This new effort builds on a previous two-dimensional model published in 2023, expanding the capability to leverage sensitive readings from NASA's James Webb Space Telescope (JWST). Researchers say that, just as traditional telescopes revealed Jupiter's Great Red Spot and clouds, JWST can now map atmospheric variations on many similar exoplanets.
"Eclipse mapping allows us to image exoplanets that we can't see directly, because their host stars are too bright," said Ryan Challener, postdoctoral associate in astronomy. "With this telescope and this new technique, we can start to understand exoplanets along the same lines as our solar system neighbors."
Detecting exoplanets remains challenging, since they emit less than 1% of the brightness of their host stars. Eclipse mapping measures the minute changes in light as the planet passes behind its star, linking those fluctuations to different regions to create a color-coded temperature map, spanning latitude, longitude, and altitude.
"You're looking for changes in tiny portions of the planet as they disappear and reappear into view," Challener explained, "so it's extraordinarily challenging."
WASP-18b, roughly ten times the mass of Jupiter, completes its orbit in just 23 hours and reaches temperatures near 5,000 degrees Fahrenheit. Its relatively strong signal made it ideal for testing the new mapping approach.
Earlier models relied on a single wavelength, but the new 3D map utilized many wavelengths from JWST's Near-Infrared Imager and Slitless Spectrograph (NIRISS), with each wavelength corresponding to a distinct temperature and altitude. This enabled researchers to create a layered map of the planet's thermal structure.
"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 said. "If you put those together, you can get a 3D map of the temperatures in this atmosphere."
JWST's observations confirmed regions with different temperatures and probably different compositions on WASP-18b's dayside-the side always facing its star due to tidal locking. A concentrated hotspot marks the spot where starlight hits most directly; winds are apparently not sufficient to spread the heat across the planet. The hotspot is surrounded by a cooler ring along the planet's visible limbs. Notably, Challener said, the team found lower amounts of water vapor in the hottest region than the planet's average, direct evidence that heat is breaking down water molecules.
"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."
Future JWST surveys could refine the 3D mapping's resolution and help document similar features on other "hot Jupiters," which represent hundreds of the more than 6,000 confirmed exoplanets.
"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."
Research Report:Horizontal and vertical exoplanet thermal structure from a JWST spectroscopic eclipse map
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