Although JWST and other space telescopes, including NASA's Hubble and Spitzer, have previously revealed isolated ingredients of this broiling planet’s atmosphere, the new readings provide a full menu of atoms, molecules, and even signs of active chemistry and clouds.

The latest data was analysed by a global team of leading scientists, including astrophysicists from the University of Bristol.

The telescope’s array of highly sensitive instruments was trained on the atmosphere of WASP-39 b, a “hot Saturn” (a planet about as massive as Saturn but in an orbit tighter than Mercury) orbiting a star some 700 light-years away. 

"When I first saw the data I cried. We had predicted what the telescope would show us, but it was more precise, more diverse, and more beautiful than I actually believed it would be,” said Dr Hannah Wakeford, an astrophysicist at the university’s School of Physics, who investigates exoplanet atmospheres and is part of the JWST research team.

The data also give a hint of how the clouds might look up close: broken up rather than a single, uniform blanket over the planet. The findings bode well for the capability of JWST’s instruments to conduct the broad range of investigations of all types of exoplanets – planets around other stars – hoped for by the science community to further understand our place in the universe.

“We observed the exoplanet with multiple instruments that, together, provide a broad swath of the infrared spectrum and a panoply of chemical fingerprints inaccessible until [this mission],” said Professor Natalie Batalha, an astronomer at the University of California, Santa Cruz, who contributed to and helped coordinate the new research. “Data like these are a game changer.”

The new discoveries are detailed in a set of five scientific papers, three of which are in press and two under review, including one led by University of Bristol astrophysicist Lili Alderson and co-authored by Dr Hannah Wakeford and research associate Dr David Grant.

Lili Alderson said: “The quality of the results we’ve seen confirms that JWST will be doing incredible exoplanet science for many years to come.”

Among the unprecedented revelations is the first detection in an exoplanet atmosphere of sulfur dioxide (SO₂), a molecule produced from chemical reactions triggered by high-energy light from the planet’s parent star. On Earth, the protective ozone layer in the upper atmosphere is created in a similar way.

“This is the first time we see concrete evidence of photochemistry – chemical reactions initiated by energetic stellar light – on exoplanets,” said Dr Shang-Min Tsai, a researcher at the University of Oxford in the United Kingdom and lead author of the paper explaining the origin of sulfur dioxide in WASP-39 b’s atmosphere. “I see this as a really promising outlook for advancing our understanding of exoplanet atmospheres with [this mission].” 

This led to another first: scientists applying computer models of photochemistry to data that requires such physics to be fully explained. The resulting improvements in modelling will help build the technological know-how to interpret potential signs of habitability in the future.

“Planets are sculpted and transformed by orbiting within the radiation bath of the host star,” Batalha said. “On Earth, those transformations allow life to thrive.”

The planet’s proximity to its host star – eight times closer than Mercury is to our Sun – also makes it a laboratory for studying the effects of radiation from host stars on exoplanets. Better knowledge of the star-planet connection should bring a deeper understanding of how these processes affect the diversity of planets observed in the galaxy.

To see light from WASP-39 b, JWST tracked the planet as it passed in front of its star, allowing some of the star’s light to filter through the planet’s atmosphere. Different types of chemicals in the atmosphere absorb different colours of the starlight spectrum, so the colours that are missing tell astronomers which molecules are present. By viewing the universe in infrared light, JWST can pick up chemical fingerprints that can’t be detected in visible light.

Other atmospheric constituents detected by the space telescope include sodium (Na), potassium (K), and water vapor (H₂O), confirming previous space- and ground-based telescope observations as well as finding additional fingerprints of water, at these longer wavelengths, that haven’t been seen before.

JWST also saw carbon dioxide (CO₂) at higher resolution, providing more than twice as much data as reported from its previous observations. Meanwhile, carbon monoxide (CO) was detected, but obvious signatures of both methane (CH₄) and hydrogen sulfide (H₂S) were absent from the data. If present, these molecules occur at very low levels.

To capture this broad spectrum of WASP-39 b’s atmosphere, an international team numbering in the hundreds independently analysed data from four of the telescope’s finely calibrated instrument modes.

Having such a complete roster of chemical ingredients in an exoplanet atmosphere also gives scientists a glimpse of the abundance of different elements in relation to each other, such as carbon-to-oxygen or potassium-to-oxygen ratios. That, in turn, provides insight into how this planet – and perhaps others – formed out of the disk of gas and dust surrounding the parent star in its younger years. 

WASP-39 b’s chemical inventory suggests a history of smash-ups and mergers of smaller bodies called planetesimals to create an eventual goliath of a planet.

“The abundance of sulfur [relative to] hydrogen indicated that the planet presumably experienced significant accretion of planetesimals that can deliver [these ingredients] to the atmosphere,” said Kazumasa Ohno, a UC Santa Cruz exoplanet researcher who worked on JWST data. “The data also indicates that the oxygen is a lot more abundant than the carbon in the atmosphere. This potentially indicates that WASP-39 b originally formed far away from the central star.” 

In so precisely parsing an exoplanet atmosphere, the telescope’s instruments performed well beyond scientists’ expectations – and promise a new phase of exploration among the broad variety of exoplanets in the galaxy.

Dr Wakeford said: “I was surprised at how well these independent observations confirmed each other's conclusions. One dataset after another confirmed what we were seeing. That is science in action!

“The data has taken us to some of the most exciting science questions and I am certain it will be the first of many new and exciting avenues of exoplanet science.”