While water vapor was previously detected in the atmosphere of exoplanet WASP-18b—an ultrahot gas giant 10 times more massive than Jupiter—University of Michigan scientists have confirmed that NASA’s James Webb Space Telescope unveiled once-hidden water features that had eluded the ground-based Hubble Space Telescope.

Located 400 light-years away, WASP-18b orbits its star at a mere 23-hour interval, defying the norms of our own solar system.

During a secondary eclipse, when WASP-18b slips behind its star and reappears, researchers successfully mapped the planet’s temperature variations and confirmed the presence of water vapor. The findings, published in Astrophysics Data System, confirm theoretical predictions and offer a glimpse into the complex atmospheric conditions of hot giant planets.

”The brightness map of WASP-18b shows a lack of east-west winds that is best matched by models with atmospheric drag. One possible explanation is that this planet has a strong magnetic field, which would be an exciting discovery,” said Ryan Challener, U-M postdoctoral researcher in astronomy.

The U-M team’s temperature mapping revealed a stark contrast, with a temperature drop of up to 1,000 degrees from the planet’s hottest point, facing the star, to the terminator, the moving line where the day and night sides meet in perpetual twilight.

One interpretation of the eclipse map is that magnetic effects force the winds to blow from the planet’s equator up over the north pole and down over the south pole, instead of east-west, as we would otherwise expect.

Researchers recorded temperature changes at different elevations of the gas giant planet’s layers of atmosphere. They saw temperatures increase with elevation, varying by hundreds of degrees.

The spectrum clearly shows multiple small but precisely measured water features present, despite the extreme temperatures of almost 5,000 degrees Fahrenheit (2,700 C). It’s so hot that it would tear most water molecules apart, so still seeing its presence speaks to Webb’s extraordinary sensitivity to detect remaining water that survived, Challener said. The amounts recorded in WASP-18b’s atmosphere indicate water vapor at various elevations.

Astrophysicist Ryan MacDonald, a NASA Sagan Fellow at U-M, played a pivotal role in the identification of water-related features, alongside other species, in the spectrum of the planet. He provided statistical confirmation affirming the genuine detection of these subtle characteristics.

The observations of WASP-18b were conducted using the Near-Infrared Imager and Slitless Spectrograph, an instrument contributed by the Canadian Space Agency.

“This was the first time the eclipse mapping technique has been applied to JWST data,” said Hayley Beltz, graduate student research assistant of astronomy and astrophysics at U-M.

“By learning about the temperature structure, we were able to see that our model with magnetic effects was a better match. This sets the stage for future observations where we can test our models even further to understand the huge diversity in exoplanet atmospheres.”

The proximity of WASP-18b, both to its parent star and to our own planet, has captivated the attention of scientists worldwide. WASP-18b stands as one of the most massive celestial bodies whose atmospheres we can explore, presenting an exceptional opportunity to unravel the mysteries of planetary formation and their positioning in the cosmos.

Early insights from the observations made by the JWST have begun shedding light on these intriguing questions, providing a glimpse into the origins and journeys of such extraordinary worlds, said Emily Rauscher, U-M associate professor of astronomy.

“It’s a thrill to see eclipse mapping with JWST and that magnetic fields are shaping the atmospheric structure of this planet,” she said.

Rauscher served in an advisory capacity to Challener and Beltz and has been thinking about eclipse mapping since 2005 and how magnetic fields could shape hot Jupiter’s circulation since 2010.

”It was a great feeling to look at WASP-18b’s JWST spectrum for the first time and see the subtle but precisely measured signature of water,” said Louis-Philippe Coulombe, a graduate student at the University of Montreal and lead author of the WASP-18b study. ”Using such measurements, we will be able to detect such molecules for a wide range of planets in the years to come.”