A groundbreaking team of astronomers has successfully generated the first three-dimensional map of a planet orbiting another star, revealing an atmosphere characterized by distinct temperature zones. This exciting research, co-led by a Cornell expert, focuses on the gas giant WASP-18b, an ultra-hot Jupiter located approximately 400 light years from Earth. The findings were published in a recent study that applies a novel technique called 3D eclipse mapping, or spectroscopic eclipse mapping.
The temperature map of WASP-18b represents a significant advancement from a previously published 2D model by the same research team in 2023. This earlier work demonstrated the potential of eclipse mapping to utilize highly sensitive observations from NASA's James Webb Space Telescope (JWST). Researchers believe that similar exoplanets observable by JWST can now be mapped for atmospheric variations, akin to how Earth-based telescopes once captured images of Jupiter's Great Red Spot and its banded cloud structure.
“Eclipse mapping allows us to image exoplanets that we can't see directly due to their host stars being too bright,” explained Ryan Challener, a postdoctoral associate in the Department of Astronomy and the first author of the study titled Horizontal and Vertical Exoplanet Thermal Structure from a JWST Spectroscopic Eclipse Map, published in Nature Astronomy.
Detecting exoplanets poses a significant challenge, as they emit less than 1% of a host star's brightness. The process of eclipse mapping involves measuring small fractions of light as a planet passes behind its star, obscuring and revealing portions of it. By linking minute changes in light to specific regions, scientists can create a brightness map, which can be transformed into a 3D temperature map encompassing latitude, longitude, and altitude.
While the earlier 2D map relied on a single light wavelength, the new 3D map re-analyzed observations from JWST's Near-Infrared Imager and Slitless Spectrograph (NIRISS) across multiple wavelengths. “Each color corresponds to different temperatures and altitudes within WASP-18b's gaseous atmosphere,” Challener explained. “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.”
This innovative approach confirmed distinct spectroscopic regions with varying temperatures and potentially different chemical compositions on WASP-18b's visible dayside, which always faces its star due to a tidally locked orbit. The planet features a circular hotspot where the most direct starlight strikes, with winds insufficient to redistribute the heat. Surrounding this hotspot is a colder ring near the planet's visible edges.
Importantly, measurements indicated lower levels of water vapor in the hotspot compared to WASP-18b's average. “We think that's evidence that the planet is so hot in this region that it's starting to break down the water,” Challener remarked. “This had been predicted by theory, but it's really exciting to actually see this with real observations.”
Further observations with JWST could enhance the spatial resolution of this pioneering 3D eclipse map. The technique is expected to illuminate temperature maps of other hot Jupiters, which number in the hundreds among the more than 6,000 confirmed exoplanets to date. “This new technique is going to be applicable to many other planets that we can observe with the James Webb Space Telescope,” Challener stated. “We can start to understand exoplanets in 3D as a population, which is very exciting.”
