Transmission strings map WASP-17b's uneven atmospheric limb

What the measurement found

The study found that WASP-17b's atmosphere is broadly circular but not symmetrical: one side reaches farther out from the planet than the other. The extended region sits on the evening side of the equator, where jets of wind carry heat from the permanently lit day side around toward the permanently dark night side. The morning side of the same equator is cooler and less inflated. That is where circulation models predicted the bulge, and the measurement found it there.

The study, co-led by Cornell University and the University of Bristol, is the first application of transmission strings to real observations. The paper, "A Spatial Map of the Atmospheric Limb of a Hot Jupiter," was published in Nature Astronomy on 14 October 2026. Lead author Lili Alderson, a Klarman Postdoctoral Fellow at Cornell, began the work during her physics doctorate at Bristol.

WASP-17b is a hot Jupiter, a gas giant in a tight, scorching orbit, and it is tidally locked, so one hemisphere faces permanent day and the other permanent night. It orbits a star about 1,300 light-years from Earth. Results on composition and temperature, from equator to poles and between the day and night sides, match earlier theoretical predictions and other modelling techniques. The team presents WASP-17b as a benchmark for comparison with the more than 6,000 confirmed exoplanets.

How transmission strings works

Traditional transmission spectroscopy reads the planet's limb as a single average. The limb is the ring of atmosphere visible as the planet crosses its star, and averaging it collapses a three-dimensional atmosphere into one radius.

Transmission strings, also called transit mapping, track the planet through the transit and read starlight filtering around the limb from many angles. The method was developed at Bristol by David Grant and Hannah Wakeford. Cornell-led work had already used JWST for eclipse mapping, which measures light changes as a planet passes behind its star.

"We're able to go around the edge," Alderson said. "So, we've gone from a ring to understanding where on the ring the gemstones are, so to speak."

What the result means

The team calls the result one of the most comprehensive three-dimensional views of an exoplanet atmosphere yet produced. The asymmetry is a direct fingerprint of heat and winds reshaping the atmosphere, the researchers said. Alderson said the result is proof of concept that planets are 3D objects and that, as they orbit their star, what theorists predict should be happening is happening.

The team says the successful demonstration paves the way for mapping many more candidates, a first step toward mapping weather-scale patterns in exoplanet climates. Wakeford, associate professor of astrophysics at Bristol, said the numbers came together into "this very structured, dynamical picture of this giant world over 1,000 light-years away".