The James-Webb telescope reveals an exoplanet with ‘sand clouds’ that challenges astronomers

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Slowly but surely, month by month, the James Webb Space Telescope provides more and more evidence of its ability to provide details about the composition of atmospheresatmospheres exoplanets. The ultimate goal, of course, is to find convincing biosignatures in the atmosphere of an exo-Earth, but that may require patience for decades. This does not mean that such signatures as such will not take place before 2030. It may just take a very long time before we can be reasonably certain that there are no natural processes. abioticabioticnot linked to the existence of life, capable of producing the observed signal.

One of the strategies to achieve this Holy Grail is to learn as much as possible about the atmospheres of exoplanets, even uninhabitable gas giants such as Hot JupitersHot Jupiters or the mini-Neptunes.

It is therefore with some interest that today we learn of the results achieved by a team ofastronomersastronomers Europeans, co-directed by researchers from the Institute for Astronomy of KU Leuven, the famous Belgian University of Leuven. This concerns observations of the JWSTJWST revealing part of the composition of the exoplanet WASP-107b’s atmosphere. The resulting spectrum shows the presence not only of water vapor molecules, but also of sulfur dioxide, SO2. The presence of cloudsclouds of silicate particles, cousins ​​of grains of sandsand, was also founded; However, no trace of methane (CH4), and that is surprising.

A planetary atmosphere has a spectral signature that represents its chemical composition, as well as its composition in clouds and ‘fog’. Thanks to various techniques, it is possible to determine the physicochemical characteristics of the atmosphere of an exoplanet. These techniques include: spectroscopic transit, secondary transit or eclipse, direct spectroscopic observation of the planet or even observation of the planet in different phases around the star to measure temporal and seasonal variations. Discover exoplanets through our 9-episode web series, available on our YouTube channel. A playlist proposed by the CEA and the University of Paris-Saclay as part of the European research project H2020 Exoplanets-A. © CEA

An absence of methane that challenges planetologists

WASP-107b was observed years ago with the Hubble telescope. In fact, we’ve known for a while that it’s one of the lowest-density exoplanets known. Although it is indeed true massmass or only 12% that of Jupiter, its diameter is similar because it is strongly heated by it starstar host (slightly colder and less massive than ours SunSun), around which it turns its loop tracktrack in just six days.

Because there are only about 200 of them light yearslight years by Solar systemSolar systemIts atmosphere being so dilated, for this reason, makes it an ideal target to analyze its composition by measuring its spectrum by transmission of the light from its star once it has passed this atmosphere (see the video above for more details ). Almost six years ago, astrophysicistsastrophysicists had already discovered its presenceheliumhelium by making observations HubbleHubble in L’infra-redinfra-red.

All these observations provide tests and constraints for the chemical and dynamical models of WASP-107b. For example, we initially did not expect to find sulfur dioxide. But new models of photochemical reactions now explain its discovery.

On the other hand, the absence of detection of methane is unexplained, prompting us to question the previously known models of exoplanet atmospheres!

We also know that the signal for water vapor and sulfur dioxide is weak enough to imply that there are clouds blocking some of this signal. These clouds must consist of small silicate particles. Although clouds have been discovered on other exoplanets before, this is the first time astronomers have been able to definitively identify the chemical composition of these clouds.

Chemistry influenced by atmospheric dynamics

The press release from KU Leuven accompanying a publication in the newspaper Nature, explains that the researchers were surprised to find that these clouds disappeared silicatesilicate are located at high altitudes. In fact, the temperature there is only about 500°C. But considering the temperature of mergermerger Silicate particles can form deeper in the atmosphere, at higher temperatures, so that the particles were even expected to form silicate rains liquidsliquids. How then is it possible for these high-altitude sand clouds to exist and persist?

The press release then gives the explanation from one of the key astrophysicists behind the discovery, Michiel Min: “ The fact that we see these sand clouds high in the atmosphere must mean that the sand raindrops are evaporating in deeper, very hot layers and the resulting silicate vapor is effectively rising upwards, where it re-condenses to form silicate clouds again. This is very similar to the water vapor and cloud cycle on our own Earth, but with sand droplets. This continuous cycle of sublimationsublimation and from condensationcondensation by vertical transport is responsible for the persistent presence of sand clouds in WASP-107b’s atmosphere. »

The press release concludes by stating that “ this groundbreaking research not only sheds light on the world exoticexotic of WASP-107b, but also pushes the boundaries of our understanding of exoplanetary atmospheres. It marks a major milestone in exoplanetary exploration, revealing the complex interplay of chemicals and climate conditions on these distant worlds » and with the following commentary from Achrène Dyrek, lead author of the discovery published in Naturestationed in the departmentAstrophysicsAstrophysics from CEA Paris: “ JWST enables an in-depth atmospheric characterization of an exoplanet that has no equivalent in our solar system: we discover new worlds! »

A conference on April 2, 2023, “First results from JWST: exoplanets in transit,” by Achrene Dyrek, CEA Astrophysics Department. © French Physical Society

The researcher is one of the winners of the L’Oréal – UNESCO Young Talents prize for women and science 2023 and a few months ago gave a conference on the first results obtained with the JWST on the atmospheres of exoplanets. In the case of WASP-107b, Achrène Dyrek and her colleagues were able to carry out their work thanks to the spectroscopespectroscope low resolutionresolution of the instrument MiriMiri of the JWST, an instrument that observes in the infrared and to which the CEA made an important contribution.

This work involves taking into account the dynamics specific to an exoplanet’s atmosphere to predict and interpret the composition that can be obtained from the study of the transmission spectrum.

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