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Webb reveals chemical profile of atmosphere of exoplanet WASP-39 b

An international team of astronomers has revealed the first 'chemical profile' of an exoplanet's atmosphere. The team, including Leiden astronomer Yamila Miguel and provenda Amy Louca, made the profile using so-called Early Release Science data from the James Webb Space Telescope. The results have been submitted for publication in five papers and are available on the preprint server.

New observations of WASP-39b with the JWST have provided a clearer picture of the exoplanet, showing the presence of sodium, potassium, water, carbon dioxide, carbon monoxide and sulfur dioxide in the planet's atmosphere.

It involves the atmosphere of 'hot Saturn' WASP-39 b, an exoplanet about the same weight as Saturn. It orbits around a star other than our sun about 700 light years from Earth. Webb and other space telescopes, including Hubble and Spitzer, have previously revealed isolated ingredients of this hot planet's atmosphere.

The new measurements with the highly sensitive instruments on Webb provide a full inventory of atoms, molecules and even signs of active chemistry and clouds. In addition, the data also gives an idea of what these clouds look like up close. Namely, they are fractured rather than a single, uniform blanket over the planet.

Changing the research field of exoplanets

'We observed the exoplanet with multiple instruments,' research coordinator Natalie Batalha of the University of California says. 'Together, these provide a wide part of the infrared spectrum and a large number of chemical fingerprints. These were inaccessible until Webb's arrival.'

'This is going to completely change the field of exoplanet research,' UvA astronomer Jean-Michel Desert adds. 'We can now observe the spectral fingerprints of the chemical composition of the atmosphere of exoplanets in an unambiguous and comprehensive way as never before.'

Sulphur dioxide in the atmosphere

One of the extraordinary finds is the first observation of sulphur dioxide in the atmosphere of an exoplanet. Sulphur dioxide is produced by chemical reactions triggered by high-energy light from the planet's parent star. On Earth, the protective ozone layer in the upper atmosphere is formed in a similar way.

Leiden astronomer Yamila Miguel made models of the atmosphere of WASP-39 b. Those models included the photochemistry used to explain the origin of sulphur dioxide, namely the chemical reactions triggered by energetic starlight. She sees JWST as a turning point in her field: 'This is the first time we see actual evidence of photochemistry on exoplanets.'

This led to another first: scientists applied computer models of photochemistry to data that requires a much  higher level of physics to be fully explained. These improved models will help build the technological knowledge to interpret possible signs of life in the future.

No life on the exoplanet

WASP-39 b has an estimated temperature of 900 degrees Celsius and an atmosphere composed mainly of hydrogen. The exoplanet will almost certainly not be able to sustainlife. But the new results could guide evidence for potential life on other exoplanets, such as those of the smaller, rocky planets like those in the TRAPPIST-1 system.

The proximity of the planet to its star - the planet is eight times closer to its star than Mercury is to our sun - makes it a great laboratory for studying the effects of radiation from stars on exoplanets. A better knowledge of the star-planet relationship should lead to a better understanding of the diversity of planets in our Galaxy.

No methane or hydrogen sulphide

Other constituents in the atmosphere discovered by the Webb telescope include sodium, potassium and water vapour, confirming previous observations from telescopes in space and on the ground. It also found additional signals of water at longer wavelengths that were not previously observed.

Webb also observed carbon dioxide at a higher resolution, providing twice as much data as previous observations. While carbon monoxide was also observed, the Webb data lacks clear signs of both methane and hydrogen sulphide.

Webb’s approach

Capturing such a broad spectrum of WASP-39 b's atmosphere involved a scientific powerhouse. An international team of hundreds of people independently analysed data from the instruments on the Webb telescope. Webb views the universe in infrared light, beyond the region in the electromagnetic spectrum that the human eye can perceive. This allows the telescope to pick up chemical fingerprints that cannot be detected in visible light. The three instruments used are NIRSpec, NIRCam and NIRISS.

In order to see WASP-39 b's light, Webb followed the planet as it passed in front of its star, filtering some of the star's light through the planet's atmosphere. Different types of molecules in the atmosphere absorb different colours of the starlight spectrum. The colours that are missing therefore tell astronomers which molecules are present.

Chemical inventory of WASP-39 b

With the complete inventory of chemical ingredients now discovered, scientists also get a glimpse into the abundance of different elements in relation to each other, such as the carbon-oxygen or potassium-oxygen ratio. This, on the other hand, provides insight into how this planet formed in its early years from the disc of gas and dust around its parent star. The chemical inventory of WASP-39 b indicates a history of collisions and clumping of smaller chunks, called planetesimals, that eventually created a giant planet.

The abundance of sulphur compared to hydrogen indicates that the planet presumably experienced a significant accretion of planetesimals that were able to supply these ingredients to the atmosphere. Oxygen is also much more abundant in the atmosphere than carbon. This possibly indicates that WASP-39 b originally formed far from its central star.

Fitting small puzzle pieces together

Other co-authors working in the Netherlands are Hinna Shivkumar and Saugata Barat from the University of Amsterdam and Amy Louca from the Leiden Observatory. Amy Louca: 'As a PhD student, it was great to work with this data and reproduce signatures of carbon dioxide and sulphur dioxide, for example, in the models. It showed us that there are so many different aspects to exoplanets. We are now finally able to fit the smaller puzzle pieces together.'

This article appeared as a press release on the website of The Netherlands Research School for Astronomy (NOVA).

Credits artist impression: Melissa Weiss/Center for Astrophysics | Harvard & Smithsonian
Credits images: NASA, ESA, CSA, Joseph Olmsted (STScI) 


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