In a major scientific milestone, NASA’s James Webb Space Telescope has directly detected carbon dioxide in the atmospheres of multiple exoplanets for the first time by observing their emitted light—marking a new chapter in the study of distant worlds.
The finding, published in The Astrophysical Journal, marks the first time scientists have identified CO₂ not through the indirect transit method, but by isolating the thermal glow from the planets themselves—a technique once considered nearly impossible due to the brightness of their host stars.
A breakthrough in exoplanet observation
The planets in question orbit HR 8799, a young star system just 130 light-years from Earth. While none of these gas giants are considered habitable, their atmospheric makeup holds clues to one of astronomy’s most compelling questions: how do planets form in other star systems?
Using Webb’s coronagraph—a device that blocks starlight—researchers filtered out the overwhelming glow of the central star to detect the faint light from the planets themselves. This allowed them to isolate and analyze the chemical composition of their atmospheres.
“It’s like putting your thumb up in front of the sun when you’re looking up at the sky,” said lead author William Balmer of Johns Hopkins University.
This technique enabled the detection of carbon dioxide across all four known planets in the HR 8799 system, providing the strongest direct evidence yet of its presence in alien atmospheres.
Carbon dioxide is more than a planetary exhaust gas—it’s a fingerprint of formation. In the cold outskirts of a star system, it can freeze into solid particles, clump together, and help form planetary cores. This supports the “core accretion” model of planet formation, in which icy material gradually merges to build a solid core that later attracts a massive gaseous envelope.
According to Balmer, the detection of CO₂ in young gas giants offers new validation that such planets may form similarly to Jupiter and Saturn—suggesting our own solar system may not be unique in how it developed.
A new way of seeing alien worlds
Previously, carbon dioxide had been observed in exoplanets such as WASP-39b, but only during transits—when a planet passes in front of its star. That approach captures how starlight filters through a planet’s atmosphere. But it has limitations: only a small subset of exoplanets transit their stars from Earth’s point of view.
By detecting the planet’s own infrared radiation, this new method drastically expands the number of targets that can be studied—and the kinds of atmospheres that can be analyzed.
Balmer likened the difficulty to using a flashlight to spot fireflies beside a lighthouse: “We’re actually seeing the light that is emitted from the planet itself, as opposed to the fingerprint of that light from the host star.”
Although these gas giants are not likely to support life, their large moons might. Some of Jupiter’s own moons, like Europa, may harbor subsurface oceans with the right conditions for microbial life. Similar moons around distant exoplanets could offer comparable environments.
NASA’s upcoming Nancy Grace Roman Space Telescope, set to launch in 2027, will also use coronagraphy to observe exoplanets, particularly smaller, Earth-sized ones. Scientists hope these new methods will one day detect biosignatures in the atmospheres of terrestrial worlds.
