A recent research has verified that an exoplanet has differences between its eternal morning and evening atmosphere. The discovery done using NASA’s James Webb Space Telescope confirmed what models have previously predicted.
Tidally locked to its parent star, WASP-39 b orbits a star about 700 light-years away from Earth. One side of the planet is always exposed to the star, while the other is covered in eternal darkness. This giant planet has a diameter 1.3 times greater than Jupiter, but a mass similar to Saturn’s.
Astronomers observed a temperature difference between the eternal morning and eternal evening on WASP-39 b, through Webb’s NIRSpec (Near-Infrared Spectrograph). The Spectrograph revealed the evening atmosphere to be hotter than the morning by roughly 300 degrees Fahrenheit (about 200 Celsius).
The lead author of the study, and an exoplanet researcher at the Space Telescope Science Institute, Néstor Espinoza comments, “WASP-39 b has become a sort of benchmark planet in studying the atmosphere of exoplanets with Webb.”
The new analysis builds 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. Data extracted from the new analysis indicates the temperature difference in the two regions. The data shows the evenings to be notably hotter, with a scorching 1,450 degrees Fahrenheit (800 degrees Celsius), and the mornings to be relatively cooler at 1,150 degrees Fahrenheit (600 degrees Celsius).
Espinoza said, “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.”
“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,” he mentions.
Astronomers confirmed gas circulation around the planet as the main culprit of the temperature difference on WASP-39 b. Researchers are to further investigate the structure of WASP-39 b’s atmosphere, and the cloud cover, through the extensive modelling of the data obtained. This can also help to understand why the evenings are hotter. Additional analysis planned by the team will study how the cloud cover may affect temperature, and vice versa.
In the current understanding, researchers generally expect the gas to be moving as the planet rotates around its star, especially for planets like WASP-39 b, which is highly irradiated and orbits relatively close to its star. This means that hotter gas from the dayside should move through the evening to the nightside via a powerful equatorial jet stream. Due to extreme temperature differences, the air pressure difference would also be significant, which would result in high wind speed.
Researchers employed a variety of models including the General Circulation Models, and 3-dimensional models (similar to the ones used to predict weather patterns on Earth), to understand the wind pattern on the planet. It was found that the prevailing winds are likely moving from the night side across the morning terminator, around the dayside, across the evening terminator and then around the nightside, on WASP-39 b. Therefore resulting into the morning side of the terminator to be cooler than the evening side.
To put it in simple terms, the morning side gets hit by the winds of air that have been cooled on the night side, while the evening is hit by winds of air heated on the dayside.
Another interesting result that the research revealed was the wind speed on WASP-39 b. Research suggests the wind speeds on WASP-39 b can reach thousands of miles an hour!
Espinoza explains, “This analysis is also particularly interesting because you’re getting 3D information on the planet that you weren’t getting before.”
“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,” he adds.
When Webb began its regular science operations in 2022, WASP-39 b became among the first targets to be analyzed. The data in this study was collected under the Early Release Science Program 1366. This program was aimed to help scientists quickly learn how to use the telescope’s instruments and realize its full scientific potential.
The results of the research have been published in the journal Nature.
Moving ahead, the researchers are planning to employ identical methods of analysis to study atmospheric differences of other tidally locked hot Jupiters, as part of Webb Cycle 2 General Observers Program 3969.
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