Astronomers have found carbon dioxide (CO2) in the atmosphere of a Saturn-sized planet 700 light-years away—the first clear detection of the gas on a planet beyond the Solar System. The discovery, made by the James Webb Space Telescope, provides clues about how the planet formed. The result also shows how quickly Webb can identify a range of other gases, such as methane and ammonia, which could hint at a planet’s potential habitability for life.

Webb “is ushering in this new era of exoplanet atmospheric science,” says Nikku Madhusudhan of the University of Cambridge, who was not involved in the study.

The Webb telescope is sensitive to infrared wavelengths of light that are mostly blocked by the Earth’s atmosphere. It has already amazed astronomers with its ability to bring into view the most distant stars and galaxies in the universe.

But infrared sensitivity is also critical for researchers studying worlds much closer to home, in the Milky Way. When an exoplanet’s orbit takes it in front of its star, some of the starlight passes through the planet’s atmosphere and fingerprints its composition. Atmospheric gases absorb specific wavelengths of light, which appear as dimming when starlight is spread out into a spectrum.

For most gases of interest, decreases occur at infrared wavelengths. The Hubble Space Telescope and its infrared sibling, the Spitzer Space Telescope, have detected water vapor, methane and carbon monoxide around some hot, giant exoplanets, but little else.

Webb promises to detect many more gases in smaller planets the size of Neptune and potentially even rocky planets similar in size to Earth, although it is unlikely to be able to confirm the existence of life.

For its first exoplanet observations, astronomers targeted the hot gas giant WASP-39b, which orbits its star every 4 days in an orbit much narrower than Mercury’s. The first data was received on July 10 and the team started working on it a few days later. Even in raw data based on a single pass through the star, the spectral decrease of CO2 “It sticks out like a sore thumb,” says Webb’s team member Jacob Bean of the University of Chicago. There have been some preliminary discoveries of gas before, he says, but none of them came under scrutiny. Webb’s spectrum was “the right size, the right shape, and in the right position,” says Bean. “CO2 it just came out.”

Bean and his colleagues reported the results yesterday on the arXiv preprint server, and they will appear in the Nature in the near future.

Hubble and Spitzer have previously found water vapor, sodium and potassium in WASP-39b’s atmosphere. Webb has now added CO2, as well as another gas whose spectral signature was initially a mystery. Subsequent observations revealed what it is, but Bean wouldn’t say anything about it until the result was peer-reviewed.

In the coming months the team will publish the planet’s full optical to mid-infrared spectrum and “do a complete chemical inventory of its atmosphere,” says team member Laura Kreidberg of the Max Planck Institute for Astronomy.

Finding CO2 it is valuable because it is a clue to a planet’s “metallicity”—the percentage of elements heavier than helium in its composition. The hydrogen and helium produced in the Big Bang are the starting materials for all visible matter in the universe, but anything heavier was forged later in stars. Researchers believe that a good supply of heavy elements is essential for the creation of giant planets. When planets form from a disk of material around a young star, the heavier elements form solid grains and pebbles that coalesce into a solid core that is eventually massive enough to pull gas under its own gravity and become transformed into a gas giant.

From CO2 WASP-39b’s signal, the team estimates that the planet’s metallicity roughly matches that of Saturn. Surprisingly, WASP-39b is about the same mass as Saturn. The planets share some commonalities even though they have extremely different orbits, Bean says. “Can we find a common history for these two objects?” he says. “I don’t know yet.”

With Webb, finding “important chemicals will be the norm rather than the exception,” says Madhusudhan. He predicts that when Webb starts studying cooler planets closer in size to Earth, there will be some real surprises — perhaps some gases that could indicate whether the planets are suitable for life. “It’s anyone’s guess,” he says. “A whole zoo of chemicals is possible.”

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