New map of the city OSA-18bThe hot Jupiter exoplanet, located about 325 light-years from Earth, exhibits an atmosphere with distinct temperature zones—so scorching that it destroys water vapor.
Hot Jupiter WASP-18b. Image credit: NASA Goddard Space Flight Center.
The WASP-18b map is the first application of a technique called 3D eclipse mapping or spectroscopic eclipse mapping.
This study is based on 2D model that members of the same team published in 2023, demonstrating the potential of eclipse mapping for use in high-sensitivity observations of the NASA/ESA/CSA James Webb Space Telescope.
“This technique is truly 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 we have ever seen in studying these celestial bodies.”
Using this technique, astronomers can now begin to map atmospheric changes for many of the same types of exoplanets observed by Webb, just as ground-based telescopes have long 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 Dr. Ryan Challener, an astronomer at Cornell University and the University of Maryland.
“With this telescope and this new technology, we can begin to understand exoplanets as well as our neighbors in the solar system.”
Detecting exoplanets is generally difficult – they usually emit much less than 1% of the brightness of their parent star.
Mapping eclipses requires measuring small fractions of this total as a planet orbits behind its star, hiding and revealing parts of it along the way.
Scientists can link minute changes in light to specific regions to create a map of brightness that, if done in multiple colors, can be converted into temperature in three dimensions: latitude, longitude and altitude.
“You're looking for changes in tiny parts of the planet as they disappear and come back into view, so it's extremely difficult,” Dr Challener said.
WASP-18b, which has a mass of about 10 Jupiters, orbits in just 23 hours and has a temperature approaching 2,760 degrees Celsius (5,000 degrees Fahrenheit), which provided a relatively strong signal, making it a good test for the new mapping technique.
While the earlier 2D map used a single wavelength of light or color, the 3D map reanalyzed the same observations obtained with the Webb Near-Infrared Imaging Instrument and the No-Slit Spectrograph (NRISS) at many wavelengths.
“Each color corresponded to different temperatures and altitudes in WASP-18b's gas atmosphere, which could be combined to create a 3D map,” Dr Challener said.
“If you map the wavelength that water absorbs, you'll see the water deck in the atmosphere, while the wavelength that water doesn't absorb will be probed deeper.”
“If you put them together, you can get a three-dimensional map of the temperatures in that atmosphere.”
The new image confirmed spectroscopically distinct regions—varying in temperature and possibly chemical composition—on the visible dayside side of WASP-18b, the side that always faces the star due to its tidally dependent orbit.
The planet has a circular “hot spot” where starlight receives the most direct light and where winds do not appear to be strong enough to redistribute heat.
The hot spot is surrounded by a cooler “ring” located closer to the outer visible edges or limbs of the planet.
Notably, the measurements showed lower water vapor levels at the hot spot than the WASP-18b average.
“We think this is evidence that the planet is so hot in this region that water is starting to split,” Dr Challener said.
“This was predicted by theory, but it's really interesting to see it in real-life observations.”
“Additional Webb observations may help improve the spatial resolution of the first 3D map of the eclipse.”
“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 really exciting to finally have the tools to see and map the temperature of another planet in such detail,” Dr Mansfield said.
“This gives us the opportunity to use this technique on other types of exoplanets. For example, if a planet has no atmosphere, we can still use this technique to map the temperature of the surface itself to possibly understand its composition.”
“Although WASP-18b was more predictable, I believe we will have a chance to see something we never expected before.”
The WASP-18b map was published in the journal paper in the magazine Nature Astronomy.
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RC Challenger etc.. Horizontal and vertical thermal structure of the exoplanet in the JWST spectroscopic eclipse map. Nat Astronpublished online October 28, 2025; doi: 10.1038/s41550-025-02666-9
