NASA’s James Webb Space Telescope has captured the first clear evidence for carbon dioxide in the atmosphere of a planet outside the solar system, according to an international research team that includes two University of Maryland astronomers. This observation of WASP-39 b, a gas giant planet orbiting a sun-like star 700 light-years away, provides important insights into the composition and formation of the planet, and suggests that Webb will be able to measure carbon dioxide in the thinner atmospheres of smaller rocky planets that could hold similarities to Earth.

Associate Professor Eliza Kempton and Assistant Professor Thaddeus Komacek are part of the group behind the finding that was accepted for publication in the journal Nature. Scores of other UMD faculty and alums have been involved with the Webb Telescope through its 30-year development leading to its launch last December.

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The work of Kempton and Komacek is part of a more extensive investigation that includes observations of the planet using multiple Webb instruments to provide the exoplanet research community with robust Webb data as soon as possible—part of the Early Release Science program overseen by the Space Telescope Science Institute in Baltimore.

Unlike the cooler, more compact gas giants of our outer solar system, WASP-39 b orbits very close to its star—only about one-eighth the distance between the sun and Mercury—completing one circuit in just over four Earth-days. The planet’s discovery, reported in 2011, was made based on ground-based detections of the subtle, periodic dimming of light from its host star as the planet transits, or passes, in front of the star.

Transits also allow astronomers to scrutinize planets’ atmospheres using minute differences in brightness of transmitted light across a spectrum of wavelengths; previous observations from other telescopes, including NASA’s Hubble and Spitzer space telescopes, had revealed the presence of water vapor, sodium and potassium in the planet’s atmosphere. Webb’s unmatched infrared sensitivity has now confirmed the presence of carbon dioxide on this planet as well.

“The reason we hadn't been able to definitively identify CO₂ in the atmosphere of WASP-39 b previously was that we never had a telescope that could produce spectra across the right wavelength range,” Kempton said. “This discovery shows us that Webb is delivering on its promise of being a transformational facility for astronomical observations.”

The research team used Webb’s Near-Infrared Spectrograph for its observations of WASP-39 b. Kempton received an early version of the spectrum—before anyone had labeled its features—and said the presence of carbon dioxide was immediately apparent.

“This is very different from all previous exoplanet atmosphere observations where we would typically go through a much more involved process of comparing many different possible atmosphere models to the data to convince ourselves that we've detected a specific atom or molecule,” she said. “For the Webb observation of WASP-39 b, the CO₂ is just sitting there in plain sight, waving, ‘Hello! I'm here!’"

No observatory has ever measured such subtle differences in brightness of so many individual colors across the 3 to 5.5-micron range in an exoplanet transmission spectrum before. Access to this part of the spectrum is crucial for measuring abundances of gases like water and methane, as well as carbon dioxide, which are thought to exist in many different types of exoplanets.

“Detecting such a clear signal of carbon dioxide on WASP-39 b bodes well for the detection of atmospheres on smaller, terrestrial-sized planets,” said Natalie Batalha of the University of California at Santa Cruz, who leads the team.

Understanding the composition of a planet’s atmosphere is important because it tells us something about the origin of the planet and how it evolved.

“Carbon dioxide molecules are sensitive tracers of the story of planet formation,” said Mike Line of Arizona State University, another member of the research team. “By measuring this carbon dioxide feature, we can determine how much solid versus how much gaseous material was used to form this gas giant planet. In the coming decade, JWST [Webb] will make this measurement for a variety of planets, providing insight into the details of how planets form and the uniqueness of our own solar system.”

Thanks to the advanced capabilities of Webb, Kempton said the discovery of carbon dioxide in WASP-39 b’s atmosphere is the beginning of a long and fruitful exploration of exoplanets.

“It is just the tip of a huge iceberg,” Kempton said. “We expect that hundreds of exoplanets will be observed by Webb over the telescope's lifetime.”

Emily C. Nunez of the College of Computer, Mathematical, and Natural Sciences contributed to this article, which is based on a release by the Space Telescope Science Institute.