An exoplanet atmosphere as never seen before

Known for beaming stunning images back to Earth, the James Webb Space Telescope (JWST) just scored another first: a molecular and chemical portrait of a distant world's skies.

A collaboration between NASA, the European Space Agency and the Canadian Space Agency, the telescope has only been operational for a few months, yet has already revolutionised the world of astronomy.

One of its recent targets was a "hot Saturn" - a planet about as massive as Saturn orbiting a star some 700 light-years away - known as WASP-39 b, for study by the Early Release Science (ERS) Transiting Exoplanets team.

This same team, which includes a group of Canadian astronomers at Université de Montréal led by Björn Benneke, previously revealed in August 2022 that WASP-39 b's broiling atmosphere undeniably contained carbon dioxide.

Many molecules and even clouds detected

New results of the team's observations - posted online yesterday on arXiv - provide a full menu of atoms, molecules, and even signs of active chemistry and clouds associated with WASP-39 b.

The findings bode well for the capability of Webb's instruments to conduct the broad range of investigations of exoplanets - planets around other stars - hoped for by the scientific community. That includes probing the atmospheres of smaller, rocky planets like those in the TRAPPIST-1 system.

The suite of discoveries is detailed in a set of five new scientific papers. The data, which have been described by the discovery team as "a game changer", were collected by three of Webb's instruments including the Canadian Near-Infrared Imager and Slitless Spectrograph, or NIRISS.

Among the revelations is the first detection in an exoplanet atmosphere of sulfur dioxide, a molecule produced from chemical reactions triggered by energetic ultraviolet light from the planet's parent star. This type of process, called photochemistry, has never been observed outside of the Solar System until now. On Earth, the protective ozone layer in the upper atmosphere is created in a similar way.

Not a habitable world

At an estimated temperature of 900 degrees Celsius and with an atmosphere made of mostly hydrogen, WASP-39 b is not believed to be habitable. But this new work establishes the groundwork to finding evidence of potential life on other planets that may be habitable.

WASP-39 b's proximity to its host star - eight times closer than Mercury is to our Sun - also makes it a great laboratory for studying the effects of radiation from host stars on exoplanets. Better knowledge of the relationship between a planet and its star should bring a deeper understanding of how these processes create the diversity of planets observed in the Galaxy.

Other atmospheric elements detected by the Webb Telescope include sodium, potassium, and water vapour, confirming previous space and ground-based telescope observations. Carbon monoxide was also detected, a new addition to WASP-39 b's roster.

A true team effort

Capturing such a broad spectrum of WASP-39 b's atmosphere was a scientific tour de force, as an international team numbering in the hundreds independently analysed data from the Webb Telescope's finely calibrated instruments. They then made detailed comparisons of their findings within the team, yielding yet more scientifically nuanced and rigorous results.

"Even from a sociological perspective, this was a great experiment," said Benneke, an UdeM physics professor and scientist at the Trottier Institute for Research on Exoplanets. "As world-leading experts in this field who are often in competition with each other, we really banded together and collaborated as one team to design the best program to provide a great data set for the community to work with. It was a great success and brought all of us closer together."

A transiting exoplanet

To see light from WASP-39 b, Webb tracked the planet as it passed in front of its star, allowing some of the star's light to filter through the planet's atmosphere. This starlight became imprinted with signatures hinting at the different chemicals found in the exoplanet's atmosphere. By studying this light through a technique called transit spectroscopy, astronomers determined the composition of the planet's atmosphere from hundreds of light-years away.

The signatures of the detected chemicals in an exoplanet atmosphere can be studied individually but can also be compared to one another. By comparing the abundance of different molecules, astronomers gain insight into how a planet may have formed. In the case of WASP-39 b, its chemical inventory suggests that it was created from the collision and mergers of several smaller bodies called planetesimals.

Upcoming analyses

In addition to Benneke, several UdeM graduate students and a postdoctoral researcher - Louis-Philippe Coulombe, Caroline Piaulet, Michael Radica, Pierre-Alexis Roy and Jake Taylor - were involved in this series of results and in the months to come will continue to analyse Webb data for several other exoplanet candidates.

"The first several weeks after our observing program's data release was probably one of the most rewarding periods of my life," said Michael Radica, an UdeM doctoral student and the second author on the Canadian instrument NIRISS paper. "Seeing the first transmission spectra from NIRISS was the moment when it really hit me that all our work in the months and years leading up to now was worth it."

The UdeM team and its international collaborators are preparing several more scientific papers whose results will be published soon.

"We can now start pushing JWST and NIRISS's capabilities even further," said Radica, "and the lessons learned from the Early Release Science observations are already proving invaluable with analyses of smaller Neptune- and even Earth-sized planets currently in the works."

• A transmission spectrum of the hot Saturn WASP-39 b captured by the Canadian NIRISS instrument on the Webb Telescope reveals the signatures of many different chemicals in its atmosphere, including potassium, water vapor, and carbon monoxide.

  Credit: NASA/ESA/CSA/J. Olmsted/STScI

About these findings

A series of five scientific papers on the exoplanet WASP-39 b by the JWST Transiting Exoplanet Community Early Release Science Team, was published online on arXiv on November 21, 2022.