The new map of WASP-18b, a hot Jupiter exoplanet about 325 light-years from Earth, reveals an atmosphere with distinct temperature zones – so scorching that they break down water vapor.
The hot Jupiter WASP-18b. Image credit: NASA Goddard Space Flight Center.
WASP-18b’s map is the first to apply a technique called 3D eclipse mapping, or spectroscopic eclipse mapping.
This study builds on a 2D model published in 2023 by members of the same team, which demonstrated the potential of eclipse mapping to exploit highly sensitive observations from the NASA/ESA/CSA James Webb Space Telescope.
“This technique is really the only one that can probe all three dimensions at once: latitude, longitude and altitude,” said Dr. Megan Weiner Mansfield, an astronomer at the University of Maryland and Arizona State University.
“This gives us a higher level of detail than ever before to study these celestial bodies.”
Using this technique, the astronomer can now begin to map the atmospheric variations of many similar types of exoplanets observable by Webb, just as terrestrial telescopes long ago observed Jupiter’s Great Red Spot and the structure of banded clouds.
“Eclipse mapping allows us to image exoplanets that we can’t see directly because their host stars are too bright,” said Dr. Ryan Challener, an astronomer at Cornell University and the University of Maryland.
“With this telescope and this new technique, we can begin to understand exoplanets in the same way as our neighbors in the solar system.”
Detecting exoplanets is difficult: they typically emit well less than 1% of the brightness of a host star.
Eclipse mapping requires measuring small fractions of this total as a planet orbits behind its star, obscuring and revealing parts of it along the way.
Scientists can relate tiny 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 parts of the planet as they disappear and reappear, so it’s extremely difficult,” Dr. Challener said.
WASP-18b – which has roughly the mass of 10 Jupiters, orbits in just 23 hours and has temperatures approaching 2,760 degrees Celsius (5,000 degrees Fahrenheit) – provided a relatively strong signal, making it a good test for the new mapping technique.
While the previous 2D map used a single wavelength of light, or color, the 3D map reanalyzed the same observations from Webb’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) instrument in many wavelengths.
“Each color corresponded to different temperatures and altitudes within WASP-18b’s gaseous atmosphere that could be pieced together to create the 3D map,” Dr Challener said.
“If you build a map at a wavelength that water absorbs, you will see water in the atmosphere, whereas a wavelength that water does not absorb will probe deeper.”
“If you put them together, you can get a 3D map of temperatures in that atmosphere.”
The new view confirmed spectroscopically distinct regions – differing in temperature and possibly chemical composition – on the visible dayside of WASP-18b, the side always facing the star due to its tidally locked orbit.
The planet has a circular “hot spot” where the most direct starlight lands and where the winds are apparently not strong enough to redistribute the heat.
Around the hot spot is a cooler “ring,” closer to the planet’s visible outer edges, or limbs.
Notably, measurements showed water vapor levels in the hotspot below the WASP-18b average.
“We think this proves that the planet is so hot in this region that it’s starting to break down water,” Dr. Challener said.
“This had been predicted by theory, but it’s really exciting to see this with real observations.”
“Additional observations from Webb could help improve the spatial resolution of the first 3D eclipse map.”
“This technique can already help illuminate temperature maps of other hot Jupiters, which make up hundreds of the more than 6,000 exoplanets confirmed to date.”
“It’s very exciting to finally have the tools to see and map the temperatures of a different planet in such detail,” Dr Mansfield said.
“This allows us to potentially use the technique on other types of exoplanets. For example, if a planet has no atmosphere, we can still use the technique to map the temperature of the surface itself to potentially understand its composition.”
“Even if WASP-18b was more predictable, I think we will have the chance to see things we could never have expected before.”
The map of WASP-18b was published in an article in the journal Natural astronomy.
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RC Challenger and others. Horizontal and vertical thermal structure of an exoplanet from a JWST spectroscopic eclipse map. Nat Astronpublished online October 28, 2025; doi: 10.1038/s41550-025-02666-9
