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Webb detects carbon dioxide in exoplanet atmosphere

The James Webb Space Telescope has found evidence of carbon dioxide (CO₂) in the atmosphere of a giant planet 700 light years away. This result provides insight into the composition and formation of this gas giant and shows what James Webb is capable of. The research by the international group of astronomers, including Leiden Associate Professor Yamila Miguel, has been accepted for a publication in Nature.

It concerns the planet WASP-39 b, a hot gas giant with a mass of about a quarter of that of Jupiter (about the same size as Saturn) and a diameter 1.3 times that of Jupiter. The fact that it is so extremely swollen is partly related to its high temperature (about 900°C). Unlike the cooler, more compact gas giants in our solar system, WASP-39 b orbits very close to its star - only about an eighth of the distance between the Sun and Mercury - and completes one orbit in just over four days.

Filtered starlight

WASP-39 b moves exactly in front of its star as seen from Earth. During such a transit, some starlight is blocked by the planet and some is transmitted through the planet's atmosphere. The atmosphere filters some colours more than others, depending on factors like its composition, thickness and the presence of clouds.

Because different gases absorb different combinations of colours, researchers can analyse small differences in the brightness of the transmitted light across a spectrum of wavelengths to determine exactly what an atmosphere is made of. WASP-39 b is an ideal object for transmission spectroscopy because its atmosphere is swollen and the planet often moves in front of its star. The team used Webb's near-infrared spectrograph NIRSpec for the observations.

First clear detection of carbon dioxide

The researchers see the first clear, detailed and irrefutable evidence for carbon dioxide on a planet outside the solar system as a good omen for detection on smaller, rocky planets. Natalie Batalha from the University of California at Santa Cruz, leads the team of researchers who are using Webb to study these so-called 'transiting' exoplanets.

A transmission spectrum from the hot gas giant exoplanet WASP-39 b is the first definitive evidence for carbon dioxide in a planet outside the solar system. Credit: NASA, ESA, CSA, Leah Hustak (STScI), Joseph Olmsted (STScI).

'Even without the robust carbon dioxide property, this spectrum would be remarkable,' says Batalha. 'No observatory has ever before measured such subtle differences in brightness from so many individual colours in the 3 to 5.5 micron range in the transmission spectrum of an exoplanet. Access to this part of the spectrum is crucial for measuring concentrations of gases such as water, methane, and carbon dioxide, which are believed to occur in many different types of exoplanets.'

Understanding the formation process of 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,' co-author and Leiden researcher Yamila Miguel, who worked on models and interpreting the data adds.

Jean-Michel Désert, Professor at the UvA: 'By measuring this carbon dioxide property, we can determine how much solid and how much gaseous material was used to form this gas giant. Over the next decade, Webb will take measurements of this kind on different types of planets. This will give us more insight into the formation process of planets and whether our own solar system is unique.

Early Release Science

The NIRSpec observations of WASP-39b are part of a larger study using several other methods and instruments, including observations of two other planets passing in front of their star. The study is part of the Early Release Science programme, which is designed to provide the exoplanet research community with Webb data as soon as possible.

'The aim is to analyse the Early Release Science observations immediately and to develop tools that will allow anyone to analyse Webb data in the future,' co-author Vivien Parmentier from Oxford University explains. 'This will ensure that the best science will come out of the observations.'

The article 'Identification of carbon dioxide in an exoplanet atmosphere' will be published in Nature on Monday 29 August.

Main image: Artistic impression of exoplanet WASP-39 b, based on current knowledge of the planet. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI).

Source: NOVA/astronomie.nl

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