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

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.