The James-Webb telescope reveals an exoplanet with “sand clouds” that challenges astronomers

2023-11-15 16:23:31

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Slowly but surely, month after month, the James Webb Space Telescope is providing more and more evidence of its ability to provide details on the composition of the atmospheresatmospheres of exoplanets. Of course, the ultimate goal is to find convincing biosignatures in the atmosphere of an exo-Earth, but for that we will have to be patient, perhaps for decades. This is not to say that such signatures as such will not occur before 2030. It simply may take a very long time to be reasonably certain that there are no natural abioticabiotic processes, unrelated to 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 when it comes to uninhabitable gas giants like hot Jupiters or mini-Neptunes.

It is therefore with a certain interest that we learn today of the results obtained by a team of European astronomers, co-directed by researchers from the Institute of Astronomy of KU Leuven, the famous Belgian university of Louvain. They concern observations of JWSTJWST revealing part of the composition of the atmosphere of the exoplanet WASP-107b. The resulting spectrum shows not only the presence of water vapor molecules, but also sulfur dioxide, SO2. The presence of clouds of silicate particles, cousins ​​of sand grains, has also been established; however, no trace of methane (CH4), and that is surprising.

A planetary atmosphere has a spectral signature which represents its chemical composition, but also its composition in clouds and “fog”. Thanks to several techniques, it is possible to determine the physicochemical characteristics of the atmosphere of an exoplanet. Among these techniques: spectroscopic transit, secondary transit or eclipse, direct spectroscopic observation of the planet or even observation of the planet at different phases around the star in order 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 H2020 Exoplanets-A European research project. © CEA

An absence of methane that challenges planetologists

WASP-107b had already been observed years ago with the Hubble telescope. In fact, we’ve known for a while now that it’s one of the lowest-density exoplanets known. Indeed, although its massmass is only 12% of that of Jupiter, its diameter is comparable because it is strongly heated by its host star (slightly colder and less massive than our SunSun), around which it loops its orbitorbit in only six days.

As it is only located about 200 light-years from the Solar SystemSolar System, its dilated atmosphere makes it, for this reason, a target of choice for analyzing its composition by measuring the spectrum by transmission of light from its star once it passes through this atmosphere (see the video above for details). Almost six years ago, astrophysicistsastrophysicists had already detected the presence of heliumhelium by carrying out observations with HubbleHubble in the infrared.

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

On the other hand, the absence of detection of methane is to date unexplained, which prompts us to question the models of exoplanet atmospheres known to date!

We also know that the signal for water vapor and sulfur dioxide is weak enough to imply that there are clouds blocking part of this signal. These clouds must precisely be made up 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, which accompanies a publication in the newspaper Nature, explains that researchers were surprised to find silicatesilicate clouds at high altitudes. In fact, the temperature there is only around 500°C, which means, given the fusion temperature of the silicate particles, they can form deeper in the atmosphere, at higher temperatures, and that the particles must even constitute rains of liquid liquid silicates. How is it then possible that these sand clouds exist at high altitude and continue to persist?

The press release then gives the explanations of one of the main astrophysicists behind the discovery, Michiel Min: “ The fact that we see these sand clouds high in the atmosphere must mean that the sand rain droplets are evaporating into deeper, very hot layers and that 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 droplets made of sand. This continuous cycle of sublimationsublimation and condensationcondensation by vertical transport is responsible for the lasting presence of sand clouds in the atmosphere of WASP-107b. »

The press release concludes by stating that “ 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 an important milestone in exoplanetary exploration, revealing the complex interplay of chemicals and climate conditions on these distant worlds » and with the following comment from Achrène Dyrek, main author of the discovery published in Natureworking at the Department of AstrophysicsAstrophysics at CEA Paris: “ JWST allows a deep atmospheric characterization of an exoplanet which has no equivalent in our Solar System, we are discovering new worlds! »

The researcher is one of the winners of the L’Oréal – Unesco Young Talents Prize for Women in Science 2023 and, a few months ago, she gave a conference on the first results obtained with the JWST concerning the atmospheres of exoplanets. In the case of WASP-107b, Achrène Dyrek and her colleagues were able to carry out their work using the low-resolution spectroscopespectroscope of the JWST’s MiriMiri instrument, an instrument observing in the infrared and of which the CEA was a major contributor.

A conference on 2/4/2023 “First results from JWST: exoplanets in transit”, by Achrène Dyrek, Department of Astrophysics of the CEA. © French Physical Society

This work implies that the dynamics specific to the atmosphere of an exoplanet must be taken into account to predict and interpret the composition that can be obtained from the study of the transmission spectrum.

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