We just got the most detailed picture of an exoplanet atmosphere ever

WASP-39b, a gas giant some 700 light-years away, turns out to be quite an exoplanetary treasure.

Earlier this year, WASP-39b was the subject of the very first detection of carbon dioxide in the atmosphere of a planet outside the solar system.

Now, an in-depth analysis of data from the James Webb Space Telescope (JWST) has provided us with an absolute gold mine of information: the most detailed look at an exoplanet’s atmosphere to date.

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

These epic discoveries have been published in five articles in Natureand paving the way for the eventual detection of the chemical signatures of life beyond the solar system.

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

“We put the telescope through its paces to test its performance, and it was almost flawless – even better than we hoped.”

Since the discovery of the first exoplanets in the early 1990s, we’ve been trying to learn more about these worlds orbiting alien stars.

But the challenges were steep. 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 guest stars.

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

And there’s something else we can see based on transit data. As starlight passes through the atmosphere of the passing exoplanet, it changes. Some wavelengths in the spectrum become dimmed or brightened depending on how molecules in the atmosphere absorb and re-emit light.

The signal is weak, but with a powerful enough telescope and a stack of transits, the changing absorption and emission characteristics on the spectrum can be decoded to determine the contents of an exoplanet’s atmosphere.

JWST is the most powerful space telescope ever launched. Using three of its four instruments, it obtained detailed infrared spectra of the star WASP-39. Scientists then set to work analyzing the colorful codes.

First there was a count of the molecules present in the atmosphere of WASP-39b. In addition to the above carbon dioxide, the researchers discovered water vapour, sodium and carbon monoxide. No methane was detected, implying WASP-39b’s metallicity is higher than Earth’s.

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

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

Photon-induced chemical reactions are known as photochemistryand they have implications for the habitability, stability of an atmosphere and the formation of aerosols.

WASP-39b, to be clear, is probably not habitable for life as we know it for a whole host of reasons, including but not limited to its scorching temperature and gaseous composition, but its photochemistry detection is one that has implications for atmospheric studies of other worlds and insight into the evolution of WASP-39b itself.

Planetary scientists have been preparing for years for the insights into atmospheres that JWST was expected to provide. With the first detailed exoplanet atmosphere analysis, it looks like the space telescope is about to 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.

With JWST, we may not be able to detect the signatures of life in an exoplanet’s atmosphere — perhaps an even more powerful telescope is needed to deliver that level of fine detail — but with WASP-39b’s analysis, that discovery feels increasingly tantalizing within easy reach .

“That kind of data,” says astronomer Natalie Batalha from the University of California Santa Cruz, “are a game changer.”

The research will be published Nature and can be read in preprints here, here, here, hereand here.