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November 23, 2022

NASA’s Webb Telescope Reveals an Exoplanet Atmosphere in ‘Once Impossible’ Detail

New observations of WASP-39b with the JWST have provided a clearer picture of the exoplanet, showing the presence of sodium, potassium, water, carbon dioxide, carbon monoxide and sulfur dioxide in the planet’s atmosphere. This artist’s illustration also displays newly detected patches of clouds scattered across the planet.

New observations of WASP-39b with the JWST have provided a clearer picture of the exoplanet, showing the presence of sodium, potassium, water, carbon dioxide, carbon monoxide and sulfur dioxide in the planet’s atmosphere. This artist’s illustration also displays newly detected patches of clouds scattered across the planet.

Credit: Melissa Weiss/Center for Astrophysics | Harvard & Smithsonian


NASA’s James Webb Space Telescope made the first identification of sulfur dioxide in an exoplanet’s atmosphere. Its presence can only be explained by photochemistry — chemical reactions triggered by high-energy particles of starlight.

NASA’s James Webb Space Telescope made the first identification of sulfur dioxide in an exoplanet’s atmosphere. Its presence can only be explained by photochemistry — chemical reactions triggered by high-energy particles of starlight.

Credit: NASA/Jet Propulsion Laboratory-Caltech/Robert Hurt, Center for Astrophysics | Harvard & Smithsonian/Melissa Weiss


A transmission spectrum of WASP-39b captured by Webb’s Near InfraRed Spectrograph instrument in July 2022. It reveals the rich soup of molecules in the planet’s steamy atmosphere, including the first detection of the light-produced molecule sulfur dioxide. The blue line is of a best-fit model, while the various rectangular colors each respectively highlight peaks attributable to a certain molecule.

A transmission spectrum of WASP-39b captured by Webb’s Near InfraRed Spectrograph instrument in July 2022. It reveals the rich soup of molecules in the planet’s steamy atmosphere, including the first detection of the light-produced molecule sulfur dioxide. The blue line is of a best-fit model, while the various rectangular colors each respectively highlight peaks attributable to a certain molecule.

Credit: NASA/European Space Agency/Canadian Space Agency/Leah Hustak (Space Telescope Science Institute)/Joseph Olmsted (Space Telescope Science Institute)

NASA’s James Webb Space Telescope is dazzling scientists yet again, this time not with stunning images of the cosmos but instead with the first comprehensive list of molecular ingredients in the atmosphere of a planet roughly 700 light-years away.

From that, an international team of researchers that included scientists from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, revealed not only the first detection of active chemistry happening in the atmosphere of an exoplanet — a planet orbiting another star — but also potentially how that exoplanet formed.

“The quality and precision of these data are just outstanding,” said Kevin Stevenson, an astrophysicist at APL, co-author on all studies, and the primary advisor on the paper that used Webb’s Near InfraRed Camera (NIRCam) during the observations. “What was once impossible a decade ago will soon become routine.”

The findings, which will be published in a series of five upcoming scientific papers, bode well for the capability of Webb’s instruments to conduct the broad range of exoplanet investigations hoped for by the science community.

“We expected JWST to be a powerful tool to study exoplanet atmospheres, and these observations are among the first real evidence that that is true,” said APL astrophysicist Erin May, who was a co-author on all of the new studies. “The precision of these measurements is unmatched by previous telescopes, and we’re really just scratching the surface of what we’ll be able to learn about exoplanets going forward.”

The studies focused on a Jupiter-sized exoplanet called WASP-39b, which, at eight times closer to its star than Mercury is to the Sun, broils at roughly 1,600 degrees Fahrenheit (900 degrees Celsius). Previous observations with ground-based and space telescopes, including NASA’s Spitzer and Hubble telescopes, had determined isolated ingredients in the planet’s sweltering atmosphere. But Webb was able to complete the picture thanks to its ability to see infrared light, which lies beyond what human eyes — and most space telescopes — can see.

Operating under NASA’s Early Release Science program, researchers used Webb to track WASP-39b as it passed in front of its star. That allowed the star’s light to filter through the planet’s atmosphere. Different molecules in the atmosphere absorb different wavelengths of the starlight, so astronomers can tell which molecules are present just by looking at what wavelengths are missing when the filtered light reaches Earth.

Among the most notable of molecules was the first detection of sulfur dioxide (SO2) — a molecule produced from chemical reactions triggered by light. Such photochemical reactions regularly occur on Earth, including the photosynthetic process of plants for generating food from light, or Earth’s ozone layer that blocks harmful radiation from reaching the ground. But this was the first time researchers had confirmed such chemistry on a planet outside the solar system.

“Finding sulfur dioxide in one of JWST’s first observed targets suggests photochemistry is likely common in the atmospheres of hot exoplanets like WASP-39b and that we can expect to find other photochemical byproducts in the near future,” Stevenson explained. “It opens up a whole new avenue of scientific inquiry.”

Other atmospheric ingredients included sodium, potassium, carbon dioxide, carbon monoxide and water vapor, confirming earlier observations while also complementing them with additional molecular signatures at other wavelengths that weren’t possible before.

With the complete roster of atmospheric makings in hand, the team could gain insight into how WASP-39b formed from the disk of gas and dust that surrounded its parent star while in its infancy. For example, the planet’s smaller carbon-to-oxygen ratio and greater potassium-to-oxygen ratio than in the Sun’s atmosphere suggests the planet has had a history of smashups of small rocky bodies called planetesimals. These collisions eventually created the core of the giant planet seen today. The ratios also suggest WASP-39b likely formed much farther from its star where water is found only as ice, and later migrated inward, possibly collecting more material along the way.

The implications of those details spread well beyond just WASP-39b, though. Because astronomers know of hundreds of hot, Jupiter-sized exoplanets throughout the galaxy, the team notes that WASP-39b’s characteristics offer important clues about how much planetesimal mergers generally contribute to such planets during their early evolution.

Overall, the results confirm Webb’s instruments perform well beyond scientists’ expectations — a development that team members say promises a new phase of exploration among the wide variety of exoplanets in the galaxy.

“We can look forward to a huge wave of new discoveries and intriguing results with JWST,” Stevenson said.

Media contact: Jeremy Rehm, 240-592-3997, Jeremy.Rehm@jhuapl.edu

The Applied Physics Laboratory, a not-for-profit division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu.

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