University of Bristol: NASA’s Webb detects carbon dioxide in exoplanet atmosphere

This observation of a gas giant planet orbiting a Sun-like star 700 light-years away provides important insights into the composition and formation of the planet.

The finding, which involved academics from the University of Bristol and is accepted for publication in the journal Nature, is also indicative of Webb’s unique ability to detect and measure carbon dioxide in the thinner atmospheres of smaller, rocky planets.

WASP-39 b is a hot gas giant with a mass roughly one-quarter that of Jupiter (about the same as Saturn) and a diameter 1.3 times greater than Jupiter.

Its extreme puffiness is related in part to its high temperature (about 1,600 degrees Fahrenheit or 900 degrees Celsius). Unlike the cooler, more compact gas giants in our 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.

Previous observations from other telescopes, including NASA’s Hubble and Spitzer space telescopes, 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.

Transiting planets like WASP-39 b, 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 transmitted through the planet’s atmosphere.

Because different gases absorb different combinations of colours, researchers can analyse small differences in brightness of the transmitted light across a spectrum of wavelengths to determine exactly what an atmosphere is made of. With its combination of inflated atmosphere and frequent transits, WASP-39 b is an ideal target for transmission spectroscopy.

The research team used Webb’s Near-Infrared Spectrograph (NIRSpec) for its observations of WASP-39 b. In the resulting spectrum of the exoplanet’s atmosphere, a small hill between 4.1 and 4.6 microns presents the first clear, detailed evidence for carbon dioxide ever detected in a planet outside the solar system.

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: “As soon as the data appeared on my screen, the whopping carbon dioxide feature grabbed me. It was a special moment, crossing an important threshold in exoplanet sciences.”

No observatory has ever measured such subtle differences in brightness of so many individual colours 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.

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

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.

Mike Line of Arizona State University, another member of this research team, said: “Carbon dioxide molecules are sensitive tracers of the story of planet formation.

“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.”

Dr Hannah Wakeford, from the University of Bristol’s School of Physics, was part of the academic team involved in the discovery. She said: “This work has been a great collaborative effort with hundreds of scientists across the world. I have been involved with the project since 2016, and seeing it all come together and to have helped coordinate the science and collaboration has been amazing.

“Observations with Hubble and Spitzer of WASP-39b revealed the presence of sodium, potassium, and water vapor in its atmosphere with only hints of potential CO2. We learnt more than ever before from that dataset and it posed many questions about the planet’s formation, with a heavy element abundance predicted much higher than expected.

“We thought we knew what we were going to see with JWST, and we were right, but I don’t think we truly believed it until we saw the exquisite quality of the JWST data and that CO2 absorption feature just jumps right out of the page. It was a beautiful moment to see the new data compared to something I predicted in 2018 and for an exoplanet I have been working on since my PhD. Now I am working with my own PhD students on the JWST data for this planet and it has all come full circle.”

This NIRSpec prism observation of WASP-39 b is just one part of a larger investigation that includes observations of the planet using multiple Webb instruments, as well as observations of two other transiting planets. The investigation, which is part of the Early Release Science program, was designed to provide the exoplanet research community with robust Webb data as soon as possible.

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

Natasha Batalha, co-author on the paper from NASA’s Ames Research Center, added: “NASA’s open science guiding principles are centred in our Early Release Science work, supporting an inclusive, transparent, and collaborative scientific process.”