Fluffy exoplanet

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Artist’s view of the WASP-107b planet with its fluffy atmosphere above its
Artist’s view of the WASP-107b planet with its fluffy atmosphere above its parent star.

Astronomers can look 50 times deeper into the atmosphere of this exoplanet than is possible with Jupiter

A team of European astronomers, with the help of researchers from the University of Vienna, has studied the atmosphere of the nearby exoplanet WASP-107b using the James Webb Space Telescope. An exoplanet is a planet orbiting a star other than our sun. Looking deep into the fluffy atmosphere of WASP-107b, they discovered not only water vapor and sulfur dioxide, but even silicate sand clouds. This discovery represents a significant milestone in the study of exoplanets, as it reveals the complicated interplay of chemicals and climatic conditions on these distant worlds. Furthermore, for the first time astronomers have been able to definitively determine the composition of clouds on an exoplanet. The results of the study were published today in the renowned journal Nature.

Astronomers around the world are using the advanced capabilities of the Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST) to make groundbreaking observations of exoplanets. Manuel Güdel, astrophysicist at the University of Vienna, is one of the developers of MIRI. His doctoral student Gwenaël van Looveren is also one of the co-authors of the new study. "JWST is revolutionizing the characterization of exoplanets and providing unprecedented insights at a remarkable speed," says Güdel, a co-principal investigator of the MIRI instrument. One of the fascinating worlds that can be studied is WASP-107b, a unique gaseous exoplanet orbiting a star that is slightly cooler and less massive than our Sun. The planet has a similar mass to Neptune, but is much larger than it, almost reaching the size of Jupiter. This characteristic makes WASP-107b rather "fluffy" compared to the gas giant planets in our solar system.

The fluffiness of this exoplanet allows astronomers to look about 50 times deeper into its atmosphere than is possible with a solar system giant like Jupiter. This opportunity opened a window to decipher the complex chemical composition of its atmosphere. The reason for this is quite simple: the signals or spectral features are much more pronounced in a less dense atmosphere than in a more compact atmosphere. In the study now published as "Fast Track" in Nature, the team was able to detect water vapor, sulfur dioxide (SO 2 ) and silicate clouds. Remarkably, however, no trace of the greenhouse gas methane (CH 4 ) could be detected. These discoveries provide crucial insights into the dynamics and chemistry of this fascinating exoplanet.

Manuel Güdel from the University of Vienna explains: "Firstly, the absence of methane indicates a possibly warm interior and provides an exciting insight into the movement of heat energy in the planet’s atmosphere. Secondly, the discovery of sulphur dioxide (known by the smell of burning matches) was a big 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 favors the formation of sulfur dioxide in its atmosphere. Although its host star only emits a relatively low proportion of high-energy photons due to its cooler nature, these photons can penetrate deep into the planet’s atmosphere thanks to its fluffy nature. This enables the chemical reactions required for the formation of sulphur dioxide.

Clouds of sand, water and sulphur dioxide discovered

Another discovery of the new study: clouds at high altitudes partially obscure the water vapor and sulfur dioxide in the atmosphere. While clouds on other exoplanets have already been suspected, this is the first time that astronomers have been able to definitively determine the chemical composition of these clouds. The clouds of WASP-107b therefore consist of small silicate particles, a substance familiar to humans that occurs in many parts of the world as the main component of sand.

"The discovery of clouds of sand, water and sulphur dioxide on this fluffy exoplanet by JWST’s MIRI instrument is a major milestone. It changes our understanding of the formation and evolution of planets and sheds new light on our own solar system," says Güdel.

In contrast to the Earth’s atmosphere, in which water freezes at low temperatures, silicate particles can freeze out and form clouds on gaseous planets with temperatures of around 1000 degrees Celsius. In the case of WASP-107b, however, with a temperature of around 500 degrees Celsius in the outer atmosphere, these silicate clouds should, according to conventional models, form deeper in the atmosphere, where the temperatures are much higher. Furthermore, sand clouds rain down high up in the atmosphere. So how is it possible for these sand clouds to exist and persist at high altitudes?

Michiel Min, lead author of the study, from the SRON Netherlands Institute for Space Research, explains: "The fact that we see these sand clouds high up in the atmosphere must mean that the sand rain droplets evaporate in lower, very hot layers and the resulting silicate vapor is efficiently transported back upwards, where it condenses again to form silicate clouds. This is very similar to the water vapor and cloud cycle on Earth, but with droplets of sand." This continuous cycle of sublimation and condensation through vertical transport is responsible for the persistent presence of sand clouds in the atmosphere of WASP-107b.

about the study

"This study combines the results of several independent analyses of the JWST observations and reflects the years of work invested not only in building the MIRI instrument, but also in the calibration and analysis tools for the observational data obtained with MIRI," says Jeroen Bouwman from the Max Planck Institute for Astronomy, Germany.

These observations were carried out as part of the 1280 program for guaranteed time observations. This result was published in the journal Nature: ’SO2, silicate clouds, but no CH4 detected in a warm Neptune’, by Dyrek, Min, Decin et al, 2023, Nature ; DOI: 10.1038/s41586’023 -06849-0

The James Webb Space Telescope is the world’s most important observatory for space research. Webb solves mysteries in our solar system, looks to distant worlds around other stars and explores the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA and its partners, the European Space Agency (ESA) and the Canadian Space Agency.

The European consortium team consists of 46 astronomers from 29 research institutions in 12 countries. Manuel Güdel, Nicole Pawellek and PhD students Gwenaël van Looveren and Rodrigo Guadarrama from the University of Vienna are part of the team. "Together with colleagues from Europe and the United States, we have been building and testing the MIRI instrument for almost 20 years. It is rewarding to see how our instrument decodes the atmosphere of this fascinating exoplanet," says instrument specialist Bart Vandenbussche from KU Leuven.

Video animation of WASP-107b orbiting its parent star