The James Webb Space Telescope — already famous for its mesmerizing images of the cosmos — has done it again. The telescope has captured the first unambiguous evidence of carbon dioxide in the atmosphere of a planet outside the Solar System.
The finding not only provides tantalizing hints about how the exoplanet formed, it is also a harbinger for what’s to come as Webb studies more and more alien worlds. It was reported in a manuscript on the preprint server arXiv1, ahead of peer review, and is expected to publish in Nature in the coming days. (Nature’s news team is independent of its journals team.)
Although the plotted data that led to the discovery lack the lustre of Webb’s previous images — which showed galaxies locked in a cosmic dance and radiant clouds in a stellar nursery — they still prompted Jessie Christiansen, an astronomer at the NASA Exoplanet Science Institute at Caltech in Pasadena, to describe them as “gorgeous”.
The plot, or spectrum, reveals detailed information about the atmosphere of the exoplanet WASP-39b, called a ‘hot Jupiter’ by scientists because it is slightly wider in diameter than Jupiter but orbits its star much more closely than Mercury orbits the Sun, making it unbearably hot. The planet, which is over 200 parsecs from Earth, was initially discovered during ground-based observations2 and later detected by NASA’s Spitzer Space Telescope, operational between 2003 and 2020. Data from the latter suggested3 that WASP-39b’s atmosphere might contain carbon dioxide, but they were inconclusive.
Then came Webb. For a little more than eight hours on 10 July, the infrared telescope observed the planet move across the face of its star. As it did, starlight shone through the planet’s atmosphere where various molecules absorbed specific wavelengths of infrared light. Astronomers wondered whether carbon dioxide would show up as a telltale blip in the spectrum. “And there it was — just jumping off of the computer screen,” says Natalie Batalha, an astronomer at the University of California at Santa Cruz (UCSC), who leads Webb’s Transiting Exoplanet Early Release Science team.
Batalha wasn’t alone. When Christiansen, who is not part of the team, saw the data, she gasped. “I was like ‘oh there it is,’” she says. “We’ve had hints of it before, but this is the first time it’s really been a ‘punch in the face’ kind of detection.”
The result has bolstered confidence that Webb is going to be revolutionary for exoplanet research. In the first year of its operation alone, the telescope is commissioned to observe 76 exoplanets, but the final tally could fall in the hundreds. It will gaze through the atmospheres of gas giants and small, rocky worlds that could be like Earth. “My very first thought when I saw that signal was ‘wow, this is going to work,’” Batalha says.
But the finding of carbon dioxide is also impressive on its own. “From a science standpoint, it’s extremely exciting,” says Jonathan Fortney, the director of the Other Worlds Laboratory at UCSC and a co-author on the paper. It’s safe to expect that a planet like Jupiter, which formed out of the same disk of material as its star, would have roughly the same chemical makeup as that star. But that clearly isn’t the case in our Solar System, nor is it true for WASP-39b. The exoplanet’s strong carbon dioxide signal suggests that it is enriched with elements heavier than hydrogen and helium, which typically make up stars. The question is why?
“That’s where the story starts to get interesting,” Batalha says. It’s possible that when WASP-39b was young, it was bombarded with comets and asteroids, which could have delivered heavier elements such as carbon and oxygen. Interestingly, the exoplanet appears to have the same amounts of heavy elements as Saturn, which astronomers also think underwent a violent youth.
Or the answer might be that WASP-39b formed from materials in the cold outer reaches of its planetary system, then migrated inward. At its final resting spot, it snuggled up to its host star, which could have blasted away some of the hydrogen in the exoplanet’s atmosphere — concentrating heavier elements to make it appear richer in carbon dioxide. Fortney, Batalha and their colleagues are currently working on four papers that will analyse the planet’s spectrum in significantly more detail and probe these possibilities further.
“It’s like archaeology,” Batalha says. “You’re trying to build up a big story — and you’re using the molecules themselves, as tracers of that story.”
The building blocks of life
Spotting carbon dioxide in a planet’s atmosphere is a stepping stone towards detecting life beyond Earth. Of course astronomers don’t expect that WASP-39b is capable of hosting life — it is far too close to its star. They don’t even expect the Webb telescope to definitely find life on another planet. But using Webb to detect carbon dioxide helps lay the foundation for future discoveries.
Astronomers think that a mixture of carbon dioxide and methane in a planet’s atmosphere could be an indicator of life — a so-called biosignature. So WASP-39b’s signal is “halfway to a good biosignature”, Christiansen says. Batalha’s team built a model predicting that the planet’s atmosphere also contains water, carbon monoxide and hydrogen sulfide — but little methane.
Ultimately, the detection of life will probably require an even more advanced observatory than Webb. But, Batalha says, “this is a really important phase that we need to pass through to be ready for that technology in the future”.