Four hundred light years away, a planet circles its star so close and so fast that a full year takes less than a day. Now astronomers have mapped it in three dimensions, and what they found is a world where temperatures climb high enough to rip water molecules into their component atoms.
The planet, WASP-18b, is what astronomers call an ultra-hot Jupiter: a gas giant roughly ten times Jupiter’s mass, locked in a 23-hour orbit that bakes its surface to nearly 5,000 degrees Fahrenheit. Using observations from the James Webb Space Telescope, a team co-led by Cornell postdoctoral researcher Ryan Challener has produced the first true 3D temperature map of an exoplanet, revealing distinct climate zones across its scorched surface.
The mapping technique, called spectroscopic eclipse mapping, works by watching the planet disappear behind its host star. As different regions slip out of view, tiny changes in the light reveal temperature variations across the planet’s face. By analyzing multiple wavelengths, each probing different altitudes in the atmosphere, the team reconstructed a three-dimensional picture.
“Eclipse mapping allows us to image exoplanets that we can’t see directly, because their host stars are too bright. With this telescope and this new technique, we can start to understand exoplanets along the same lines as our solar system neighbors.”
A World of Extremes
The map reveals a planet divided into climate zones, though nothing like Earth’s temperate bands. WASP-18b’s dayside, perpetually facing its star due to tidal locking, features a circular hotspot where starlight hits most directly. This region registers the highest temperatures, and curiously, shows depleted water vapor compared to surrounding areas.
That finding supports a prediction that had remained theoretical until now: at extreme temperatures, water molecules break apart. The heat literally tears H2O into hydrogen and oxygen atoms, a process called thermal dissociation. Surrounding the central hotspot is a relatively cooler ring, where temperatures drop enough for water to remain intact.
The technique required extraordinary precision. Exoplanets typically emit less than 1% of their host star’s brightness, and eclipse mapping demands measuring tiny fractions of that already-faint signal. Each measurement captures light from specific regions as they rotate into and out of view, allowing researchers to build up a brightness map pixel by pixel.
Challener describes the challenge plainly: “You’re looking for changes in tiny portions of the planet as they disappear and reappear into view, so it’s extraordinarily challenging.”
From Flat to Full
The new work builds on a 2D map the same team published in 2023, which used a single wavelength of light. By expanding to multiple wavelengths, each corresponding to different atmospheric depths, the researchers could stack these layers into a full three-dimensional picture. Different wavelengths penetrate to different depths: colors absorbed by water reveal the upper atmosphere’s water deck, while wavelengths that pass through water probe deeper layers.
The technique transforms how astronomers can study distant worlds. Where previous methods averaged temperatures across an entire hemisphere, eclipse mapping distinguishes regional features. For WASP-18b, this meant discovering that winds apparently fail to redistribute heat from the hotspot, leaving the temperature gradient intact.
“We think that’s evidence that the planet is so hot in this region that it’s starting to break down the water. That had been predicted by theory, but it’s really exciting to actually see this with real observations.”
Hot Jupiters like WASP-18b represent hundreds of the more than 6,000 confirmed exoplanets, and many orbit close enough to their stars for detailed observation. The technique promises to work across this population, potentially revealing whether each world develops similar hotspots or shows different atmospheric circulation patterns.
More observations from JWST could sharpen the resolution of these maps, potentially revealing smaller-scale features. The telescope’s sensitivity makes it possible to detect atmospheric details that were pure speculation just a few years ago. Now astronomers can begin comparing exoplanet atmospheres the way they compare cloud patterns on Jupiter or dust storms on Mars, bringing alien worlds into focus one temperature map at a time.
Nature Astronomy: 10.1038/s41550-025-02666-9
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