The analysis of exoplanets, a long-term task

This artist's concept shows what the exoplanet WASP-17 b could look like.  WASP-17 b, also called Ditso, is a hot gas giant that orbits its star at a distance of only 0.051 AU, completing one revolution in about 3.7 Earth days.

What about exoplanets? During the first year of observation by the James Webb Space Telescope (JWST), while the press releases presenting spectacular discoveries and images on the first galaxies in the history of the Universe accumulated, the contrast with the domain of extrasolar planets was striking. Not radio silence, but almost. This is while 30% of the JWST’s observation time must be devoted to the study of exoplanets, their possible atmospheres and the gas and dust disks where these stars are born. Excluding the presence of an atmosphere around the first planet of the dwarf star Trappist-1 (which has seven), the main announcement in this sector was nothing to write home about and even had the effect of a cold shower as we have so many hopes for this nearby system. The hope of detecting a sign of extraterrestrial life there.

However, to be surprised by this meager harvest is to forget that exoplanetology is a marathon in stages. Its specialists began by essing the qualities of the JWST. First satisfaction, “the telescope is extremely stable, which is an important quality for the study of exoplanets”, explain Pierre-Olivier Lagage, research director at the CEA and co-responsible for MIRI, one of the four James-Webb instruments. He adds : “I was amazed at how easy it was to see a transit”that is to say the very slight drop in luminosity of the star when a planet pes in front of it, which makes it possible, for example, to estimate its diameter.

Next comes the analysis of the light spectrum measured on an exoplanet, which determines which molecules are present in its atmosphere. “With Hubble, we essentially had access to waterrecalls Pierre-Olivier Lagage. Since JWST has the entire infrared range, it gives us almost all the molecules. We look for water, methane, carbon monoxide and dioxide, ammonia, etc. With the final challenge of discovering biosignatures. »

Sulfur dioxide detection

The term “biosignature” designates a molecule whose presence can only be explained by the presence of life. A concept to be handled with caution, underlines the CEA researcher, referring to the case of ozone which, on Earth, is the result of biological activity: “We have long talked about ozone as a biosignature, but on certain planets it is possible to produce it abiotically”, by physical and chemical processes not involving any living being. Pierre-Olivier Lagage therefore remains ” very careful “ vis-à-vis an announcement made in September by a team presenting the discovery, thanks to JWST, of dimethyl sulfide in the atmosphere of K2-18 b, supposed to be an ocean planet. On Earth, dimethyl sulfide is only produced on a large scale by phytoplankton or by its decomposition… However, its presence on K2-18 b remains questionable.

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