Scientists map a fiery exoplanet that tears water apart

For the first time, scientists have created a 3D temperature map of a planet outside of our solar system. The exoplanet, WASP-18b, is a massive gas giant that whips around its star in less than a day.

The planet is nearly 10 times the mass of Jupiter and so intensely hot that it can rip water vapor molecules apart.

The new map doesn’t just show where the heat is – it shows how temperature changes in every direction: north to south, east to west, and top to bottom.

The planet that breaks water

WASP-18b is what scientists call an “ultra-hot Jupiter.” It orbits a star about 400 light-years from Earth, completing one full lap in just 23 hours. That’s faster than Earth’s rotation.

Because it’s so close to its star, the exoplanet is locked in place – one side always faces the heat, and the other stays in shadow.

That sunny side gets blasted with so much radiation that temperatures can hit nearly 5,000°F. That’s hot enough to destroy water molecules. For scientists, that made WASP-18b the perfect testing ground.

In 2023, a group of astronomers made a 2D map of this planet using NASA’s James Webb Space Telescope. Now, they’ve taken it a step further.

By re-analyzing the same data in multiple colors – or wavelengths – they created a full 3D view.

Reading light in the dark

The trick to making a 3D map of something you can’t see directly is in the light – or more precisely, in the minute shifts that occur as the planet slips behind its star.

This technique is called “eclipse mapping.” It’s like taking a silhouette and filling it in with detail.

“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 at Cornell University.

Mapping temperatures in the atmosphere

As the planet moves behind the star, scientists track how light from different parts of the planet fades.

These differences can be connected to specific spots on the surface. And by looking at different wavelengths – some that water vapor absorbs and some that it doesn’t – experts can build a 3D temperature map.

“If you build a map at a wavelength that water absorbs, you’ll see the water deck in the atmosphere, whereas a wavelength that water does not absorb will probe deeper,” Challener explained.

“If you put those together, you can get a 3D map of the temperatures in this atmosphere.”

What the map revealed

The new map of WASP-18b’s dayside showed a round, super-hot region where the star’s rays hit most directly. This is the hottest spot on the planet and, as the data showed, it’s too hot for water vapor to survive.

Surrounding that hotspot is a cooler band stretching toward the outer edges. These areas showed more signs of water vapor.

The difference confirms something scientists had long predicted: heat can break apart water in an atmosphere. On this planet, it’s happening on a global scale.

Megan Weiner Mansfield, an assistant professor of astronomy at the University of Maryland, said that on a population level, scientists have observed a pattern in which cooler planets tend to contain water, while hotter planets generally do not.

“But this is the first time we’ve seen this be broken across one planet instead. It’s one atmosphere, but we see cooler regions that have water and hotter regions where the water’s being broken apart,” noted Professor Mansfield.

“That had been predicted by theory, but it’s really exciting to actually see this with real observations.”

A new way to study exoplanets

Until now, the best we could do with most exoplanets was estimate an average temperature.

This new method lets scientists look at specific regions and layers of a planet’s atmosphere. It’s like going from a blurry weather forecast to a live radar map.

“This technique is really the only one that can probe all three dimensions at once: latitude, longitude and altitude,” Mansfield said. “This gives us a higher level of detail than we’ve ever had to study these celestial bodies.”

That level of detail means researchers can start comparing these faraway worlds to planets in our own solar system.

Even though we can’t see exoplanets directly, the James Webb Space Telescope can spot subtle shifts in light that ground telescopes never could.

“With this telescope and this new technique, we can start to understand exoplanets along the same lines as our solar system neighbors,” Challener said.

A step toward more complex planets

This is just the beginning. WASP-18b was a clear target because of its size and heat. The strong signal made it easier to map. But there are thousands more exoplanets already confirmed, and hundreds of them fall into the “hot Jupiter” category.

Professor Mansfield and her team believe they’ll soon be able to apply the same 3D technique to smaller, rockier planets. That could eventually lead to temperature maps of planets that resemble Earth.

Even if those planets don’t have atmospheres, the technique could still reveal surface temperatures and possible hints about what they’re made of.

“It’s very exciting to finally have the tools to see and map out the temperatures of a different planet in this much detail. It’s set us up to possibly use the technique on other types of exoplanets,” noted Mansfield.

“For example, if a planet doesn’t have an atmosphere, we can still use the 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 the chance to see things that we could never have expected before.”

The full study was published in the journal Nature Astronomy.

Image Credit: NASA/GSFC

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