JWST captures stunning 3D view of a planet’s scorching atmosphere

Led by researchers at the University of Maryland and Cornell University, the study charts temperatures across WASP-18b, a massive gas giant classified as an "ultra-hot Jupiter" located 400 light-years from Earth. The team applied a method known as 3D eclipse mapping, also called spectroscopic eclipse mapping, marking the first time this technique has been used to build a full 3D temperature map. The work expands on a 2D eclipse map the group released in 2023 using highly sensitive observations from NASA's James Webb Space Telescope (JWST).

"This technique is really the only one that can probe all three dimensions at once: latitude, longitude and altitude," said the paper's co-lead author Megan Weiner Mansfield, an assistant professor of astronomy at UMD. "This gives us a higher level of detail than we've ever had to study these celestial bodies."

With this approach, scientists can begin charting atmospheric differences across many exoplanets observable by JWST, much as ground-based telescopes once documented Jupiter's Great Red Spot and its banded clouds.

"Eclipse mapping allows us to image exoplanets that we can't see directly, because their host stars are too bright," said the paper's co-lead author Ryan Challener, a postdoctoral associate in Cornell University's Department of Astronomy. "With this telescope and this new technique, we can start to understand exoplanets along the same lines as our solar system neighbors."

Spotting exoplanets is difficult because they are typically far dimmer than their stars, often contributing less than 1% of the total light. Eclipse mapping measures tiny variations in that light as the planet moves behind its star, alternately hiding and revealing different regions. By tying these small brightness changes to specific locations on the planet and analyzing them in multiple colors, scientists can reconstruct temperatures across latitude, longitude, and altitude.

WASP-18b is well suited for this test because it has about the mass of 10 Jupiters, completes an orbit in only 23 hours, and reaches temperatures near 5,000 degrees Fahrenheit. Those properties provide a comparatively strong signal for the new mapping method.

The team's earlier 2D map used a single color of light. For the 3D version, they reanalyzed the same JWST data from the Near-Infrared Imager and Slitless Spectrograph (NIRISS) across many wavelengths. Each color probes different layers of WASP-18b's atmosphere and corresponds to a specific temperature and altitude. Combining these layers yields a detailed three-dimensional temperature structure.

"If you build a map at a wavelength that water absorbs, you'll see the water deck in the atmosphere, whereas a wavelength that water does not absorb will probe deeper," Challener explained. "If you put those together, you can get a 3D map of the temperatures in this atmosphere."

The 3D analysis identifies spectroscopically distinct zones on the planet's permanent dayside, which always faces the star because the planet is tidally locked. A circular hot spot sits where the star's light strikes most directly, and winds appear too weak to spread that heat efficiently. A cooler ring encircles the hot center near the limb of the planet. Measurements also show reduced water vapor within the hot spot compared with the planet's average.

"We've seen this happen on a population level, where you can see a cooler planet that has water and then a hotter planet that doesn't have water," Weiner Mansfield explained. "But this is the first time we've seen this be broken across one planet instead. It's one atmosphere, but we see cooler regions that have water and hotter regions where the water's being broken apart. That had been predicted by theory, but it's really exciting to actually see this with real observations."

Additional JWST observations could sharpen the spatial detail in future 3D eclipse maps. Weiner Mansfield noted that the method opens new opportunities to study many "hot Jupiters," which number in the hundreds among the more than 6,000 confirmed exoplanets. She also aims to apply 3D eclipse mapping to smaller, rocky worlds beyond gas giants like WASP-18b.

"It's very exciting to finally have the tools to see and map out the temperatures of a different planet in this much detail. It's set us up to possibly use the technique on other types of exoplanets. For example, if a planet doesn't have an atmosphere, we can still use the technique to map the temperature of the surface itself to possibly understand its composition," Mansfield said. "Although WASP-18b was more predictable, I believe we will have the chance to see things that we could never have expected before."

This research was supported by the James Webb Space Telescope's Transiting Exoplanet Community Early Release Science Program.