A newly released map of WASP-18b, a hot Jupiter exoplanet located approximately 325 light-years from Earth, showcases an atmosphere characterized by distinct temperature zones. Within this region, the scorching temperatures are capable of decomposing water vapor.
Hot Jupiter WASP-18b. Image credit: NASA’s Goddard Space Flight Center.
The WASP-18b map represents the first implementation of a method known as 3D eclipse mapping, or spectroscopic eclipse mapping.
This study features a 2D model. The paper, published in 2023 by members of the same research team, illustrated how eclipse mapping can leverage the sensitive observations from the NASA/ESA/CSA James Webb Space Telescope.
“This technique is unique in that it can simultaneously survey all three dimensions: latitude, longitude, and altitude,” stated Dr. Megan Weiner Mansfield, an astronomer at the University of Maryland and Arizona State University.
“This enables a greater level of detail than previously possible for studying these celestial objects.”
With this technology, astronomers can now begin to chart the atmospheric variations of many similar exoplanets observable through Webb, resembling how Earth-based telescopes once scrutinized Jupiter’s Great Red Spot and its striped cloud formations.
“Eclipse mapping allows us to capture images of exoplanets whose host stars are too bright for direct observation,” remarked Dr. Ryan Challenor, an astronomer at Cornell University and the University of Maryland.
“Thanks to this telescope and groundbreaking technology, we can start to understand exoplanets similarly to the neighboring worlds in our solar system.”
Detecting exoplanets is quite challenging as they typically emit less than 1% of the brightness of their host star.
Mapping a solar eclipse involves measuring a small fraction of the total brightness as the planet orbits behind the star, obscuring and revealing areas of the star in the process.
Scientists can link minute changes in light to specific regions, creating brightness maps. These maps can be rendered in various colors and translated into three-dimensional temperature readings based on latitude, longitude, and altitude.
“It’s quite difficult because you’re looking for changes where small sections of the Earth become obscured and then revealed,” Challenor explained.
WASP-18b has a mass approximately 10 times that of Jupiter, completes its orbit in just 23 hours, and achieves temperatures around 2,760 degrees Celsius (5,000 degrees Fahrenheit). Its strong signal makes it an excellent candidate for testing new mapping techniques.
While previous 2D maps relied on a single wavelength or color of light, the 3D map re-evaluated the same observations using Webb’s Near Infrared Imager and Slitless Spectrometer (NIRISS) across multiple wavelengths.
“Each color corresponds to different temperatures and altitudes within WASP-18b’s gaseous atmosphere, allowing them to be combined into a 3D map,” Dr. Challenor noted.
“Mapping at wavelengths that water absorbs can indicate the layers of water in the atmosphere, while wavelengths that water doesn’t absorb facilitate deeper probing.”
“When combined, these provide a three-dimensional temperature map of the atmosphere.”
The new perspective uncovered spectroscopically distinct zones (with varying temperatures and potentially different chemical compositions) on the visible dayside of WASP-18b (the side that perpetually faces its star due to its tidally locked orbit).
The planet exhibits a circular “hotspot” that receives the most direct stellar light, with winds insufficient to redistribute the heat.
Surrounding the hotspot is a cooler “ring” located closer to the planet’s visible outer edge.
Interestingly, the measurements indicated that water vapor levels within the hotspot were lower than the average for WASP-18b.
“We believe this suggests that the heat in this area is so intense that water is beginning to decompose,” explained Challenor.
“This was anticipated by theory, but it’s exhilarating to confirm it through actual observations.”
“Further observations from Webb could enhance the spatial resolution of this pioneering 3D eclipse map.”
“Already, this technique will aid in refining temperature maps of other hot Jupiters, which comprise hundreds of the more than 6,000 exoplanets discovered to date.”
Dr. Mansfield expressed: “It’s thrilling that we now possess the tools to visualize and map the temperature of another planet in such intricate detail.”
“We can apply this technique to other exoplanet types. For instance, even if a planet lacks an atmosphere, we might be able to use this method to map surface temperatures and discern its composition.”
“While WASP-18b was more predictable, we believe there’s potential to observe phenomena we never anticipated before.”
The map of WASP-18b is detailed in a paper published in the journal Nature Astronomy.
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RC Challenor et al.. Horizontal and vertical exoplanet thermal structures from JWST spectroscopic eclipse maps. Nat Astron published online October 28, 2025. doi: 10.1038/s41550-025-02666-9
Source: www.sci.news
