Webb telescope finds water vapor and maps temperature of ultra-hot Jupiter exoplanet

Temperature map of WASP-18

Using Webb's high-sensitivity instruments, the team created a temperature map of WASP-18 as it “slipped behind, and reappeared from its star” - referred to as a secondary eclipse.

The temperature map was created by recording temperature changes found at the various elevations of the planet's atmospheric layers.

Like our Moon, this planet is tidally locked. The same side of the exoplanet, known as the dayside, always faces its star.

The map data revealed a remarkable difference in temperature between the two sides - the one that directly faces the star and the terminator side, which remains in perpetual darkness. The temperature difference observed is most likely around 1,000 degrees.

Simply put, the side facing the star is extremely hot, whereas the terminator side is significantly cooler — as predicted by computer models earlier. 

The team suspects these drastic temperature changes are caused by a mysterious wind phenomenon. There could be a wind barrier preventing heat from being redistributed to the other side of the exoplanet. 

"The brightness map of WASP-18 b 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 co-author Ryan Challener, of the University of Michigan.

They hypothesize that the magnetic effects force the winds to travel only in the direction of the North pole to the South pole, rather than East-West. 

Clues of water vapor

Another interesting discovery made by Webb is the presence of water vapor in the atmosphere of the exoplanet WASP-18 b. Despite temperatures reaching 2,700 degrees Celsius, the spectrum data of this gas giant's atmosphere show clear signs of water vapor. 

It is unusual to find water molecules at such extreme temperatures because most molecules would be ripped apart. According to the team, Webb's extraordinary sensitivity allowed it to detect remaining water in its atmosphere. The data show the presence of water vapor at various atmospheric elevations. 

"Because the water features in this spectrum are so subtle, they were difficult to identify in previous observations. That made it really exciting to finally see water features with these JWST observations," said Anjali Piette, a postdoctoral fellow at the Carnegie Institution for Science and one of the authors of the new research.

The team observed the exoplanet for about six hours using Webb's Near-Infrared Imager and Slitless Spectrograph (NIRISS). This knowledge is critical for understanding how these hot, massive exoplanets form. 

The results have been published in the journal Nature.

Study Abstract:

Close-in giant exoplanets with temperatures greater than 2,000 K (“ultra-hot Jupiters”) have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble and Spitzer Space Telescopes1–3. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis3–12. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS13 instrument on JWST. The data span 0.85 to 2.85 μm in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at <6σ confidence) and evidence for optical opacity, possibly due to H-, TiO, and VO (combined significance of 3.8σ). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy-element abundance (“metallicity”, M/H = 1.03+1.11−0.511.03−0.51+1.11 x solar), and a carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside brightness temperature map, which shows a peak in temperature near the sub-stellar point that decreases steeply and symmetrically with longitude toward the terminators.