Webb telescope analyzes atmosphere of an exoplanet 10 times more massive than Jupiter; finds traces of water

Researchers using NASA's James Webb Space Telescope closely observed WASP-18 b, a gas giant exoplanet 10 times more massive than Jupiter, and found water vapour in its atmosphere. The team also made a temperature map of the planet as it slipped behind, and reappeared from its host star - an event called secondary eclipse.

Discovered in 2009, WASP-18 b orbits its star in just 23 hours and its same side always faces the star. The planet's brightness map shows a huge change in temperature - up to 1,000 degrees - from the hottest point facing the star to the terminator, where day and night sides of the tidally-locked planet meet in permanent twilight.

The researchers found that the planet is considerably cooler at the terminator, suggesting that there is an obstacle preventing the winds from effectively transferring heat to the night side. However, the exact factor influencing the winds remains unknown.

The team observed temperature changes at different altitudes within the layers of the planet's atmosphere. They noticed that temperatures rose as the elevation increased, with variations reaching several hundred degrees.

Despite the extreme temperatures reaching approximately 5,000 degrees Fahrenheit (2,700 degrees Celsius), the planet's atmospheric spectrum clearly shows multiple small water features that were precisely measured. This is noteworthy because such high temperatures would typically cause water molecules to disintegrate. The fact that these water features were still detected underscores the remarkable sensitivity of the Webb telescope in identifying remaining water.

The measurements taken on WASP-18 b indicate the presence of water vapor at various altitudes within its atmosphere. 

"This is the first time a planet has been mapped with JWST, and it’s really exciting to see that some of what our models predicted, such as a sharp drop in temperature away from the point on the planet directly facing the star, is actually seen in the data!" said Megan Mansfield, a Sagan Fellow at the University of Arizona, and one of the authors of the paper describing the results.​

The researchers used Webb's Near-Infrared Imager and Slitless Spectrograph (NIRISS) to look at WASP-18 b for about six hours.

hot exoplanet summer! 🔥Webb analyzed the atmosphere of an ultrahot gas giant and mapped its temperatures. Despite scorching heat (nearly 5000 F or 2700 C), WASP-18 b has small amounts of atmospheric water — precisely measured due to Webb's sensitivity: https://t.co/KF8oFxFRC7

— NASA Webb Telescope (@NASAWebb) May 31, 2023