A collection of five research studies delve into the atmosphere of exoplanet WASP-39b as astronomers seek to better understand the chemistry of a world beyond the solar system.
WASP-39b is a gas giant located about 700 light-years from Earth in the constellation Virgo. It is similar in mass to Saturn, but without rings, and it orbits its Sun-like star in just four days, putting it closer in than Mercury is to the Sun. This makes it sizzling hot (around 900ºC), providing a glimpse of gas giant atmospheres under extreme conditions.
The proximity also makes the system a good target for observations by the recently launched James Webb Space Telescope (JWST) as the planet transits across the face of its star, as recently reported five papers posted on the astronomy preprint arXiv.
The studies are part of NASA’s Early Release Science Program. There was no question that once JWST finally launched, a whole world of astronomers would want in on the action. But the question was, how to allocate observation time both fairly, and in a way that maximized the potential for new scientific discoveries. The early release program acts to shift data from “space to face” in a streamlined and egalitarian fashion — making data publicly available right away to anyone who wants to take a look.
A group of exoplanet experts won a coveted spot on the early release list, and the five studies on WASP-39b are part of that effort. Astronomers had already observed WASP-39b with several ground- and space-based telescopes, and it was visible at the start of science operations, making it a perfect target for immediate observation.
The studies focus on WASP-39b’s atmosphere. JWST observed this planet at near-infrared wavelengths using three instruments — one camera, one spectrograph, and one instrument that’s a combination of the two.
The first study was released in August 2022, reporting carbon dioxide in the planet’s atmosphere. The Spitzer Space Telescope had previously hinted at the gas’s presence, but JWST’s Near Infrared Spectrograph saw a much clearer signature, verifying the detection for the first time on an exoplanet.
In other papers, the astronomers note sulfur dioxide in the atmosphere, which is evidence that radiation from the host star is interacting with the molecules in the atmosphere. This is the first time such photochemistry has been observed outside the solar system.
And a team led by graduate student Adina Feinstein (University of Chicago) used JWST’s Near Infrared Imager and Slitless Spectrograph to check out water vapor on WASP-39b. Though previously detected, JWST confirmed its presence, as well as evidence of clouds. But the team argued about the detections at first.
“There's this feature in the data, where the transit depth was a lot shallower than what our initial models had expected,” Feinstein says. “So there was a lot of back and forth between the data reducers, who needed to prove the robustness of the measurements, and the modelers.”
“At the end of the day,” she says, “I think we, the data reducers, proved that the actual transit depth was shallower than originally expected. And we think this is because of patchy cloud cover, very similar to what we see on Earth, which was really exciting.”
The data Feinstein and her colleagues gathered also enabled them to get a better handle on the world’s composition. They found signs of potassium and calculated the relative amounts of potassium to oxygen and carbon to oxygen in the planet’s atmosphere. These ratios can indicate to scientists where and how a world arose within its planetary disk. In the case of WASP-39b, it’s possible the world originally formed much farther away from its star and has since moved inwards.
Nikku Madhusudhan (Cambridge University, UK), an exoplanet scientist who was not involved in the WASP-39b studies, says that eventually all this data will be put together to tell a coherent story. “But this is still in very early stages,” he says. “With the exception of the photochemistry paper, which had a bit more theoretical analysis, the rest were primarily observational papers demonstrating the spectacular performance of different JWST instruments for exoplanet spectroscopy.”
WASP-39b is now the world that we know the most about outside of the planets in our solar system. And these observations are just the beginning: More studies on the exoplanet are upcoming, and astronomers have high hopes that this collaborative and open-source approach to JWST data will eventually help us make more complex detections, perhaps even signatures that might indicate life.
Rustamkulov, Z. et al. Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM
Alderson, L. et al. Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H
Feinstein, A. D. et al. Early Release Science of the exoplanet WASP-39b with JWST NIRISS