We just got the most detailed view of an exoplanet’s atmosphere yet

WASP-39b, a gas giant about 700 light-years away, turns out to be a veritable exoplanet treasure.

Earlier this year, WASP-39b was the subject of the first detection of carbon dioxide in the atmosphere of an exoplanet.

Now, an in-depth analysis of data from the James Webb Space Telescope (JWST) has given us an absolute goldmine of information: the most detailed look at an exoplanet’s atmosphere yet.

The results include information about the clouds of WASP-39b, the first-ever direct detection of photochemistry in an exoplanet’s atmosphere, and a near-complete inventory of the chemical content of the atmosphere that reveals tantalizing hints about the exoplanet’s formation history.

These epic findings were published in five papers in Natureand paved the way for the eventual discovery of the chemical signatures of life outside the solar system.

“These early observations are a harbinger of more amazing science to come with JWST,” said astrophysicist Laura Kreidberg, director of the Max Planck Institute for Astronomy in Germany.

“We tried out the telescope to test its performance, and it was almost flawless—even better than we had hoped.”

Ever since the first exoplanets were discovered in the early 1990s, we have sought to learn more about these worlds orbiting alien stars.

But the challenges were great. Exoplanets can be extremely small and extremely distant. We’ve never even seen most of them: we only know of their existence based on the effect they have on their host stars.

One of these effects occurs when the exoplanet passes between us and the star, an event known as a transit. This results in a slight dimming of the starlight; periodic dimming events suggest the presence of an orbiting body. We can even tell how big this orbiting body is based on the dimming and gravitational effects on the star.

And there is one more thing we can say based on the transit data. As starlight passes through the transiting exoplanet’s atmosphere, it changes. Certain wavelengths in the spectrum are dimmed or brightened depending on how molecules in the atmosphere absorb and re-emit light.

The signal is weak, but with a sufficiently powerful telescope and a set of transits, the changing absorption and emission features in the spectrum can be decoded to determine the content of an exoplanet’s atmosphere.

JWST is the most powerful space telescope ever launched. With three of its four instruments, it obtained detailed infrared spectra of the star WASP-39. The scientists then set about analyzing the color codes.

First was a count of the molecules present in WASP-39b’s atmosphere. In addition to the aforementioned carbon dioxide, the researchers found water vapor, sodium and carbon monoxide. There was no detection of methane, suggesting that the metallicity of WASP-39b is higher than that of Earth.

The abundance of these elements is also telling. In particular, the carbon-to-oxygen ratio suggests that the exoplanet formed much farther from its host star than its current nearby position, occupying a four-day orbit. And modeling and observational data suggest that the exoplanet’s sky is populated by ragged clouds—not water, but silicates and sulfites.

Finally, observations revealed the presence of a compound called sulfur dioxide. Here in the Solar System, on rocky worlds like Venus and the Jovian moon Io, sulfur dioxide is the result of volcanic activity. But on the gaseous worlds, sulfur dioxide has a different origin story: it is produced when hydrogen sulfide is broken down by light into its component parts and the resulting sulfur is oxidized.

Chemical reactions induced by photons are known as photochemistry and are important for habitability, atmospheric stability and aerosol formation.

WASP-39b, to be clear, is unlikely to be habitable for life as we know it for a bunch of reasons, including but not limited to its scorching temperature and gas composition, but the detection of photochemistry is what matters for atmospheric research on other worlds and understanding the evolution of WASP-39b itself.

Planetary scientists have been preparing for years for the atmospheric insights JWST was expected to provide. With the first detailed analysis of an exoplanet’s atmosphere, it looks like the space telescope will deliver on its promise.

In addition, the teams involved in this research are preparing documentation so that other scientists can apply their techniques to future JWST observations of exoplanets.

We may not find the signs of life in an exoplanet’s atmosphere with JWST – perhaps an even more powerful telescope will be needed to deliver this level of fine detail – but with the analysis of WASP-39b, this discovery feels increasingly tantalizing within reach.

“Data like this,” says astronomer Natalie Batala of the University of California, Santa Cruz, “is a game changer.”

The research will be published Nature and can be read in the reprints here, here, here, here and here.

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