The latest observations from NASA’s James Webb Space Telescope helped scientists prove atmospheric variation on exoplanet WASP-39 b, 700 light-years from Earth. This observation was in keeping with theoretical models of what would be seen at the junctions of eternal morning and eternal evening on this gas-giant planet.
WASP-39 b is a planet 1.3 times Jupiter’s diameter, though it’s only about as massive as Saturn, and it’s tidally locked to its parent star. One side perpetually faces the star and is bathed in constant daylight, while the other is in perpetual darkness. Using Webb’s Near-Infrared Spectrograph, astronomers have now revealed there is a dramatic temperature difference between those two sides. The evening side is about 300 degrees Fahrenheit hotter than the morning side.
The remainder of the work is the brainchild of a team of scientists who used the 2- to 5-micron transmission spectrum technique to probe the terminator, the day-night boundary on the exoplanet, as led by Néstor Espinoza of the Space Telescope Science Institute. It is, in some sense, a relative comparison between the starlight filtered through the planet’s atmosphere during transit in front of the star and the light while it is beside the star. This comparison informs us about the temperature, composition, and other properties of the planet’s atmosphere.
Previous work by Webb on WASP-39 b had already detected carbon dioxide, sulfur dioxide, water vapor, and sodium in the atmosphere across the whole day/night boundary. The new analysis, however, separated the morning and evening sides of the planet and discovered that the evening side is scorching hot, reaching temperatures as high as 1,450 degrees Fahrenheit, while the morning side remained much cooler, at 1,150 degrees Fahrenheit.
Espinoza noted the precision required to make the discovery, “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.”
The research also surveyed the atmospheric structure and cloud cover of WASP-39 b. It still came down to the gas flow around the exoplanet as the clue to explaining the temperature difference. What happens is that, in a so strongly radiated exoplanet, hot gas from the dayside gets transferred to the nightside through strong equatorial jet streaming, creating large differences in air pressure and hence high wind speeds.
Running simulations with general circulation models, very similar to those used to simulate the formation of weather patterns on Earth, the researchers deduced that these winds most likely travel from the night side, across the morning terminator, around the dayside, and then across the evening terminator. That would form the circulation pattern observed: Winds are cooled on the night side, so the morning side ends up the coolest, and the evening side becomes the hottest as it gets hit with the winds heated on the dayside.
“This analysis is also particularly interesting because you’re getting 3D information on the planet that you weren’t getting before,” Espinoza said. Their findings revealed a small swell at the terminator as it approaches the night side of the planet due to the hotter, puffier evening edge.
Their work is published in Nature today, and the team plans to continue studying other tidally locked hot Jupiters with the same method as part of Webb Cycle 2 General Observers Program 3969. WASP-39 b was one of the first targets Webb observed after it began regular science operations in 2022.