James Webb measures the temperature of a planet close to Earth, for the first time

Discovered in 2017, the Trappist-1 system contains seven planets orbiting a “cool” young star, a red dwarf, half the heat from the sun.

This planetary system is a favorite target for the James Webb Telescope (JWST), developed by NASA and in service since July 2022. One of its missions is to probe the atmospheres of potentially habitable exoplanets outside the solar system.

Trappist-1 is an “excellent laboratory” for this endeavor, as NASA points out in a press release: It is close to the solar system and contains only rocky planets, all of the same size and mass as Earth.

But it is difficult to know their properties because exoplanets cannot be observed directly from such a great distance, unlike the stars they orbit. To detect them, astronomers use the transit method, which captures variations in luminosity caused by a planet passing in front of its host star, like a mini eclipse.

Capable of observing in the mid-infrared, JWST’s Mirim imager captured so-called secondary eclipses, when a planet passes behind its star. In this case, it is the planet Trappist -1b, which is closest to the star Trappist-1, and thus the easiest to study because its transits are more numerous.

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

A “new era” for the outer planets

By comparing the amount of light detected before and during the occultation, scientists infer what part of the planet is being emitted. This light can only be detected in the mid-infrared, a wavelength not yet exploited by astronomers, which makes it possible to detect the planet’s heat emission: the JWST “works like a giant non-contact thermometer,” NASA comments, It is one of the astrophysicists, Thomas Green, is the lead author of the study.

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Trappist-1-b’s temperature measurement is the first measurement of a rocky exoplanet. The temperature is about 230 degrees Celsius on the day side, which indicates that “there is no redistribution of heat over the whole planet, a role provided by the atmosphere,” specifies CEA, which designed the Mirim imager.

Conclusion: Trappist-1b “has little to no atmosphere,” Elsa Ducrot developed, emphasizing that it would be necessary to look at other wavelengths to make a decision. What is certain, however, is that if there is an atmosphere, it does not contain carbon dioxide, the astrophysicist continues.

Many details such as a previous telescope, Spitzer, failed to reveal “despite observing 28 secondary Trappist-1b eclipses.” The scientist says: “James Webb saw them in one eclipse!”

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 very close to its star and likely harbors life as we know it. But observing it could provide valuable information about sister planets, NASA abounds.

Including Trappist-1e, Trappist-1f, and Trappist-1g found in the habitable zone. An area that is neither too hot nor too cold to have liquid water, a condition conducive to extraterrestrial life.

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