NASA's Webb Explores Atmospheric Differences on Exoplanet WASP-39 b

NASA's Webb Explores Atmospheric Differences on Exoplanet WASP-39 b
by Erica Marchand
Paris, France (SPX) Jul 16, 2024

Researchers using NASA's James Webb Space Telescope have confirmed atmospheric differences between the perpetual morning and evening sides of the exoplanet WASP-39 b. This gas giant, with a diameter 1.3 times that of Jupiter but a mass similar to Saturn, orbits a star 700 light-years away and is tidally locked, meaning it has a permanent dayside and nightside.

Using Webb's Near-Infrared Spectrograph (NIRSpec), astronomers observed a temperature difference between the two sides of WASP-39 b. The evening side is about 300 Fahrenheit degrees (200 Celsius degrees) hotter than the morning side. Additionally, they detected more cloud cover on the morning side compared to the evening side.

Astronomers examined the 2- to 5-micron transmission spectrum of WASP-39 b, focusing on the terminator-the boundary between the dayside and nightside. By comparing the starlight filtered through the planet's atmosphere during its transit in front of the star to the unfiltered starlight, researchers gathered information on the temperature, composition, and properties of the atmosphere.

"WASP-39 b has become a sort of benchmark planet in studying the atmosphere of exoplanets with Webb," said Nestor Espinoza, an exoplanet researcher at the Space Telescope Science Institute and lead author on the study. "It has an inflated, puffy atmosphere, so the signal coming from starlight filtered through the planet's atmosphere is quite strong."

Previous Webb observations of WASP-39 b revealed carbon dioxide, sulfur dioxide, water vapor, and sodium but did not distinguish between the day and night sides. The new analysis, however, separates the terminator region into two spectra, showing the evening side at a scorching 1,450 degrees Fahrenheit (800 degrees Celsius) and the morning side at a cooler 1,150 degrees Fahrenheit (600 degrees Celsius).

"It's really stunning that we are able to parse this small difference out, and it's only possible due Webb's sensitivity across near-infrared wavelengths and its extremely stable photometric sensors," said Espinoza. "Any tiny movement in the instrument or with the observatory while collecting data would have severely limited our ability to make this detection. It must be extraordinarily precise, and Webb is just that."

Extensive modeling of the data also explored the structure of WASP-39 b's atmosphere, cloud cover, and the reasons behind the temperature difference. Future studies will examine how cloud cover affects temperature, but researchers have identified gas circulation as the main cause of the temperature disparity on WASP-39 b.

On a highly irradiated exoplanet like WASP-39 b, which orbits close to its star, the gas is expected to move with the planet's rotation: hotter gas from the dayside flows to the nightside via an equatorial jet stream. The significant temperature difference causes substantial air pressure differences, resulting in high wind speeds.

Using General Circulation Models, similar to those predicting Earth's weather patterns, researchers found that WASP-39 b's winds likely move from the night side across the morning terminator, around the dayside, across the evening terminator, and back to the night side. This results in cooler winds hitting the morning side and hotter winds hitting the evening side. Wind speeds on WASP-39 b can reach thousands of miles per hour.

"This analysis is also particularly interesting because you're getting 3D information on the planet that you weren't getting before," added Espinoza. "Because we can tell that the evening edge is hotter, that means it's a little puffier. So, theoretically, there is a small swell at the terminator approaching the nightside of the planet."

The team's results have been published in Nature. Researchers plan to use this analysis method to study atmospheric differences in other tidally locked hot Jupiters as part of Webb Cycle 2 General Observers Program 3969.

WASP-39 b was one of the first targets analyzed by Webb when it began regular science operations in 2022. Data for this study was collected under Early Release Science program 1366, aimed at helping scientists quickly learn to use the telescope's instruments and realize its full potential.

The James Webb Space Telescope is the world's premier space science observatory. Webb explores our solar system, distant worlds, and the universe's structures and origins. Webb is an international program led by NASA, with partners ESA (European Space Agency) and CSA (Canadian Space Agency).