The James Webb Space Telescope has made a groundbreaking discovery about the atmosphere of WASP-121 b, an ultrahot Jupiter. This gas giant, more than 1.5 times the width of Jupiter, has a unique temperature distribution where the evening sky is hotter than the morning sky. This phenomenon is due to the planet's rotation and the resulting temperature variations in its atmosphere.
The key to this discovery lies in the planet's proximity to its star, which is tidally locked, meaning one face is permanently facing the star. This close proximity causes the planet to rotate far enough during a single transit that the slice of atmosphere backlit at the start of the crossing no longer overlaps the slice backlit at the end. This allowed astronomers to observe the planet's rotation and measure its temperature distribution.
The team, led by Cyril Gapp, built a light-curve model that accounted for the planet's apparent size changes during the transit. They found that the side of the planet rotating into view absorbed more carbon monoxide, indicating higher temperatures. This is because water breaks apart in scorching dayside air, while carbon monoxide holds together even in extreme heat.
The study revealed that the dayside of WASP-121 b is lopsided, with the eastern half hotter than the western half. This is due to a deep circulation of heat from the dayside to the night side and a strong equatorial jet that piles heat onto the evening side. The James Webb Space Telescope was able to catch this lopsidedness in motion by observing the planet's rotation.
While the detection is statistically strong, the team acknowledges some caveats. The interpretation relies on a three-dimensional climate simulation, and the model does not fully reproduce the observed asymmetry. Additionally, the study does not provide a precise temperature gap between dawn and dusk, but rather focuses on the direction of the temperature difference.
This discovery opens up new possibilities for studying distant weather patterns. Until now, the difference between a planet's morning and evening had to be teased out from static silhouettes or ground-based Doppler shifts. The James Webb Space Telescope's ability to observe a planet's rotation during a single transit provides a third handle for astronomers, allowing them to study lower-resolution data.
The team suggests that this method could be applied to other fast-rotating ultrahot worlds, such as WASP-33 b and KELT-9 b. However, these planets orbit rapidly spinning stars, which will require further analysis to untangle their distortions. WASP-121 b remains the test case, demonstrating the power of space telescopes in studying distant weather patterns.
