James Webb Telescope Detects CO2 in Exoplanet Atmosphere

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The James Webb Space Telescope smelled carbon dioxide in the atmosphere of another solar system planet for the first time.

“It’s undeniable. It’s there. It’s definitely there,” says planetary scientist and study co-author Peter Gao of the Carnegie Institute of Science in Washington, DC. “Previous observations had evidence of carbon dioxide, but it had never been confirmed on such a scale.”

The discovery, which was submitted to arXiv.org on August 24, marks the first detailed scientific results published from the new telescope. It also shows how to find the same greenhouse gases in the atmospheres of smaller, rockier planets that are more Earth-like. Dubbed WASP-39b, the planet is huge and expanding. It is slightly wider than Jupiter and about the same size as Saturn. And because it orbits the star every four days of her on Earth, it gets extremely hot. These features make it a terrifying place to look for evidence of extraterrestrial life (SN: 4/19/16). But this expanded atmosphere combined with frequent flybys in front of its host star make it easier to observe, making it the perfect planet for new telescopes to keep pace with. James Webb (JWST) started in December 2021 and published the first image in July 2022 (SN: 7/11/22). In July, the telescope observed starlight penetrating the dense atmosphere of the planet for about eight hours as the planet flitted back and forth between its star and her JWST. Carbon dioxide molecules in the atmosphere have absorbed certain wavelengths of the star’s light.

His previous observations of WASP-39b by NASA’s now-defunct Spitzer Space Telescope detected only a faint absorption at the same wavelength. But it wasn’t enough to convince astronomers that carbon dioxide was really there. Astronomer Nicholas Cowan of Montreal’s McGill University says, “I wouldn’t bet more than a beer, maybe six packs, on this strange, tentative reference to carbon dioxide from Spitzer.” JWST’s detection, on the other hand, is “robust,” he says. “I love him too much, so I won’t bet on the firstborn, but have a nice holiday.”

The JWST data also showed extra absorption at wavelengths close to those absorbed by carbon dioxide. “This is a mystery molecule,” says astronomer Natalie Batalha of the University of California, Santa Cruz, who led the observation team. “There are several suspects under investigation.”

The spectrum of light filtered through the exoplanet WASP-39 b’s atmosphere provides strong evidence for the presence of carbon dioxide. The large peak in the center of the spectrum indicates that the planet’s atmosphere absorbed light at a wavelength of about 4.3 micrometers. This is a clear sign of CO2. A small bump about 4 microns to the left of CO2 (shown as three dots above the best-fit line) could represent a mysterious molecule.

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The amount of carbon dioxide in the exoplanet’s atmosphere can reveal details about how the planet formed (SN: 5/11/18). If the planet had been bombed with an asteroid, it could have brought in more carbon and enriched the atmosphere with carbon dioxide. If the star’s radiation stripped away some of the lighter elements in the planet’s atmosphere, it could also make carbon dioxide appear richer.

You’d need a telescope as powerful as the JWST to find it, but carbon dioxide is in the atmospheres of entire galaxies and can be hidden out of sight. It’s one of the few molecules in the atmosphere of every planet in our solar system,” says Batalha. “It’s your front line molecule.”

Finally, astronomers hope to use his JWST to find carbon dioxide and other molecules in the atmosphere of a small rocky planet orbiting the star TRAPPIST-1 (SN: 12/13/17) . Some of these planets are just the right distance from their stars to hold liquid water, which could make them good places to look for signs of life. It remains to be seen if JWST will detect these signs of life, but it will be able to detect carbon dioxide.

“When I saw this data, my first thought was, ‘This is going to work,’” says Batalha.

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