Discovered in 2017, the Trappist-1 system has seven planets revolving around a small "cold" star, a red dwarf, half as hot as the Sun.

This planetary system is a prime target of the James Webb Telescope (JWST), developed by NASA and in service since July 2022, one of whose missions is to probe the atmospheres of potentially habitable exoplanets, beyond the solar system.

Trappist-1 is an "excellent laboratory" for this quest, NASA said in a statement: it is close to the solar system and has only rocky planets, all similar in size and mass to Earth.

But it is difficult to know their characteristics because exoplanets cannot be observed directly at such a great distance, unlike the stars around which they orbit. To detect them, astronomers use the transit method, which captures variations in brightness caused by the planet passing in front of its host star, such as a micro-eclipse.

The Trappist-1 © planetary system Emmanuelle MICHEL / AFP

The JWST Mirim imager, capable of observing in the mid-infrared, was able to capture a so-called secondary eclipse, when the planet passes behind its star. In this case the planet Trappist -1b, the closest to the star Trappist-1 and therefore the easiest to study because its transits are more numerous.

"It is just before disappearing behind the star that the planet adds the most light (to that of the star) because it shows almost exclusively its face + day +," explains to AFP Elsa Ducrot, astrophysicist at the Atomic Energy Commission (CEA), co-author of the study published in Nature.

"New era" for exoplanets

By comparing the amount of light detected before and during the occultation, scientists deduce the share emitted by the planet. This is light detectable only in the mid-infrared, wavelength hitherto untapped by astronomers, which makes it possible to detect the thermal emission of the planet: the JWST acts "like a giant thermometer without contact", comments NASA whose one of the astrophysicists, Thomas Greene, is the main author of the study.

Artist's impression provided by the European Southern Observatory (ESO) showing the seven planets of the Trappist-1 © HO system / European Southern Observatory / AFP / Archives

The temperature measurement of Trappist-1-b is a first for a rocky exoplanet. It is about 230 degrees Celsius on the day, suggesting "that there is no redistribution of heat over the entire planet, a role provided by an atmosphere," says the CEA, which designed the Mirim imager.

Conclusion: Trappist-1b "has no or little atmosphere," says Elsa Ducrot, stressing that it will be necessary to dig at other wavelengths to decide. What is certain, however, is that if there is an atmosphere, it does not contain carbon dioxide, continues the astrophysicist.

So many details that a previous telescope, Spitzer, had failed to detect "despite the observation of 28 secondary eclipses of Trappist-1b". "The James Webb saw them in a single eclipse!" says the scientist.

By revealing for the first time the atmosphere around a rocky planet, the telescope developed by NASA opens "a new era" for the study of exoplanets, she adds.

Trappist-1b is too close to its star to be likely to harbor life forms as we know it. But its observation can provide valuable information about its sister planets, abounds NASA.

Including Trappist-1e, Trappist-1f and Trappist-1g which are in habitable zone. A region neither too hot nor too cold to have liquid water, a condition conducive to extraterrestrial life.

© 2023 AFP