Exoplanet WASP-39 b Gets Detailed Look From JWST

By Michael Horton •  Updated: 11/24/22 •  7 min read

Another first has just been achieved by the James Webb Space Telescope (JWST): a thorough molecular and chemical analysis of the skies of a far-off planet. The telescope’s array of highly sensitive instruments was trained on the atmosphere of WASP-39 b, a “hot Saturn” — a planet roughly the size of Saturn orbiting a star 700 light-years away.

In contrast to earlier readings from JWST and other space telescopes like Hubble and Spitzer, which only showed a few parts of this hot planet’s atmosphere, the new data show a full range of atoms, molecules, and even signs of active chemistry and clouds.

“The clarity of the signals from a number of different molecules in the data is remarkable. We had predicted that we were going to see many of those signals, but still, when I first saw the data, I was in awe,”

said Mercedes López-Morales, an astronomer at the Center for Astrophysics, Harvard & Smithsonian.

Cloud Fragments Around WASP-39 b

wasp 39 b

WASP 39 b – artist’s illustration displays newly detected patches of clouds scattered across the planet.
Credit: Melissa Weiss/Center for Astrophysics | Harvard & Smithsonian

The most recent data also show how these clouds might look close up on exoplanets: they are broken up and don’t cover the planet uniformly.

The results indicate that the James Webb Space Telescope will be able to carry out the wide range of investigations on exoplanets — planets around other stars — that scientists had hoped for. The atmospheres of smaller, rocky planets like those in the TRAPPIST-1 system can be probed.

“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. Data like these are a game changer,”

said Natalie Batalha, who contributed to and helped coordinate the new research. Batalha is an astronomer at the University of California, Santa Cruz.

Hot Saturns And Earth

A collection of five recently submitted scientific papers that are available on the preprint website arXiv provide a detailed account of the discoveries.

One of the ground-breaking discoveries is the discovery of sulfur dioxide for the first time in an exoplanet’s atmosphere. This molecule is the result of chemical reactions started by high-energy light from the planet’s parent star. Similar processes are used on Earth to produce the protective ozone layer in the upper atmosphere.

“The surprising detection of sulfur dioxide finally confirms that photochemistry shapes the climate of ‘hot Saturns,'”

said Diana Powell, a NASA Hubble fellow, and astronomer at the Center for Astrophysics.  Powell is one of the core members of the team that made the sulfur dioxide discovery.

Since Earth’s climate is also influenced by photochemistry, our planet has more in common with “hot Saturns” than previously thought.

WASP-39 b Not Habitable

WASP-39 b is not thought to be habitable, with an estimated temperature of 1,600 degrees Fahrenheit and an atmosphere primarily composed of hydrogen.

The exoplanet has been compared to Saturn and Jupiter, with a mass comparable to Saturn but an overall size comparable to Jupiter. However, the new research points the way to discovering evidence of potential life on a habitable planet.

The planet’s nearness to its host star — eight times closer than Mercury is to our sun — further makes it an ideal laboratory for studying the effects of host star radiation on exoplanets. A better understanding of the star-planet connection should contribute to a greater understanding of how these processes result in the diversity of planets seen in the galaxy.

Sodium, potassium, and water vapour are some of the other atmospheric components that JWST has detected. These observations have been confirmed by ground- and space-based telescopes, and JWST has also discovered new water features at longer wavelengths.

No Methane Or Hydrogen Sulfide

wasp 39 b atmosphere data

The atmospheric composition of the hot gas giant exoplanet WASP-39 b.
This graphic shows four transmission spectra from three of Webb’s instruments operated in four instrument modes. All are plotted on a common scale extending from 0.5 to 5.5 microns.
A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves in front of the star, to the unfiltered starlight detected when the planet is beside the star.
Each of the data points (white circles) on these graphs represents the amount of a specific wavelength of light that is blocked by the planet and absorbed by its atmosphere. Wavelengths that are preferentially absorbed by the atmosphere appear as peaks in the transmission spectrum. The blue line is a best-fit model that takes into account the data, the known properties of WASP-39 b and its star (e.g., size, mass, temperature), and assumed characteristics of the atmosphere.
At upper left, data from NIRISS shows fingerprints of potassium (K), water (H2O), and carbon monoxide (CO).
At upper right, data from NIRCam shows a prominent water signature.
At lower left, data from NIRSpec indicates water, sulfur dioxide (SO2), carbon dioxide (CO2), and carbon monoxide (CO).
At lower right, additional NIRSpec data reveals all of these molecules as well as sodium (Na).
Credit: NASA, ESA, CSA, J. Olmsted (STScI)

Additionally, with twice as much data as reported from its prior observations, the James Webb Space Telescope observed carbon dioxide with higher resolution. Meanwhile, carbon monoxide was found, but the data lacked methane or hydrogen sulfide obvious signatures.

If these molecules are there, they are at very low levels. This is important information for scientists compliling lists of the chemistry of exoplanets to learn more about how they formed and changed over time.

Capturing such a broad spectrum of WASP-39 b’s atmosphere required an international team of hundreds of people to independently analyze data from four of JWST’s finely calibrated instrument modes. Then, they compared their findings in detail with each other, which led to even more scientifically nuanced results.

Infrared Chemical Fingerprints

JWST observes the universe in infrared light, which is at the red end of the light spectrum and beyond what human eyes can see; this allows the telescope to detect chemical fingerprints that are not visible in visible light.

Each of the three instruments is named after an infrared wavelength: NIRSpec, NIRCam, and NIRISS, along with MIRI.

JWST tracked WASP-39 b as it passed in front of its star, allowing some of the star’s light to filter through the planet’s atmosphere. Because different types of chemicals in the atmosphere absorb different colors of the starlight spectrum, the colors that are absent indicate which molecules are present.

The JWST instruments outperformed scientists’ expectations by precisely parsing an exoplanet’s atmosphere, ushering in a new era of exploration among the galaxy’s vast array of exoplanets.


  1. Adina D. Feinstein et al, Early Release Science of the exoplanet WASP-39b with JWST NIRISS, arXiv (2022) arXiv:2211.10493 [astro-ph.EP]
  2. Eva-Maria Ahrer et al, Early Release Science of the exoplanet WASP-39b with JWST NIRCam, arXiv (2022). arXiv:2211.10489 [astro-ph.EP]
  3. Lili Alderson et al, Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H, arXiv (2022). arXiv:2211.10488 [astro-ph.EP]
  4. Shang-Min Tsai et al, Direct Evidence of Photochemistry in an Exoplanet Atmosphere, arXiv (2022). arXiv:2211.10490 [astro-ph.EP]
  5. Z. Rustamkulov et al, Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM, arXiv (2022). arXiv:2211.10487 [astro-ph.EP]

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