Institute of Astronomy

New from Webb Telescope: An Exoplanet Atmosphere as Never Seen Before

Published on 22/11/2022 

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

The telescope’s array of highly sensitive instruments was trained on the atmosphere of a “hot Saturn” – a planet about as massive as Saturn orbiting a star some 700 light-years away – known as WASP-39 b. While Webb and other space telescopes, including Hubble and Spitzer, previously have revealed isolated ingredients of this broiling planet’s atmosphere, the new readings provide a full menu of atoms, molecules, and even signs of active chemistry and clouds.

The latest data also give a hint of how these clouds might look up close: broken up rather than a single, uniform blanket over the planet.

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 science community. That includes probing the atmospheres of smaller, rocky planets like those in the TRAPPIST-1 system.

“We observed the exoplanet with multiple instruments that, together, provide a broad swath of the infrared spectrum and a panoply of chemical fingerprints inaccessible until JWST,” said Natalie Batalha, an astronomer at the University of California, Santa Cruz, who contributed to and helped coordinate the new research. “Data like these are a game changer.”

The suite of discoveries is detailed in a set of five new scientific papers, to be published in the journal Nature. Among the unprecedented revelations is the first detection in an exoplanet atmosphere of sulfur dioxide, a molecule produced from chemical reactions triggered 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.

Cambridge astronomers Sean Jordan and Oliver Shorttle made up one of four teams worldwide who performed photochemical model calculations of the atmosphere of WASP-39b. All four models found that SO2 was being produced via photochemistry in the atmosphere and could reproduce the transmission spectrum that was observed with JWST.

“This is the first time we see concrete evidence of photochemistry – chemical reactions initialized by energetic stellar light – on exoplanets,” said Shang-Min Tsai, a researcher at the University of Oxford in the United Kingdom and lead author of the paper explaining the origin of sulfur dioxide in WASP-39 b’s atmosphere. “I see this as a really promising outlook for advancing our understanding of exoplanet atmospheres with JWST.” 

This led to another first: scientists applying computer models of photochemistry to data that requires such higher level physics to be fully explained. The resulting improvements in modeling will help build the technological know-how to interpret potential signs of life in the future.

“Planets are sculpted and transformed by orbiting within the radiation bath of the host star,” Batalha said. “On Earth, those transformations allow life to thrive.”

At an estimated temperature of 1,600 degrees Fahrenheit (900 degrees Celsius) and an atmosphere made mostly of hydrogen, WASP-39 b is not believed to be habitable. But the new work points the way to finding evidence of potential life on a habitable planet. 

“We have found the first evidence for an exoplanet's atmosphere being modified by starlight, a key step in our ongoing characterisation of other worlds that will one day yield insight into their geology and potential life.", said IoA astronomer Dr Oliver Shorttle.

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 bring a deeper understanding of how these processes create the diversity of planets observed in the galaxy.

Other atmospheric constituents detected by the Webb telescope include sodium, potassium, and water vapor, confirming previous space and ground-based telescope observations as well as finding additional water features, at longer wavelengths, that haven’t been seen before.

Webb also saw carbon dioxide at higher resolution, providing twice as much data as reported from its previous observations. Meanwhile, carbon monoxide was detected, but obvious signatures of both methane and hydrogen sulfide were absent from the Webb data. If present, these molecules occur at very low levels, a significant finding for scientists making inventories of exoplanet chemistry in order to better understand the formation and development of these distant worlds.

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 analyzed data from four of the Webb telescope’s finely calibrated instrument modes. They then made detailed inter-comparisons of their findings, yielding yet more scientifically nuanced results.

Webb views the universe in infrared light, on the red end of the light spectrum beyond what human eyes can see; that allows the telescope to pick up chemical fingerprints that can’t be detected in visible light.

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

“You can really accurately constrain the properties of these planets by having such a wide spectrum,” said Adina Feinstein, a University of Chicago graduate student and first author of the paper focusing on spectrum observations using NIRISS. “So you start to get the full picture [of the atmospheres] that you couldn’t get before.”

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. Different types of chemicals in the atmosphere absorb different colors of the starlight spectrum, so the colors that are missing tell astronomers which molecules are present. 

Scientists among the first to see the spectrum data said it was nothing short of breathtaking.

“I am not going to lie: When I first saw the data, I cried,” said Hannah Wakeford, an astrophysicist at the University of Bristol in the United Kingdom who investigates exoplanet atmospheres. “We had predicted what JWST would show us, but it was more precise, more diverse and more beautiful than I think I actually believed it would be.”

Having such a complete roster of chemical ingredients in an exoplanet atmosphere also gives scientists a glimpse of the abundance of different elements in relation to each other, such as carbon-to-oxygen or potassium-to-oxygen ratios. That, in turn, provides insight into how this planet – and perhaps others – formed out of the disk of gas and dust surrounding the parent star in its younger years.

WASP-39 b’s chemical inventory suggests a history of smashups and mergers of smaller bodies called planetesimals to create an eventual goliath of a planet.

“The abundance of sulfur [relative to] hydrogen indicated that the planet presumably experienced significant accretion of planetesimals that can deliver [these ingredients] to the atmosphere,” said Kazumasa Ohno, a UC Santa Cruz exoplanet researcher who worked on Webb data. “The data also indicates that the oxygen is a lot more abundant than the carbon in the atmosphere. This potentially indicates that WASP-39 b originally formed far away from the central star.”

By so precisely parsing an exoplanet atmosphere, the Webb telescope’s instruments performed well beyond scientists’ expectations – and promise a new phase of exploration among the broad variety of exoplanets in the galaxy.

“We are going to be able to see the big picture of exoplanet atmospheres,” said Laura Flagg, a researcher at Cornell University and a member of the international team. “It is incredibly exciting to know that everything is going to be rewritten. That is one of the best parts of being a scientist.”


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Page last updated: 22 November 2022 at 20:54