Webb finds two different twilights on a scorching exoplanet
New research shows that the morning and evening sides of the scorching exoplanet WASP-121 b behave differently, as strong winds spread heat unevenly across the world.
- A team led by the Max Planck Institute for Astronomy measured the gap between the morning and evening sides of the scorching exoplanet WASP-121 b.
- The dayside reaches about 2,500 degrees Celsius, while the nightside drops to roughly 725 degrees.
- Strong eastward winds heat the planet's evening edge more than its morning edge, so the two twilights behave differently.
- The paper was published in the journal Nature Astronomy in June 2026.
A team of astronomers led by the Max Planck Institute for Astronomy, with Cyril Gapp of the University of Heidelberg as lead author, has measured that the two sides of the scorching exoplanet WASP-121 b behave in markedly different ways. The results were published in the journal Nature Astronomy in June 2026. The planet belongs to the class known as hot Jupiters, gas giants the size of Jupiter that orbit very close to their star, leaving one side permanently turned toward the light and the other toward the dark.
What sets this study apart is its focus on the terminator, the narrow zone at the boundary between day and night. Rather than treating the planet as a single whole, the researchers analysed the morning and evening edges separately, the places where, if someone stood on the planet, they would see the star rise and set. The two twilights, it turns out, are not the same.
Why are morning and evening so different on this planet?
The key is the wind. Extremely strong eastward winds blow across WASP-121 b, carrying heat from the dayside to the nightside, but they do not do so evenly. According to the team, these winds heat the evening terminator more than the morning one, and the temperature difference between the two edges is measurable in the very spectrum of light that passes through the atmosphere.
The contrast on this planet is extreme. The dayside reaches about 2,500 degrees Celsius, hot enough that many materials which would be solid on Earth exist here as vapour. The nightside is far cooler, around 725 degrees. That enormous difference drives the circulation of the atmosphere, which is exactly why the boundary zones become interesting to measure, because they reveal how heat is distributed.
In the planet’s hottest regions, water molecules break apart into their components, so water as a stable compound is effectively absent. On the cooler morning terminator, conditions are different, and there clouds made of silicates appear, materials related to rock and glass. The picture that emerges is not a snapshot of one kind of weather, but of two distinct regimes only a few thousand kilometres apart.
What does this measurement mean, and what does it not yet prove?
The value of studies like this lies in the method. Separating the morning and evening sides shows that exoplanet atmospheres cannot be treated as uniform spheres, but as systems with internal dynamics, winds and chemical differences from place to place. That is a step toward more detailed circulation models of worlds we will never see up close.
It is worth being precise about the limits, though. This is a single planet of a very specific kind, a hot Jupiter with little in common with the rocky worlds where conditions for life are sought. The conclusions about terminator asymmetry here say nothing about habitability, nor do they automatically transfer to other types of planets. What the study does reliably show is that today’s instruments are sensitive enough to tell morning from evening on a planet hundreds of light years away, which is itself a significant technical advance.
How much could measurements like these, repeated across many different exoplanets, reveal about the way wind shapes atmospheres throughout the galaxy?
