The James Webb Space Telescope has obtained the first sniff of carbon dioxide in the atmosphere of a planet in another solar system.
“It is incontrovertible. It’s there. It’s definitely there,” says planetary scientist and study co-author Peter Gao of the Carnegie Institution for Science in Washington, DC. “There have been hints of carbon dioxide in previous observations, but it was never confirmed to such an extent.”
The finding, posted to arXiv.org on August 24, marks the first published detailed scientific result of the new telescope. It also points the way to finding the same greenhouse gas in the atmospheres of smaller, rockier planets that are more like Earth.
The planet, named WASP-39b, is huge and bloated. It is slightly wider than Jupiter and about as massive as Saturn. And it orbits its star every four Earth days, making it very hot. Those features make it a terrible place to look. evidence of extraterrestrial life (Serial number: 04/19/16). But that combination of puffy atmosphere and frequent passes in front of its star makes it easy to observe, a perfect planet to put the new telescope to the test.
James Webb, or JWST, launched in December 2021 and released his first images in July 2022 (Serial number: 7/11/22). For about eight hours in July, the telescope observed starlight filtering through the planet’s thick atmosphere as the planet crossed between its star and JWST. As it did so, carbon dioxide molecules in the atmosphere absorbed specific wavelengths of that starlight.
Previous observations of WASP-39b with NASA’s now-defunct Spitzer Space Telescope had detected only one puff of absorption at that same wavelength. But it wasn’t enough to convince astronomers that the carbon dioxide was really there.
“I wouldn’t have bet more than a beer, at most a six-pack, on that weird tentative hint of carbon dioxide from Spitzer,” says astronomer Nicolas Cowan of McGill University in Montreal, who was not involved in the new study. JWST detection, on the other hand, “is rock solid,” he says. “I wouldn’t bet on my firstborn because I love him too much. But I’d bet on a good vacation.
The JWST data also showed slightly more absorption at wavelengths close to those absorbed by carbon dioxide. “It’s a mystery molecule,” says astronomer Natalie Batalha of the University of California, Santa Cruz, who led the team behind the observation. “We have several suspects that we are questioning.”
The amount of carbon dioxide in an exoplanet’s atmosphere can reveal details about how the planet was formed (Serial number: 05/11/18). If the planet was bombarded with asteroids, that could have brought more carbon and enriched the atmosphere with carbon dioxide. If the star’s radiation removed some of the lighter elements from the planet’s atmosphere, that could also make it appear richer in carbon dioxide.
Although it takes a telescope as powerful as JWST to detect it, carbon dioxide could be in atmospheres throughout the galaxy, hiding in plain sight. “Carbon dioxide is one of the few molecules that is present in the atmospheres of all the planets in the solar system that have atmospheres,” says Batalha. “It’s their first-line molecule.”
Eventually, astronomers hope to use JWST to find carbon dioxide and other molecules in the atmospheres of small rocky planets, like the orbiting the star TRAPPIST-1 (Serial Number: 12/13/17). Some of those planets, at just the right distances from their star to harbor liquid water, might be good places to look for signs of life. Whether JWST will detect those signs of life remains to be seen, but it will be able to detect carbon dioxide.
“My first thought when I saw this data was, ‘Wow, this is going to work,’” says Batalha.