Astronomers have successfully measured the rotation of the ultrahot Jupiter WASP-121 b by observing its shadow as it crosses its host star, revealing that the planet’s evening side is significantly hotter than its morning side. According to a study published in Nature Astronomy on June 10 by a team led by Cyril Gapp, this temperature asymmetry is caused by a powerful equatorial jet that pushes heat from the planet’s dayside toward the evening terminator, where water molecules are torn apart by intense heat.
How Astronomers Map Weather on Distant Exoplanets
Scientists identified the temperature difference by monitoring how the planet’s silhouette changed shape during a single transit. Because WASP-121 b orbits its star in just 30 hours, the planet rotates fast enough that the portion of the atmosphere backlit at the start of the transit differs from the portion backlit at the end. By modeling these changing longitudes, the team led by Cyril Gapp detected a distinct chemical signature: an increase in carbon monoxide levels on the evening side. According to the study, water signals dipped in these same regions, a phenomenon known as thermal dissociation where heat rips water molecules into their constituent atoms.
WASP-121 b is tidally locked, meaning one hemisphere faces its star permanently, similar to how the same side of the Moon always faces Earth. This creates a permanent, blistering dayside and a comparatively cooler night side.
The Challenge of Distinguishing Planet Weather from Stellar Noise
Measuring an exoplanet’s climate from light-years away requires separating the planet’s signal from the host star’s interference. WASP-121 b presents a complex target because it orbits a fast-spinning, tilted star that appears brighter at its poles than at its equator. To ensure the results were accurate, the team compared data from two separate James Webb Space Telescope (JWST) observations: one in October 2022 using a near-infrared spectrograph and another in October 2023 using a slitless imager. By modeling out the star’s gravitational distortions and brightness variations, the researchers confirmed that the observed asymmetry was a result of the planet’s own atmospheric circulation rather than an artifact of the telescope or the star.
Future Trends in Exoplanetary Atmospheric Research
This technique provides a new blueprint for studying the weather on other fast-rotating worlds. Astronomers intend to apply this “transit rotation” method to other ultrahot Jupiters, such as WASP-33 b and KELT-9 b. While current climate simulations—which estimate the dayside at roughly 2,800 Kelvin—successfully predict the direction of the temperature difference, they struggle to replicate the exact scale of the heat gap. Moving forward, the refinement of these three-dimensional climate models will be essential to translate raw spectral data into precise, localized weather maps for planets outside our solar system.
When reading exoplanet studies, look for the distinction between “modeled values” and “thermometer readings.” Current technology allows us to infer atmospheric chemistry and heat distribution, but we are still developing the tools to measure exact temperatures at specific surface points on distant worlds.
Frequently Asked Questions
What is an ultrahot Jupiter?
An ultrahot Jupiter is a gas giant exoplanet with an equilibrium temperature exceeding 2,000 Kelvin, often orbiting its host star so closely that its year lasts only a few days or even hours.
How does the James Webb Space Telescope “see” weather on a planet?
The telescope measures the light spectrum as a planet passes in front of its star. By detecting changes in chemical signatures like carbon monoxide and water as the planet rotates, astronomers can map which side of the planet is hotter.
Is the asymmetry on WASP-121 b permanent?
Yes, because the planet is tidally locked, the circulation patterns—including the jet that drives heat to the evening side—are expected to remain consistent over time, according to the research published in Nature Astronomy.
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