Webb investigates eternal sunrises, sunsets in the distant world

Researchers using the James Webb Space Telescope have finally confirmed what models have previously predicted: An exoplanet has differences between its eternal morning and eternal evening atmospheres. WASP-39 b, a giant planet with a diameter 1.3 times that of Jupiter but a mass similar to Saturn orbiting a star about 700 light-years from Earth, is tidally bound to its star mother. This means that it has a continuous day and a continuous night – one side of the planet is always exposed to its star, while the other is always covered by darkness.

Using Webb’s NIRSpec (Near-Infrared Spectrograph), astronomers confirmed a temperature difference between eternal morning and eternal evening in WASP-39 b, with the evening appearing hotter by approximately 300 degrees Fahrenheit (about 200 degrees Celsius). They also found evidence for different cloud cover, with the permanent morning part of the planet likely to be cloudier than the evening.

Image A: Artist’s concept WASP-39 b

Astronomers analyzed the 2- to 5-micron transmission spectrum of WASP-39 b, a technique that studies the exoplanet terminator, the boundary that separates the planet’s day and night sides. A transmission spectrum is created by comparing the starlight filtered through a planet’s atmosphere as it moves in front of the star, with the unfiltered starlight detected when the planet is near the star. When making this comparison, researchers can obtain information about the temperature, composition, and other properties of the planet’s atmosphere.

“WASP-39 b has become a standard type of planet in the study of exoplanet atmospheres with Webb,” said Néstor Espinoza, an exoplanet researcher at the Space Telescope Science Institute and lead author of the study. “It has a puffy, puffy atmosphere, so the signal from starlight filtered through the planet’s atmosphere is quite strong.”

Previously published Webb spectra of WASP-39b’s atmosphere, which revealed the presence of carbon dioxide, sulfur dioxide, water vapor and sodium, represent the entire day/night boundary – there was no detailed attempt to distinguish one side and the other.

Now, the new analysis constructs two different spectra from the terminator region, essentially splitting the day/night boundary into two semicircles, one from the evening and the other from the morning. The data shows that the evening is significantly hotter, 1450 degrees Fahrenheit (800 degrees Celsius) and the morning is relatively cooler 1150 degrees Fahrenheit (600 degrees Celsius).

Image B: Transmission spectra

“It’s truly amazing that we’re able to analyze this small change, and it’s only possible because of Webb’s sensitivity at near-infrared wavelengths and its extremely stable photometric sensors,” Espinoza said. “Any small movement in the instrument or the observatory during data collection would have severely limited our ability to make this discovery. It has to be extremely precise, and Webb is exactly that.”

Extensive modeling of the obtained data also allows researchers to investigate the structure of WASP-39 b’s atmosphere, cloud cover and why the evening is hotter. While future work by the team will study how cloud cover can affect temperature and vice versa, astronomers confirmed the circulation of gas around the planet as the main culprit of the temperature change in WASP-39 b.

In a highly irradiated exoplanet like WASP-39 b that orbits relatively close to its star, researchers generally expect the gas to move as the planet orbits its star: The hottest gas from the day should move during the evening to the nightside through a powerful gas. equatorial jet stream. Since the temperature difference is so extreme, the air pressure difference would also be significant, which in turn would cause high wind speeds.

Image C: Transient light curve

Using General Circulation Models, 3-dimensional models similar to those used to predict weather patterns on Earth, the researchers found that in WASP-39 b the prevailing winds are likely to move from the night side along the morning terminator , around the day, across the evening terminator and then around the night side.

As a result, the morning side of the terminator is cooler than the evening side. In other words, the morning side is buffeted by winds of air that have cooled on the night side, while the evening is buffeted by winds of air that has warmed during the day. Research suggests that wind speeds in WASP-39 b could reach thousands of miles per hour.

“This analysis is also particularly interesting because you’re getting 3D information about the planet that you weren’t getting before,” Espinoza added. “Because we can say that the evening edge is hotter, that means it’s a little more puffy. So, theoretically, there’s a small bulge in the terminator that approaches the night side of the planet.”

The results of the team are published in Nature.

The researchers will now seek to use the same analysis method to study the atmospheric changes of other hot, tidally locked Jupiters as part of the Webb Cycle 2 General Observers Program 3969.

WASP-39 b was among the first targets analyzed by Webb after it began regular science operations in 2022. The data in this study was collected under the Early Release Science 1366 program, designed to help scientists quickly learn how to use telescope instruments and realize full scientific potential.