James Webb Maps and Finds Traces of Water in a Super-hot Gas Giant’s Atmosphere

Exoplanet WASP-39 b and Its Star (Illustration). Credits - NASA, ESA, CSA, and L. Hustak (STScI)

NYU Abu Dhabi scientist at the Center for Astro, Particle, and Planetary Physics, Jasmina Blecic, together with a team of international experts, identified water vapor in the atmosphere of WASP-18 b - an ultra-hot gas giant 10 times more massive than Jupiter - and made a temperature map of the planet as it slipped behind and reappeared from its star.


“We have studied WASP-18 b since 2009 using other telescopes, but James Webb was the first to spot multiple small but precisely measured water features in the atmosphere of this scorching planet. Despite its extremely high temperatures of nearly 5,000 degrees Fahrenheit (2,700°C), where most water molecules are destroyed, we detected a few remaining ones. This is a major advancement, indicating that the James Webb Space Telescope (JWST's) remarkable precision and sensitivity can detect water on much smaller Earth-like planets that might harbor life. Those planets are expected to have comparable-sized features."

Jasmina Blecic

The dayside of WASP-18 b always faces the star, just as the same side of the Moon always faces Earth. This leads to huge change in temperature – up to 1,000 degrees – from the hottest point facing the star to the terminator, where day and night sides meet in permanent twilight.

“For the first time also this configuration enabled us to observe detailed temperature variations on the dayside of the planet. We can differentiate between a cooler terminator area and a roasting central point directly facing the star,” added Blecic, one of the authors of the paper describing the results.

Given how much cooler the planet is at the terminator, there is likely something hindering winds from efficiently redistributing heat to the night side. But what is hindering the winds is still a mystery.

‘‘One possible explanation is that this planet has a strong magnetic field, which would be an exciting discovery! Magnetic effects could 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,” said co-author Ryan Challener from the University of Michigan.

WASP-18 b was observed for around six hours with the Near-Infrared Imager and Slitless Spectrograph (NIRISS), an instrument with the highest resolution on board James Webb, which allowed this exciting discovery.

Proximity, both to its star and to us, as well as its large mass, helped make WASP-18 b such an intriguing target for scientists. WASP-18 b is one of the most massive worlds whose atmospheres we can investigate. We want to know how such planets form and come to be where they are. This, too, has some early answers from James Webb.

Blecic with her team found that WASP-18 b’s composition is very similar to that of its star, meaning it most likely formed from the leftover gas that was present just after the star was born. Those results are very valuable to get a clear picture of how strange planets like WASP-18 b, which have no counterpart in our solar system, come to exist.