A planet glows in the light of a nearby sun.

'A new era in exoplanet research,' says Princeton member of the Webb Space Telescope team that found carbon dioxide out there

WASP-39b is a hot, puffy gas giant in the constellation Virgo. Data collected by the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope show unambiguous evidence for carbon dioxide in its atmosphere. This artist’s illustration suggests what the exoplanet might look like, based on direct and indirect transit observations from space- and ground-based telescopes; JWST has not captured a direct image of it.

NASA’s James Webb Space Telescope (JWST) has captured definitive evidence of carbon dioxide in the atmosphere of WASP-39b, a gas giant planet orbiting a sun-like star 700 light-years away.

“This discovery heralds the dawn of a new era in exoplanet research,” said Tansu Daylan, a Princeton astrophysicist who is one of the co-authors of the paper in Nature detailing the discovery. “JWST has started giving us an extraordinary opportunity to characterize exoplanets in unprecedented detail.”

WASP-39b is known as a “hot Jupiter” because it is a gas giant that is incredibly close to its sun — much closer than Mercury is to our sun.

The exoplanet has a mass roughly one-quarter that of Jupiter (about the same as Saturn) and a diameter 1.3 times greater than Jupiter. This extreme "puffiness" is related in part to its high temperature — about 1600°F, or 900°C. Unlike the cooler, more compact gas giants in our solar system, WASP-39b orbits very close to its star — only about one-eighth the distance between the Sun and Mercury — completing one trip around its star in just over four Earth days.

That close proximity to its star almost certainly rules out the possibility that this carbon dioxide was created by lifeforms, say the scientists. “WASP-39b is unlikely to harbor the set of conditions necessary to support life as we know it,” said Daylan, a postdoctoral research associate in astrophysical sciences. “However, JWST will observe smaller, potentially terrestrial exoplanets, and our result confirms that JWST has the potential to probe the relevant biosignatures on rocky planets beyond our solar system.”

Filtered starlight

WASP-39b’s discovery, reported in 2011, was based on ground-based detections of the subtle, periodic dimming of light from its host star as the planet passes in front of the star, or transits.

Transiting planets like WASP-39b, whose orbits we observe edge-on rather than from above, can provide researchers with ideal opportunities to probe planetary atmospheres.

During a transit, some of the starlight is eclipsed by the planet completely — causing the overall dimming — and some is filtered through the planet’s atmosphere. The atmosphere scatters and absorbs some colors more than others, depending on factors such as what the atmosphere is made of, how thick it is, and whether or not there are clouds. (We observe this effect in our own atmosphere as the color and quality of sunlight changes depending on how hazy or humid the air is, or where the sun is in the sky.)

Because different gases absorb different combinations of colors, researchers can analyze small differences in brightness of the transmitted light across a spectrum of wavelengths to determine exactly what an atmosphere is made of.

First clear detection of carbon dioxide

In the resulting spectrum of WASP-39b’s atmosphere, the small hill between 4.1 and 4.6 microns is sparking Everest-sized delight in exoplanet researchers. It is the first clear, detailed, indisputable evidence for carbon dioxide ever detected in a planet outside our own solar system.

“As soon as the data appeared on my screen, the whopping carbon dioxide feature grabbed me,” said Zafar Rustamkulov, a graduate student at Johns Hopkins University and member of the transiting exoplanet team. “It was a special moment, crossing an important threshold in exoplanet sciences.”

Earlier, the Spitzer Space Telescope had indirectly suggested the presence of carbon dioxide in this exoplanet’s atmosphere, Daylan said, but the JWST data provided “the first direct evidence of carbon dioxide in an exoplanet atmosphere, which was made possible by the wavelength resolution and coverage as well as the measurement precision of the Near-Infrared Spectrograph (NIRSpec) on JWST.”

Even without the strong carbon dioxide feature, this spectrum would be remarkable, say the researchers. 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.

We generally expect carbon dioxide to be abundant in exoplanet atmospheres, although the carbon dioxide content of an atmosphere will depend on where the planet formed and how it evolved,” Daylan said. “It is plausible to think that JWST will regularly detect or reliably rule out the existence of carbon dioxide and similar molecules such as water and methane. This will be transformative for the field of exoplanet research, significantly increase the detail with which we can characterize exoplanets, and enhance our understanding of pathways in planet formation and evolution.”

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

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 Michael Line of Arizona State University, a 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 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.”

Early release science

This spectrum was shared through the Early Release Science program, designed to provide the exoplanet research community with robust space telescope data as soon as possible.

“The goal is to analyze the Early Release Science observations quickly and develop open-source tools for the science community to use,” said Vivien Parmentier of the University of Oxford. “This enables contributions from all over the world and ensures that the best possible science will come out of the coming decades of observations.”

It is a tremendously exciting experience to be working on the analysis of the JWST data,” said Daylan, “witnessing the high data quality and wide wavelength coverage that will transform our research field and enhance our understanding of planet formation and evolution.”

Identification of carbon dioxide in an exoplanet atmosphere,” by the JWST Transiting Exoplanet Community Early Release Science Team, appears in the current issue of the journal Nature (DOI: 10.1038/s41586-022-05269-w)This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program JWST-ERS-01366. Support for program JWST-ERS-01366 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127.