Artist rendition of WASP-18. (Credit: NASA/JPL-Caltech)

Four hundred light-years away, a world twice the size of Jupiter glows like a forge. Now, thanks to the James Webb Space Telescope, astronomers have charted that fiery planet in three dimensions — the first detailed 3D map ever made of a world beyond our solar system.

Hot and Gassy

The gas giant WASP-18b belongs to a class known as ultra-hot Jupiters — giant, searing, gaseous planets that orbit perilously close to their stars. Researchers used a new technique called three-dimensional eclipse mapping, or spectroscopic eclipse mapping, which turns tiny shifts in light into a detailed picture of an alien atmosphere.

WASP-18b is a bruiser: roughly ten times the mass of Jupiter and whipping around its star in just 23 hours. The planet is tidally locked to its star, much like the Moon is to Earth, so one side is locked in perpetual daylight. Temperatures on that dayside soar near 5,000°F (2,760°C) — hot enough to tear water molecules apart.

The new 3D map reveals two main features. At the center of the dayside lies a circular hotspot — the bull’s-eye where starlight hits most directly. Around the edges, near the planet’s “horizon,” sits a cooler ring. The hotspot burns so fiercely that water vapor appears scarcer there than elsewhere, consistent with basic chemistry: under such heat, H₂O breaks apart into hydrogen and oxygen and only recombines in cooler regions.

“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 and first author of the study published in Nature Astronomy. “With this telescope and this new technique, we can start to understand exoplanets along the same lines as our solar system neighbors.”

What is Eclipse mapping and Why Does a 3D Map Matter?

The idea of mapping an exoplanet hundreds of light-years away sounds impossible — after all, how do you chart a place you can’t actually see?

In eclipse mapping, astronomers take advantage of the planet’s orbit. As the planet slips behind its star, the glowing dayside disappears bit by bit until it’s fully hidden. When the planet reemerges, the light returns in reverse order. These changes are minuscule — just fractions of a percent — but the James Webb Space Telescope can measure them across many infrared wavelengths.

Each wavelength probes a different depth in the atmosphere, a bit like how medical imaging reveals layers of tissue. Stack those slices together, and you get a 3D model showing temperature variations by latitude, longitude, and altitude.

That map carries clues about the planet’s winds and chemistry. On some hot, tidally locked worlds, fast jet streams shove the hottest zone east or west of center. Here, the hotspot stays close to dead center, which hints that powerful drag — possibly tied to the planet’s strong gravity and magnetic effects — keeps the winds in check. Higher up, the atmosphere seems to warm with height, a sign that starlight is being absorbed in the upper layers before it can heat deeper ones.

A New Way of Looking at Planets

Until now, many exoplanet studies have had to rely on broad averages, such as a single temperature for half a planet or an estimate of the direction of heat. That’s like saying “Europe is warm” without distinguishing between Madrid and Helsinki. A 3D map lets researchers ask better questions: Where does the heat pool? How quickly does it fall off toward the edges? At what heights do molecules survive, and where do they come apart?

WASP-18b was a clever first choice. It’s so hot that it shines brightly in infrared, giving JWST a strong planetary signal to pry from the star’s glare. But the same method can be used on many other “hot Jupiters,” of which astronomers have found hundreds. As similar maps accumulate, patterns should emerge. Do the hottest worlds always have pinned hotspots? How often do we see upper layers that warm with height? Where does water survive, and where does it fail?

There are still puzzles to work through. The chemistry out near the planet’s limb — the visible edge — can be hard to pin down because the geometry is tricky, and some standard tools tend to underestimate water there. More JWST observations will sharpen the view and help sort out those edge cases. The team already cross-checked their work with two different mapping approaches, and both pointed to the same big picture: a central furnace, a cooler ring and water signatures that sag where the heat is most severe.

However, astronomers are very optimistic about what might lie ahead in the future thanks to the JWST.

“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.”