The James Webb Space Telescope is studying an exoplanet's eternal day — and eternal night

An extreme close-up of a purple world. To the left, the entire image is shrouded in shadow.

What's the James Webb Space Telescope's favorite movie? Eternal sunshine of the exoplanet WASP-39 b, perhaps.

In a new study, data from the James Webb Space Telescope reveals details of this mysterious gas giant exoplanet, which orbits a star about 700 light-years away from Earth. WASP-39 b is tidally locked to that star, which means the same side of the planet always faces its star. In other words, half of the planet experiences perpetual day, and the other experiences perpetual night.

But the dividing line isn't black and white — there's a proverbial gray area between the two halves that experience the equivalent of eternal dawn and eternal twilight, and it's in these areas that the JWST is revealing new information.

Using data from the JWST's Near-Infrared Spectrograph, astronomers determined the temperature difference between these two transitional areas of the planet: a whopping 300 degrees Fahrenheit (about 200 degrees Celsius). The morning is cooler at 1,150 degrees Fahrenheit (600 degrees Celsius), while the evening is warmer at 1,450 degrees Fahrenheit (800 degrees Celsius).

“It's really stunning that we are able to parse this small difference out, and it's only possible due to Webb's sensitivity across near-infrared wavelengths and its extremely stable photometric sensors,” Néstor Espinoza, an exoplanet researcher at the Space Telescope Science Institute and lead author on the study, said in a statement. "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."

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What's more, models based on the JWST's data also revealed weather patterns in these regions. For one thing, the dawn side is cloudier than the evening side; for another, the prevailing winds on WASP-39 b appear to move from the night side, to the morning, to the day side, then to the evening — at speeds up to thousands of miles per hour.

Astronomers will continue to dive into this dataset to learn more about WASP-39 b and other exoplanets. "WASP-39 b has become a sort of benchmark planet in studying the atmosphere of exoplanets with Webb," said Espinoza.

A paper about these results was published on July 15 in the journal Nature.