The James Webb Telescope was the first to detect carbon dioxide (CO₂) in the atmosphere of a planet outside our solar system. The scientists involved rejoice that this marks the beginning of a new era in exoplanet research. On the one hand, carbon dioxide is a key molecule whose presence can say a lot about the history of the formation of a planet.
On the other hand, the successful measurement of CO₂ shows that James-Webb can also determine the concentration of water and methane in the atmosphere of distant planets. Because these substances reveal themselves through signatures in the same range of the infrared spectrum in which the James Webb telescope is sensitive.
“I felt like a kid in a candy store when I first saw this spectrum,” says Professor Michael Line of Arizona State University, “Hey, that’s definitely carbon dioxide! This telescope opens up tremendous potential for exploring worlds outside of our solar system. This is truly a game changer.”
That’s exactly what the scientists from the James Webb telescope had hoped for – that molecules could be detected on distant planets that might indicate the existence of life. Water, methane and carbon dioxide can be important indicators. In the case of the exoplanet WASP-39 b, which is around 700 light-years away from us, the existence of carbon dioxide is by no means an indication of possible life forms.
WASP-39 b is a hot gas giant with a mass about one-fourth that of Jupiter orbiting its star at a very close distance — just one-eighth the distance between the Sun and Mercury. Temperatures of 900 degrees Celsius prevail on WASP-39 b. It’s all very hostile to life.
But the measurement of carbon dioxide proves that James Webb is able to detect this and other molecules on other exoplanets – for example on smaller and cooler rocky planets that could offer habitable environmental conditions.
“The discovery of such a clear signal of carbon dioxide on WASP-39 bodes well for the discovery of atmospheres on smaller, Earth-sized planets,” said Natalie Batalha of the University of California at Santa Cruz, who leads the research team. This gives the opportunity to measure carbon dioxide in the thinner atmospheres of smaller planets.
The measurement data, now published in the journal “Nature”, were recorded by the James Webb telescope on July 10, 2022. It is a so-called transmission spectrum in the wavelength range of 3 and 5.5 microns.
A transmission spectrum is created by comparing the starlight filtered through a planet’s atmosphere as it moves in front of the star to the unfiltered starlight collected when the planet is adjacent to the star. However, this requires that the planet – as seen by the James Webb telescope – periodically moves in front of the star as it orbits its central star, so that part of the light is absorbed in the planet’s atmosphere.
During the transit, the planet obscures some of the starlight, resulting in an overall dimming. A smaller portion of the light passes through the planet’s atmosphere. The increase in the absorption of the atmosphere at a wavelength of 4.3 microns represents the light being swallowed up by carbon dioxide.
To obtain the infrared spectrum with the CO₂ detection, the James Webb telescope gazed at the WASP-39 system for more than eight hours, starting three hours before the transit and ending two hours after the transit. The transit itself took about three hours.
The star WASP-39 in the constellation Virgo is about the same size, mass, temperature and color as the Sun. The researchers have calculated a model of the planet’s atmosphere from the current and previous measurements of the “Hubble” space telescope. It consists mainly of hydrogen and helium as well as small amounts of water, carbon dioxide and a thin veil of clouds.
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