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Researchers using NASA’s James Webb Space Telescope have finally confirmed what models previously predicted: An exoplanet has an eternal morning and eternal evening atmosphere. WASP-39 b, a giant planet 1.3 times the diameter of Jupiter but with a mass similar to that of Saturn, orbits a star about 700 light-years from Earth. It is called “tidally locked” to its parent star. That means it has a constant day and night side: One side of the planet is always facing its star, while the other side is always in darkness.
Using Webb’s NIRSpec (Near-Infrared Spectrometer), astronomers confirmed that there is a temperature difference between perpetual morning and perpetual evening on WASP-39 b. Evenings appear to be about 300 degrees Fahrenheit (about 200 degrees Celsius) warmer. They also found evidence of differential cloud cover, with the perpetual morning part of the planet likely cloudier than the evening. The astronomers analyzed WASP-39 b’s 2- to 5-micron transmission spectrum, a technique that studies the exoplanet’s tail, the boundary separating the day and night sides of the planet. The transmission spectrum is created by comparing starlight filtered through the planet’s atmosphere as it passes in front of the star with the unfiltered starlight observed when the planet is next to the star. This comparison can provide researchers with information about the temperature, composition, and other properties of the planet’s atmosphere.
“WASP-39 b has become a benchmark planet for studying exoplanet atmospheres using Webb,” said Nestor Espinosa, an exoplanet researcher at the Space Telescope Science Institute and lead author of the study. “It has a puffy, puffy atmosphere, so the starlight signal streaming through the planet’s atmosphere is very strong.” Previously published Webb spectra of WASP-39 b’s atmosphere, which showed carbon dioxide, sulfur dioxide, water vapor and sodium, represented the entire day/night boundary — and there was no detailed attempt to distinguish between the two sides. Now, the new analysis constructs two different spectra of the termination region, effectively dividing the day/night boundary into two halves, one from the evening and one from the morning. The data show that the evenings are significantly hotter, at 800 degrees Celsius, and the mornings are relatively cooler, at 600 degrees Celsius.
“It’s really amazing that we can distinguish such a small difference, and this is only possible thanks to Webb’s sensitivity to near-infrared wavelengths and very stable optical sensors,” Espinosa said. “Any small movement of the instrument or the observatory during data collection would severely limit our ability to make this detection. It has to be extremely precise, and Webb is exactly that. Comprehensive modeling of the data obtained will also allow the researchers to investigate the structure of WASP-39 b’s atmosphere, its cloud cover, and why it gets so hot in the evening. Although the team will study in the future how cloud cover might affect temperature and vice versa, the astronomers confirmed that the rotation of gas around the planet is the main reason for the temperature differences on WASP-39 b. On a very radiant exoplanet like WASP-39 b, which orbits relatively close to its star, researchers would generally expect the gas to be in motion as the planet orbits its star: the hotter gas from the day side would travel through a strong equatorial jet stream to the night side during the evening. Because the temperature difference is so large, the air pressure difference would also be large, which in turn would drive up the wind speeds.
Using general circulation models, three-dimensional models similar to those used to predict weather patterns on Earth, the researchers found that the prevailing winds on WASP-39 b likely move from the night side over the end of the morning, around the day side, over the end of the evening and then around the night. As a result, the morning side of the terminator is cooler than the evening side. That is, the morning side is swept by air that has cooled from the night side, while the evening side is swept by air that has warmed from the day side. Research suggests that wind speeds on WASP-39 b can reach thousands of kilometers per hour! “This analysis is also particularly interesting because you’re getting three-dimensional information about the planet that you haven’t had before,” Espinosa adds. “Because we can see that the evening edge is hotter, it means that it’s a little bit bulging. So theoretically, there’s a little bulge at the terminator that’s approaching the dark side of the planet.”
Researchers will now use the same analysis method to study atmospheric differences among other tidally locked hot Jupiters, as part of the Webb 3969 Public Observer Program. WASP-39 b was one of the first targets Webb analyzed when it began its regular science activities in 2022. The data in this study were collected as part of the Early Science Launch Program 1366, which is designed to help scientists quickly learn how to use the telescope’s instruments and realize their full scientific potential.
The James Webb Space Telescope is the world’s premier space science observatory. Webb solves mysteries in our solar system, peers into distant worlds around other stars, and investigates the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners ESA (European Space Agency) and CSA (Canadian Space Agency).
source: NASA
