The James Webb Space Telescope reveals an exoplanet’s atmosphere like never before

The James Webb Space Telescope (JWST) has just scored another first: a detailed molecular and chemical image of the world’s distant sky.

The telescope’s highly sensitive instrument cluster trained on the atmosphere of “hot Saturn” – a Saturn-mass planet orbiting a star about 700 light-years away – known as WASP-39 b. While JWST and other space telescopes, including Hubble and Spitzer, had previously detected isolated components of this planet’s atmosphere, the new readout provides a full catalog of atoms, molecules, and even signs of active chemistry and clouds.

“The clarity of the signals from a number of different particles in the data is remarkable,” says Mercedes Lopez-Morales, an astrophysicist at the Center for Astrophysics. Harvard and Smithsonian and one of the scientists who contributed to the new findings.

“We expected we would see many of these signals, but when I first saw the data, I was in awe,” adds Lopez-Morales.

The latest data also gives a hint of how these exoplanet clouds might appear up close: loose rather than a single, uniform shroud over the planet.

The results bode well for JWST’s ability to perform the wide range of investigations on exoplanets — planets around other stars — that scientists have been hoping for. This includes examining the atmospheres of smaller, rocky planets such as those in the TRAPPIST-1 system.

said Natalie Batalha, an astronomer at the University of California, Santa Cruz, who co-authored and helped coordinate the new research. “Data like this is a game changer.”

The collection of discoveries is detailed in a set of five newly submitted scientific papers, available on preprint site arXiv. Among the unprecedented discoveries is the first detection in the atmosphere of an exoplanet of sulfur dioxide, a molecule resulting from chemical reactions generated by high-energy light from the planet’s parent star. On Earth, the protective ozone layer in the upper atmosphere is created in a similar way.

“The surprising discovery of sulfur dioxide finally confirms that photochemistry is shaping a ‘hot Saturn’ climate,” says Diana Powell, a NASA Hubble Fellow, an astronomer at the Astrophysical Center and a core member of the team that made the discovery of sulfur dioxide. Earth is also through photochemistry, so our planet has more in common with ‘hot Saturn’ than we previously knew.”

Gia Adams, a Harvard graduate student and researcher at the Center for Astrophysics, analyzed data that confirmed the sulfur dioxide signal.

“As an early career researcher in the field of exoplanet atmospheres, it’s very exciting to be a part of a discovery like this,” Adams says. “The process of analyzing this data seemed magical. We saw hints of this feature in early data, but this high-resolution tool revealed the signature of SO.”₂ clearly and helped us solve the mystery.”

At an estimated temperature of 1,600 degrees Fahrenheit and an atmosphere made mostly of hydrogen, WASP-39 b is not believed to be habitable. The exoplanet has been compared to both Saturn and Jupiter, with a similar mass to Saturn, but with an overall size the size of Jupiter. But the new work points the way to finding evidence of possible life on a habitable planet.

The planet’s proximity to its host star – eight times closer than Mercury is to our sun – also makes it a laboratory for studying the effects of radiation from host stars on exoplanets. Better knowledge of the star-planet connection should lead to a deeper understanding of how these processes create the diversity of planets observed in the galaxy.

Other components of the atmosphere detected by JWST include sodium, potassium and water vapor, confirming previous space and ground-based telescope observations as well as finding additional water features, at longer wavelengths, not seen before.

JWST also saw CO at a higher resolution, providing twice as much data as was reported from its previous observations. Meanwhile, carbon monoxide was detected, but there were no clear signals of both methane and hydrogen sulfide in the data. If they exist, these molecules occur at very low levels, an important finding for scientists inventorying the chemistry of exoplanets in order to better understand the formation and evolution of these distant worlds.

Capturing such a broad spectrum of WASP-39 b’s atmosphere was a powerful science tour de force, as an international team digitized hundreds of independently analyzed data from four modes of JWST’s precisely calibrated instruments. Then they made detailed comparisons of their findings, yielding more scientifically accurate results.

JWST sees the universe in infrared light, on the red end of the light spectrum beyond what the human eye can see; It allows the telescope to pick up chemical signatures that cannot be detected in visible light.

Each of the three instruments even has a version of infrared “IR” in its name: NIRSpec, NIRCam, and NIRISS.

To see the light from WASP-39 b, JWST 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. Different types of chemicals in the atmosphere absorb different colors of the starlight spectrum, so the missing colors tell astronomers which molecules are present.

By accurately analyzing an exoplanet’s atmosphere, the JWST instruments have performed beyond scientists’ expectations — and promise a new phase of exploration among the galaxy’s wide range of exoplanets.

“I’m looking forward to seeing what we find in the atmospheres of small terrestrial planets,” says López-Morales.