JWST Finds a Clear, Unambiguous Signal for Carbon Dioxide in an Exoplanet’s Atmosphere

An early – and exciting — science result from the James Webb Space Telescope (JWST) was announced today: the first unambiguous detection of carbon dioxide in the atmosphere of an exoplanet. This is the first detailed evidence for carbon dioxide ever detected in a planet outside our Solar System.

The planet is named WASP-39b and is a hot gas giant with a mass roughly that of Saturn, in a very close-in orbit of a Sun-like star 700 light-years away. The planet was discovered in 2011 by the WASP (Wide Angle Search for Planets) consortium using a ground-based telescopes as the planet transited, or passed in front of its host star.

Subsequent observations of the exoplanet with the Hubble and Spitzer space telescopes revealed a substantial amount of water vapor in its atmosphere, as well as sodium, and potassium. But now, JWST’s unparalleled infrared sensitivity has 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,” said Eliza Kempton, an associate professor of astronomy at the University of Maryland, who was part of the research team that made this discovery. “This discovery shows us that Webb is delivering on its promise of being a transformational facility for astronomical observations.”

“This was also an unparalleled experiment in open science that we’re very proud of,” said Natalie Batalha of the University of California at Santa Cruz, who leads the team. “Over 300 scientists from around the world are participating,” she said on Twitter.  

The team’s paper is now on arXiv, but it has also been accepted for publication in Nature and should appear online next week.

Carbon dioxide is naturally present in Earth’s atmosphere as part of the carbon cycle, but is also being emitted at increasing levels by human activities. Scientists feel that finding CO2 in the atmosphere of another world is an important step towards finding chemical biotracers of extraterrestrial life.

Studying exoplanet atmospheres was one of the highly anticipated areas of research for JWST. The research team used the telescope’s Near-Infrared Spectrograph (NIRSpec) for its observations of WASP-39 b. In a press release, Kempton said they received an early version of the spectrum—before anyone had labeled its features—and the presence of carbon dioxide was immediately apparent.

“The shape of the spectrum is unmistakable to someone like me who models exoplanet atmospheres for a living,” she said. “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. For the Webb observation of WASP-39 b, the CO2 is just sitting there in plain sight, waving, ‘Hello! I’m here!’”

The spectrum of the exoplanet’s atmosphere show’s a small ‘hill’ between 4.1 and 4.6 microns. Kempton explained that no other 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,” she said.

On July 10, 2022, JWST observed WASP-39 for 8 hours, measuring its brightness and waiting for it to be eclipsed by the planet WASP-39b. This is the same transit technique used by NASA’s Kepler and TESS missions to find exoplanets, with one key difference, Batalha explained on Twitter.

“Kepler and TESS observe transits in ‘white light’ whereas JWST can observe a transit in hundreds of colors all at once. JWST can do this because it’s equipped with spectrographs that spread white light into infrared rainbows of color,” she said. “Detecting such a clear signal of carbon dioxide on WASP-39 b bodes well for the detection of atmospheres on smaller, terrestrial-sized planets.”

During a transit, some of the starlight is eclipsed by the planet completely, which causes a dimming of the starlight. But some light is transmitted through the planet’s atmosphere. Because different gases absorb different combinations of colors, researchers can analyze small differences in the brightness of the transmitted light across a spectrum of wavelengths and determine exactly what an atmosphere is made of.

WASP-39 b orbits very close to its star—only about one-eighth the distance between the Sun and Mercury—completing one orbit in just over four Earth-days. With its known atmosphere and frequent transits, the team knew that WASP-39 b was an ideal early target.

“It’s amazing to see the ESA NIRSpec instrument producing this incredible data so early in the mission, when we know we can still improve on the data quality moving forward,” said Sarah Kendrew, ESA Webb MIRI Instrument and Calibration Scientist at the Space Telescope Science Institute in Baltimore.

The JWST team said that 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.

“Seeing the data for the first time was like reading a poem in its entirety, when before we only had every third word,” added team member Laura Kreidberg of the Max Planck Institute for Astronomy in Heidelberg, Germany. “These first results are just the beginning; the Early Release Science data have shown that Webb performs beautifully, and smaller and cooler exoplanets (more like our own Earth) are within its reach.”

And the team hopes this early release will enable and encourage other teams to collaborate and share the plethora of data that JWST is now starting to produce.  

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

Sources and further reading:
ESA Webb
University of Maryland
STScI/Webb
arXiv