Indisputable signs of the presence of CO2 on an exoplanet

This is the first detailed and indisputable evidence for the presence of carbon dioxide ever found on a planet outside the solar system.

Professor Björn Benneke and a team of doctoral students from the University of Montreal and the Institute for Research on Exoplanets (iREx) participated in this work led by astronomer Natalie Batalha, from the University of California in Santa Cruz, and which will be published in the magazine Nature (New window) (in English).

Astrophysicists used James Webb’s Near Infrared Spectrograph (NIRSpec) to observe the planet WASP-39b. And what they discovered is clear: there is indeed CO2 in its atmosphere.

Data analyzed in Montreal detected a huge signature of carbon dioxide around the planet, no less than 26 times louder than any noise in the datanotes Professor Benneke.

Noise can be compared to inaccuracies in the field of vision. To illustrate this, the professor compares the arrival of James Webb to the transition from analog TV to 4k TV.

It was a very, very special moment for me. As a scientist, I’m always very skeptical, but in this case it was like seeing something with my own eyes. »

A quote from Björn Benneke, University of Montreal and iREx

Professor Benneke says this study shows the ability of the space telescope to detect and measure CO2 in very thin atmospheres, even those of small and rocky planets like Earth.

It’s really a new era in astronomy that opensenthuses the astrophysicist.

This detection was achieved thanks to five hours of use of the NIRSpec instrument, which the international team obtained as part of the program Early Release Science. This started at the end of June; its first results were announced in July.

Artist’s rendering of the James Webb Space Telescope.

Photo: iStock/dima_zel

WASP-39 b, a perfect object of study

The target of the observation program, the planet WASP-39 b, is in orbit around a Sun-like star located 700 light years from Earth.

The mass of this hot gas giant is about a quarter of that of Jupiter, and its diameter is 1.3 times larger.

Unlike the cooler, more compact gas giants in our solar system, WASP-39 b orbits very close to its star, barely one-eighth the distance between the Sun and Mercury. In addition, it completes the tour in just over four Earth days.

Its atmosphere appears puffy compared to other planets, a phenomenon probably linked in part to its high temperature of around 900°C.

The planet was detected in 2011 using the transit technique which shows a subtle, periodic dimming of a star’s light as the planet transits, or passes, in front of the star.

Because of its swollen atmosphere and its frequent transits, WASP-39 b was therefore a perfect target for carrying out transmission spectroscopy, which also makes it possible to specify the composition of a planet’s atmosphere thanks to transit. Some light from the star passes through the planet’s atmosphere. This light is filtered by the atmosphere and allows us to measure its compositionexplains Professor Benneke.

Gases absorb different combinations of colors, allowing us to analyze small differences in brightness across various wavelengths to determine exactly what the planet’s atmosphere is made of. »

A quote from Björn Benneke, University of Montreal and iREx

Indisputable proof

Detection of the telltale signal, an absorption line, was performed at wavelengths between 4.1 and 4.6 microns in the infrared.

A graph shows the transmission spectrum of the hot gas giant exoplanet WASP-39b.

A transmission spectrum of the hot gas giant exoplanet WASP-39b captured by ‘James-Webb’s’ near-infrared spectrograph (NIRSpec) on July 10, 2022 reveals the first definitive evidence for the presence of carbon dioxide on a planet outside the solar system.


No telescope or observatory has ever before measured such subtle differences in the brightness of so many individual infrared colors in an exoplanet’s transmission spectrum. Access to this part of the spectrum (from 3 to 5.5 microns) is crucial for determining the abundance of gases such as water and methane, as well as CO2, which could exist in many types of exoplanets.

Water vapor has also been detected in the atmosphere of the exoplanet.

On Earth and elsewhere

On our planet, carbon dioxide plays an extremely important role, since it influences the climate, a climate that allows the presence of life. It could play a similar role in the appearance of life elsewhere.

As a molecule, it is very important. For example, Venus is a very hot planet due to the presence of a lot of CO2 which, due to the greenhouse effect, heats the surface. We begin to study distant planets like WASP-39 b in the same way as those of the solar systemunderlines the professor.

The fact that CO2 exists gives us an idea of ​​the formation of the planet. The abundance of CO2 in its atmosphere is similar to that of Saturn, but not like Jupiter. »

A quote from Björn Benneke, University of Montreal and iREx

A mysterious signature

The teams that have analyzed the spectrum of the planet have also detected the presence there of something which they do not explain and which has never been detected elsewhere. There’s a mysterious molecule in its spectrum, something we’re still trying to figure outis surprised Professor Benneke.

Predictive models of planetary chemistry are currently being modified in an attempt to interpret this observation.

This is a big surprise for all researchers. There is probably something about the high temperature chemistry of certain elements that remains to be understood.supports the professor.

Analyzes carried out in the coming months should explain this mysterious chemical signature.

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