The atmosphere of alien planets provides valuable information about their chemistry, history and, last but not least, their environment. Now, with the help of the new James Webb Space Telescope, astronomers have achieved an important achievement: For the first time, they have detected carbon dioxide in the atmosphere of an extrasolar planet. The high-resolution near-infrared spectrograph (NIRSpec) detected the spectral lines of the CO2 in the gas envelope of the gas giant WASP-39 b, which is 700 light-years away. This hot gaseous planet orbits its parent star very closely and has a bloated atmosphere that left prominent absorption lines in the star's light spectrum. The first detection of CO2 demonstrates the great benefit of the new telescope for planetary research.
The James Webb Space Telescope, which was launched into space at the end of December 2021, only started its scientific operation in early summer 2022 - and already with its first images it met and even exceeded all expectations. Because his cameras and spectrographs, which work in the near-infrared and mid-infrared, have already looked further into the early cosmos than any telescope before him and have also shown closer astronomical objects with a completely new sharpness. However, another task of the telescope is to provide new information about the atmospheres of extrasolar planets. In particular, the Webb telescope's Near-Infrared Spectrograph (NIRSpec) and Near-Infrared Imager and Slitless Spectrograph (NIRISS) are specially designed for this. The data on exoplanet atmospheres are important because they can reveal what their gas envelope and surface are like, what processes are taking place on the planets, how they developed and, last but not least, whether life exists on them.
Spectral view of hot gas giants
Astronomers with the James Webb Telescope have now made an important breakthrough in this area. The team, led by Natalie Batalha from the University of California at Santa Cruz, had targeted the exoplanet WASP-39 b, which is around 700 light-years away, for their study. This has about the mass of Saturn, but is inflated to 1.3 times the size of Jupiter. The reason for this is the small distance to its sun-like parent star. It orbits it at a distance of about one eighth of the distance of Mercury from the Sun and takes just over four days to complete one orbit. This heats up the gas giant to around 900 degrees. "The heat causes the planet's atmosphere to expand, so WASP-39b is a third larger than Jupiter, the largest gas giant in our solar system," explains co-author Monika Lendl from the University of Geneva.
From our perspective, the gas giant discovered in 2011 passes directly in front of its star during its orbits. During these transits, the star's light shines through the planet's bloated atmosphere. The molecules and elements contained in the gas envelope absorb specific parts of the light and leave characteristic absorption lines in the light spectrum. The Hubble and Spitzer space telescopes had already detected spectral lines of potassium, water vapor and sodium for WASP-39 b. However, their resolution was not sufficient to clearly identify other molecules. The near-infrared spectrograph of the James Webb telescope has now succeeded in doing this.
CO2 signature unmistakably recognizable
The transmission spectrum of WASP-39 b revealed a group of fine absorption lines around 4.3 micrometers wavelength - the region where the signature of carbon dioxide lies. "When the data appeared on my screen, I was immediately excited by the huge carbon dioxide signal," says co-author Zafar Rustamkulov of Johns Hopkins University in Baltimore. "It was a special moment that crosses an important threshold in exoplanet research." With a standard deviation of an enormous 26 sigma, the spectral CO2 signature was unmissable and highly specific. The James Webb telescope has thus provided the first clear and detailed detection of carbon dioxide in the atmosphere of an exoplanet. "This clear detection of CO2 is an important milestone for characterizing the atmosphere of exoplanets," explains co-author Laura Kreidberg from the Max Planck Institute for Astronomy in Heidelberg.
The results show that the Webb telescope lives up to expectations and is capable of detecting key molecules such as CO2 on exoplanets. This provides valuable insight into the composition, formation and evolution of planets. "CO2 is an important indicator of the formation history of planets: it helps us to measure the complete carbon and oxygen inventory of the atmosphere, which is very sensitive to the conditions in the disc in which the planet formed," explains Kreidberg. With the help of the CO2 data, astronomers can, for example, better narrow down the original place of formation of the planet, the proportions of solid and gaseous planetary building blocks and later migrations of a planet. CO2 and other molecules in the same spectral range can also provide information about chemical processes in the atmosphere and on the surface of the celestial body and also reveal whether there may be life there.
“The exoplanet community has been searching for the signature of carbon dioxide for decades. With the extraordinary new capabilities of the JWST, it will be possible to routinely detect carbon dioxide on both hot Jupiters and smaller, cooler planets similar to our Earth,” says Kreidberg.
Source: JWST Transiting Exoplanet Community Early Release Science Team , Nature accepted, doi: 10.48550/arXiv.2208.11692