Now in 3D, maps begin to bring exoplanets into focus

Astronomers have generated the first three-dimensional map of a planet orbiting another star, revealing an atmosphere with distinct temperature zones—one so scorching that it breaks down water vapor, a team co-led by a Cornell expert reports in new research.

The temperature map of WASP-18b—a gas giant known as an “ultra-hot Jupiter,” located 400 light years from Earth—is the first applying a technique called 3D eclipse mapping, or spectroscopic eclipse mapping.

The effort builds on a 2D model that members of the same team published in 2023, which demonstrated eclipse mapping’s potential to leverage highly sensitive observations by NASA’s James Webb Space Telescope (JWST).

The researchers say that for many similar types of exoplanets observable by JWST, they can now begin mapping atmospheric variations just as, for example, Earth-based telescopes long ago observed Jupiter’s Great Red Spot and banded cloud structure.

“Eclipse mapping allows us to image exoplanets that we can’t see directly, because their host stars are too bright,” said Ryan Challener, a postdoctoral associate in the 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.”

Challener is the first author of “Horizontal and Vertical Exoplanet Thermal Structure from a JWST Spectroscopic Eclipse Map,” published in Nature Astronomy.

More than 30 co-authors include Megan Wiener Mansfield, assistant professor of astronomy at the University of Maryland, who co-led the project, and Jake Turner, a research associate in the Cornell Center for Astrophysics and Planetary Science.

Detecting exoplanets at all is difficult—they typically emit much less than 1% of a host star’s brightness. Eclipse mapping requires measuring small fractions of that total as a planet circles behind its star, obscuring and revealing parts of it along the way. Scientists can link minute changes in light to specific regions to produce a brightness map that, when done in multiple colors, can be converted to temperatures in three dimensions: latitude, longitude and altitude.

“You’re looking for changes in tiny portions of the planet as they disappear and reappear into view,” Challener said, “so it’s extraordinarily challenging.”

WASP-18b, which has roughly the mass of 10 Jupiters, orbits in just 23 hours and has temperatures approaching 5,000 degrees Fahrenheit—provided a relatively strong signal, making it a good test case for the new mapping technique.

While the earlier 2D map utilized a single light wavelength, or color, the 3D map re-analyzed the same observations from JWST’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) instrument at many wavelengths.

Challener said each color corresponded to different temperatures and altitudes within WASP-18b’s gaseous atmosphere that could be pieced together to create the 3D map.

“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 said. “If you put those together, you can get a 3D map of the temperatures in this atmosphere.”

The new view confirmed spectroscopically distinct regions—differing in temperature and possibly in chemical composition—in WASP-18b’s visible “dayside,” the side always facing the star due to its tidally locked orbit.

The planet features a circular “hotspot” where the most direct starlight lands, and where winds apparently aren’t strong enough to redistribute the heat.

Surrounding the hotspot is a colder “ring” nearer the planet’s outer visible edges, or limbs. Notably, Challener said, measurements showed lower levels of water vapor in the hotspot than WASP-18b’s average.

“We think that’s evidence that the planet is so hot in this region that it’s starting to break down the water,” Challener said. “That had been predicted by theory, but it’s really exciting to actually see this with real observations.”

Challener said additional JWST observations could help improve the first 3D eclipse map’s spatial resolution. Already the technique can help illuminate the temperature maps of other hot Jupiters, which make up hundreds of the more than 6,000 exoplanets confirmed to date.

“This new technique is going to be applicable to many, many other planets that we can observe with the James Webb Space Telescope,” Challener said.

“We can start to understand exoplanets in 3D as a population, which is very exciting.”