webb space telescope avoids these detours. With its sensitive instruments, it finds signs of a weather cycle with sand rain in the atmosphere of a distant gas giant.

A team of European astronomers with the participation of the Max Planck Institute for Astronomy recently used observations with the James Webb Space Telescope (JWST) to study the atmosphere of the nearby exoplanet WASP-107b. Shining a light through its cloudy atmosphere, they discovered water vapour, sulphur dioxide and even silicate sand clouds. These particles are found in a dynamic atmosphere with energetic material transport. This research shows that the JWST data can be used to describe how complex chemicals behave under the climatic conditions on distant worlds.

The exoplanet WASP-107b is a unique gas planet. It orbits a star that is somewhat cooler and less massive than our sun. While the planet's mass is similar to that of Neptune, it has a significantly larger diameter and is almost as large as that of Jupiter. This characteristic makes WASP-107b rather "airy" compared to the gaseous planets of the solar system. As a result, the research team was able to look 50 times deeper into its atmosphere than when exploring a gas giant in the solar system such as Jupiter.

Webb’s mid-infrared instrument reveals the chemistry

The team of European astronomers took full advantage of the remarkable fluffiness of this exoplanet by observing it with the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope. This opportunity opened a window to look deep into its atmosphere, unravelling its complex chemical composition. 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 vapor, 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 tantalizing glimpse into the transport of heat energy in the planet’s atmosphere. Secondly, the discovery of sulfur dioxide (known for the odor of burnt matches) was a major surprise. Previous computations had predicted its absence, but novel climate models of WASP-107b’s atmosphere now show that its fluffy nature accommodates the formation of sulfur dioxide. Even though its rather cool host star emits a relatively small fraction of high-energy photons, they can reach deep into the planet’s atmosphere. This circumstance enables the chemical reactions required to produce sulfur dioxide.

WASP-107b’s weather report predicts sand clouds

But this is not the complete picture. The spectral characteristics of sulphur dioxide and water vapour are considerably reduced compared to a cloudless scenario. High-altitude clouds, on the other hand, partially obscure the water vapour and sulphur dioxide in the atmosphere. While astronomers have been able to indirectly deduce clouds of various substances on other exoplanets, this is the first case in which researchers have been able to directly determine the chemical composition of these clouds. In this case, the clouds consist of small silicate particles, a familiar substance that occurs almost everywhere on Earth as the main component of sand. “The discovery of clouds of sand, water, and sulfur dioxide on this fluffy exoplanet reshapes our understanding of planetary formation and evolution, shedding new light on our own Solar System”, says Leen Decin.

Co-author Paul Mollière from the Max Planck Institute of Astronomy agrees: „The value of JWST cannot be overstated: wherever we look with this telescope, we always see something new and unexpected. This latest result is no exception.“

An exotic atmospheric cycle of silicate droplets

In contrast to Earth’s atmosphere, where water freezes at low temperatures, silicate particles can freeze out to form clouds in gaseous planets that attain temperatures around 1000 degrees Celsius. However, in the case of WASP-107b, where the outer atmosphere becomes as hot as approximately 500 degrees Celsius, traditional models predicted that these silicate clouds should form deeper within the atmosphere, where temperatures are substantially higher. In addition, high-altitude sand clouds rain down to lower layers. How is it then possible that these sand clouds exist at high altitudes and continue to endure? 

“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. The resulting silicate vapour is efficiently lifted up”, Michiel Min from SRON (Netherlands Institute for Space Research) explains. “Here, they recondense to form silicate clouds once more. This is similar to Earth’s water vapour and cloud cycle but with sand droplets.” This continuous cycle of sublimation and condensation through vertical transport is responsible for the sustained presence of sand clouds in WASP-107b’s atmosphere.


Background information

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

JWST’s Mid-InfraRed Instrument (MIRI), built by a European consortium of research institutions over a course of 20 years, is a multi-purpose scientific instrument for infrared wavelengths between 5 and 28 microns. It combines an imaging camera with a spectrograph. With the support of industrial partners, Max Planck Institute for Astronomy provided the mechanisms of all the wavelength range control elements, such as filter and grating wheels, and led the electrical design of MIRI.

The European consortium comprises 46 astronomers from 29 research institutions across 12 countries. The Max Planck Institute for Astronomy team comprises Jeroen Bouwman, Paul Mollière, Thomas Henning, Oliver Krause, and Silvia Scheithauer.


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