Ann Arbor Times

An international team of astronomers has created the first three-dimensional map of an exoplanet’s atmosphere, revealing dramatic temperature differences and regions so hot that water vapor is destroyed. The research, published in Nature Astronomy, focused on WASP-18b, a gas giant located 400 light years from Earth.

Emily Rauscher, associate professor of astronomy at the University of Michigan, was among the scientists involved in the study. “Partially, this result is really cool because it’s a demonstration of a new technique,” said Rauscher. “Scientifically, this result is also really cool because, for this particular planet, it gets incredibly hot.”

The project was co-led by Ryan Challener from Cornell University and Megan Weiner Mansfield from the University of Maryland. Their work introduces a method called 3D eclipse mapping or spectroscopic eclipse mapping. This builds upon earlier two-dimensional models and uses highly sensitive data from the James Webb Space Telescope (JWST), which is operated by NASA with support from the European Space Agency and Canadian Space Agency.

“Eclipse mapping allows us to image exoplanets that we can’t see directly, because their host stars are too bright,” said Challener. “With this telescope and this new technique, we can start to understand exoplanets along the same lines as our solar system neighbors.”

The team compared their approach to how telescopes on Earth helped astronomers study features like Jupiter’s Great Red Spot. With JWST’s capabilities, similar atmospheric studies are now possible for planets outside our solar system.

Challener previously worked in Rauscher’s group at Michigan, where he began developing software to extract 3D thermal information from JWST data. Mansfield contributed methodology she developed during a workshop hosted by Rauscher in 2018.

“I definitely have a bit of pride in that,” Rauscher said. “Here at Michigan, we’ve been able to contribute toward making this observation happen.”

The collaboration included about three dozen researchers from more than 20 institutions worldwide and was supported by JWST’s Transiting Exoplanet Community Early Release Science Program.

Detecting exoplanets remains difficult due to their faintness compared to their host stars; exoplanets typically emit less than 0.1% as much light as their stars. “The analogy is a firefly in front of a lighthouse,” Rauscher explained. “But we know when the planet is in front of and when it’s behind the star. We can actually measure that tiny little dip that had been light coming from the planet.”

By tracking how light dims as WASP-18b moves behind its star, JWST enables scientists to link subtle changes in brightness to specific areas on the planet across different wavelengths of light—information that translates into temperature maps covering latitude, longitude, and altitude.

WASP-18b has about ten times Jupiter’s mass and completes an orbit every 23 hours. Its temperatures reach nearly 5,000 degrees Fahrenheit—conditions that provided strong signals for testing this new mapping technique.

The resulting map revealed distinct zones on WASP-18b’s dayside: a central hotspot exposed directly to starlight with weak winds unable to spread heat away efficiently, surrounded by cooler regions near the planet's visible edges. Measurements showed lower water vapor levels in the hotspot area than elsewhere on WASP-18b.

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

Researchers believe further observations could improve map resolution and help apply these methods to other gas giants among thousands of known exoplanets.

“One of the really cool things about exoplanets is that most of the planets we found are completely unlike the ones in our solar system,” Rauscher said. “And so it’s a lot of fun to try and figure out how we take what we understand, more or less, in our solar system and stretch it to these very different situations.”