At 1,300 light-years from Earth, a giant planet has an atmosphere that is much more extended on one side than the other.
This planet is called WASP-17b. It is almost twice as wide as Jupiter, while being far less massive. It orbits very close to its star, completing its year in just a few days. One side remains constantly exposed to the scorching light of its sun, while the other stays in darkness.
Multidimensional temperature map of WASP-17b.
Credit: Alderson et al., Nature Astronomy (2026), figure 5, CC BY 4.0.
Researchers already knew that powerful winds must transport heat from one side to the other. But it is difficult to verify what happens on a planet so distant. Even the James Webb Space Telescope cannot directly photograph its clouds or winds as it would for Jupiter.
The team therefore studied the light from the star as the planet passed in front of it. A small part of this light passes through the outer layers of its atmosphere before reaching us. By measuring the absorbed colors, astronomers can determine information about the gases present and their temperature.
Usually, these measurements provide mainly an average for the entire edge of the atmosphere. This time, the researchers used a new method to distinguish different regions around this edge. They found that the air is much more extended on one side than on the other.
The difference is mainly due to heat. The team estimates that these two regions differ in temperature by about 650 degrees. On the dark side, the gases arrive after being heated and expanded on the illuminated side, then carried along by the winds. They eventually contract again as they cool.
Seen through its atmosphere, the planet therefore has a slightly asymmetrical silhouette, almost like an egg. WASP-17b is a gas giant, and the measured difference concerns its atmospheric layers, not a surface.
This new analysis also made it possible to study the presence of water vapor and carbon dioxide around the planet. The observations broadly match models that already predicted powerful atmospheric currents capable of moving heat between the two sides.
The significance extends beyond this single planet. The method could be applied to other worlds observed by James Webb. Instead of describing an atmosphere by an average temperature, astronomers will be able to begin understanding where it is hotter, where gases expand, and how winds circulate.