Every planet you have ever seen drawn was a perfect circle. That was a lie — at least for WASP-17 b. Using the James Webb Space Telescope, astronomers have produced the most detailed spatial map yet of an exoplanet’s atmospheric limb, revealing a dramatic 650-kelvin temperature divide between the morning and evening sides of the puffy hot Jupiter 1,300 light-years away. The work, published in Nature Astronomy, pushes the study of exoplanet atmospheres decisively into three dimensions.
A team led by Lili Alderson of Cornell University and the University of Bristol used a novel technique called “transmission strings” — extracting spectra at multiple angles around the planet’s terminator as it transits its star. They measured a morning-side temperature of about 858 kelvin and an evening side of roughly 1,522 kelvin. The hotter evening limb is also more extended, stretching farther into space, consistent with heat-carrying winds dragging gas from the permanently irradiated dayside. Water and carbon dioxide abundances remained broadly consistent around the limb.
Why the egg shape matters
For decades, astronomers treated the atmosphere visible during a transit as a single uniform ring, averaging the whole circle. That approach can identify molecules but washes out spatial variation. The new method fits the planet’s outline as a shape free to differ from a circle — and WASP-17 b is subtly egg-shaped. “Having this very robust mathematical model that we can apply to this state-of-the-art data is incredibly exciting,” said Bristol astrophysicist Hannah Wakeford, who developed the technique with David Grant.
A new era for exoplanet weather
Astronomers have confirmed more than 6,000 exoplanets, but reconstructing the 3D structure of their atmospheres remains difficult. Transmission strings demonstrate a way to extract spatial information from transit light, opening the door to distinguishing conditions across latitudes, investigating heat transport, and testing whether theoretical models accurately describe distant climates. It is not yet a weather map — but it is the first real measurement of an alien world’s atmospheric asymmetry.
