In a first, James Webb Space Telescope reveals distant gassy atmosphere is filled with carbon dioxide

Scientists used transmission of light to determine the makeup of the gas giant's atmosphere.
An illustration of what exoplanet WASP-39 b could look like. NASA, ESA, CSA, and J. Olmsted (STScI)

NASA’s James Webb Space Telescope (JWST) continues its sizzling summer of scientific discovery, finding the first clear evidence of carbon dioxide in the atmosphere of an exoplanet. The findings have been accepted for publication in the journal Nature. Extrasolar planets, or exoplanets, are any planet outside of our solar system. Most of them orbit other stars the way Earth orbits the sun, but some free-floating exoplanets (aka rogue planets) orbit a galactic center, untethered to any other stars.

This new finding means that the groundbreaking JWST may be able to detect and measure key molecules like carbon dioxide in the thinner atmospheres of smaller rocky planets in the future. This kind of data gives scientists insight into the formation, composition, and evolution of the galaxy’s planets.

Exoplanet WASP-39b was first discovered in 2011. Seven years later, NASA’s Hubble and Spitzer space telescopes detected water vapor, sodium, and potassium in WASP-39b’s atmosphere, offering a glimpse at what’s going on around the planet. In 2022, it became the first exoplanet to be studied by JWST.

Spinning about 700 light-years away from Earth, WASP-39b is a hot gas giant with a mass about the same as Saturn, but a diameter about 1.3 larger than Jupiter (our solar system’s biggest planet). The planet’s puffiness is partially due to the fact that it’s about 1,600 degrees Fahrenheit (900 degrees Celsius), giving it the nickname “hot Saturn.” WASP-39b is in an endless summer because it orbits its home star very closely, unlike the cooler and more compact gas giants in our solar system. It’s so close that it completes a complete orbit of its star, or one “year,” in just over four Earth-days.

[Related: NASA’s official exoplanet tally has passed 5,000 worlds.]

WASP-39b was first reported using ground-based detections of periodic dimming of light from its host star. This is when the light from the planet’s host star dims as the the planet passes in front of it, like during an eclipse. Transiting, or this eclipse-like event, can provide researchers with ideal opportunities to probe planetary atmospheres.

Different gases absorb different combinations of colors, which means 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” according to NASA. WASP-39b’s combination of inflated atmosphere and frequent transits makes it a perfect target for a technique called transmission spectroscopy.

A series of light curves from JWST’s Near-Infrared Spectrograph (NIRSpec) shows the change in brightness of three different wavelengths (colors) of light from the WASP-39 star system over time as the planet transited the star July 10, 2022.
Credits: Illustration: NASA, ESA, CSA, and L. Hustak (STScI); Science: The JWST Transiting Exoplanet Community Early Release Science Team

The team used JWST’s Near-Infrared Spectrograph (NIRSpec) for these observations. “As soon as the data appeared on my screen, the whopping carbon dioxide feature grabbed me,” Zafar Rustamkulov, a graduate student at Johns Hopkins University and member of the JWST Transiting Exoplanet Community Early Release Science team, which undertook this investigation, said in a statement. “It was a special moment, crossing an important threshold in exoplanet sciences.”

[Related: Newly discovered exoplanet may be a ‘Super Earth’ covered in water.]

Measuring such subtle difference in the brightness of so many single colors across the 3 to 5.5-micron range in an exoplanet transmission spectrum is a first for researchers, NASA reports. It’s critical to access this part of the spectrum when measuring how much gas, water, methane, and carbon dioxide in 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,” team leader Natalie Batalha of the University of California at Santa Cruz said in the NASA statement.

For scientists, understanding what makes up a planet’s atmosphere is important because it offers a window into its origin and evolution. “Carbon dioxide molecules are sensitive tracers of the story of planet formation,” research team member Mike Line of Arizona State University said in the NASA statement. “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.”