An artists impression of the exoplanet WASP-39 b, a gassy giant orbiting close to its sun

WASP-39b orbits close to its star, making it a hot exoplanet (artist’s rendering).Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

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, but is also a harbinger for what’s to come as Webb studies more and more alien worlds. It was reported in a manuscript posted on the preprint server arXiv1, ahead of peer review, and is expected to be published in Nature in the coming days. (Nature’s news team is independent of its journals team.)

The discovery is presented in a data plot with none of the lustre of Webb’s previous images — which showed galaxies locked in a cosmic dance and radiant clouds in a stellar nursery. But Jessie Christiansen, an astronomer at the NASA Exoplanet Science Institute at the California Institute of Technology in Pasadena, describes the data 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 has a diameter similar to Jupiter’s but orbits its star much more closely than Mercury orbits the Sun, making it incredibly hot. The planet, which is more than 200 parsecs from Earth, was initially discovered during ground-based observations2 and later detected by NASA’s Spitzer Space Telescope, which operated between 2003 and 2020. Data from the latter suggested3 that WASP-39b’s atmosphere might contain carbon dioxide, but they were inconclusive.

A transmission spectrum of the exoplanet WASP-39b superimposed on an illustration of the planet and its star.

Researchers detected carbon dioxide in WASP-39b’s atmosphere when the exoplanet crossed in front of its star. The data plot shows a telltale blip where infrared wavelengths from the star’s light were absorbed by carbon dioxide on the exoplanet.Credit: NASA, ESA, CSA, Leah Hustak (STScI), Joseph Olmsted (STScI)

Then came Webb. For a little more than eight hours on 10 July, the infrared telescope observed the planet moving across the face of its star. In this time, 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 study co-author Natalie Batalha, an astronomer at the University of California, 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.”

Mysterious origins

The result has bolstered confidence that Webb is going to be revolutionary for exoplanet research. In its first year of operation alone, the telescope is commissioned to observe 76 exoplanets; the final tally could be in the hundreds over its lifetime. It will gaze through the atmospheres of gas giants and small, rocky worlds that could be similar to Earth. “My very first thought when I saw that signal was, ‘Wow, this is going to work,’” Batalha says.

But finding carbon dioxide is also impressive on its own. “From a science standpoint, it’s extremely exciting,” says Jonathan Fortney, director of the Other Worlds Laboratory at UCSC and a co-author of the paper. It would be reasonable to expect that a planet similar to Jupiter, which formed from the same disk of material as its star, would have roughly the same chemical make-up as that star. But that 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 the hydrogen and helium that typically constitute 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 seems to have the same amounts of heavy elements as Saturn, which astronomers also think endured 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 inwards. 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 — causing the heavier elements to become more concentrated, and making it richer in carbon dioxide than it was originally. Fortney, Batalha and their colleagues are working on four papers that will analyse the planet’s spectrum in significantly more detail, and probe these possibilities.

“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. Astronomers don’t expect WASP-39b to be capable of hosting life — it is much too close to its star. They don’t even expect the Webb telescope to find definitive signs of life on another planet. But using Webb to detect carbon dioxide helps to 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 — what’s known as a biosignature. WASP-39b’s signal is “halfway to a good biosignature”, says Christiansen. Batalha’s team has 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 observatory even more advanced 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”.