James Webb Produces First 3D Atmospheric Map of Exoplanet WASP-18b — Hotspot May Dissociate Water

James Webb produces the first three-dimensional map of an exoplanet's atmosphere

Scientists have used the James Webb Space Telescope (JWST) to produce the first three-dimensional map of an exoplanet's atmosphere. The target, WASP-18b, is a gas giant roughly 400 light-years away with about 10 times the mass of Jupiter and a blisteringly short 23-hour orbit. The new map, presented in a study published Oct. 28 in Nature Astronomy, reveals how temperature and composition vary across the planet and with altitude.

How eclipse mapping with JWST works

Eclipse mapping exploits the moment a planet passes behind its host star. As the star progressively blocks different regions of the planet, tiny changes in the system's measured light encode information about the planet's brightness and spectrum across different locations. By combining those changes with JWST's spectral sensitivity at multiple wavelengths, researchers can infer temperature and chemical differences at different atmospheric depths.

“Eclipse mapping allows us to image exoplanets that we can't see directly, because their host stars are too bright,” said Ryan Challener, a co-author and exoplanet researcher at Cornell University. “With this telescope and this new technique, we can start to understand exoplanets along the same lines as our solar system neighbors.”

Key findings for WASP-18b

The JWST eclipse map shows two distinct thermal regions on the planet's permanently sunlit hemisphere. A circular "hotspot" sits near the substellar point (the location that faces the star most directly), and a cooler ring surrounds it toward the planet's limb. This pattern indicates that atmospheric circulation does not fully redistribute the incoming stellar heat across the planet.

By observing at wavelengths that are absorbed by water and at wavelengths that pass through water, the team probed different atmospheric altitudes. They found a reduced water signal in the hotspot compared with the planet’s average atmosphere. The simplest explanation is thermal dissociation: temperatures in the hotspot may be high enough to break water molecules into constituent atoms, reducing observable water absorption.

“We think that's evidence that the planet is so hot in this region that it's starting to break down the water,” Challener said, noting that this effect had been predicted by theory but is now supported by observations.

Why this matters

This demonstration of 3D eclipse mapping opens a new avenue for characterizing exoplanet atmospheres. Applying the technique to other worlds observed by JWST can reveal vertical and horizontal variations in temperature, composition and cloud structure, helping to build a population-level understanding of how atmospheres behave under different stellar irradiation and gravity regimes.

Future JWST observations can sharpen WASP-18b's map, improve spatial resolution, and extend the method to other gas giants and, potentially, smaller planets. The result is a step toward treating exoplanets as three-dimensional, dynamic worlds rather than one-dimensional averages.

Reference: Study published Oct. 28 in Nature Astronomy; lead quotes from co-author Ryan Challener (Cornell University).