Astronomers using the James Webb Space Telescope have mapped the three-dimensional, slightly egg-shaped atmosphere of the hot Jupiter WASP-17b. Published on October 14, 2026, in Nature Astronomy, the Cornell-led team applied a new transmission strings technique to reveal how stellar heat and high-speed winds distort the distant exoplanet’s outer edges.
For decades, mathematical models of distant exoplanets treated their outer edges as idealized circles. A Cornell-led international research team has shattered that assumption by mapping the uneven contours of WASP-17b, a gas giant orbiting a star more than 1,000 light-years from Earth. The resulting shape is slightly egg-shaped, featuring an extended profile on one side that captures how intense heat and whipping winds influence the atmosphere.
Transmission Strings Technique Replaces Averaged Spectra
Conventional transit spectroscopy analyzes stellar illumination passing through an exoplanet’s gas envelope as one combined spectrum averaged across the entire outer boundary, or limb. While effective for basic chemical identification, that method obscures spatial variations caused by equatorial jets, temperature gradients, and weather patterns. To overcome these limitations, Bristol astrophysicists David Grant and Hannah Wakeford developed the transmission strings methodology.
Rather than assuming a uniform circular radius, the innovative software calculates different distances from the center of the world out to the atmospheric boundary across multiple polar angles. By following the path of starlight at precise angles around the planet’s perimeter during a transit, the method distinguishes the atmospheric profile of the evening limb from that of the morning limb. Observations of WASP-17 b were taken with JWST NIRISS/SOSS, NIRSpec/G395H, and MIRI/LRS in March 2023 as part of the JWST TST DREAMS program.
Atmospheric Asymmetry on WASP-17b
Situated 1,300 light-years away from our planet, WASP-17b is an excessively puffed-up gas giant locked in synchronous rotation with its parent star. The transmission maps verified several key dynamical features predicted by three-dimensional general circulation models. High-speed wind streams carry extremely hot gas from the eternally sunny day side toward the night side, causing the atmosphere on the evening side of the equator to puff out and extend much farther than on the morning side.
This is proof of concept that planets are in fact 3D objects, and that consistently, as they go around their star, what we predict should be happening is happening.
Lili Alderson, Cornell University
The analysis confirmed steep temperature and structural shifts between the hot equatorial band and cooler polar regions. Researchers found water and carbon dioxide distributed fairly consistently around the atmospheric edge, even while temperature and shape varied across the limb.
Future Implications for Exoplanet Research
The successful demonstration of transmission strings on WASP-17b paves the way for mapping many more candidates across the broader population of confirmed exoplanets. Lead author Lili Alderson emphasized that the framework provides a scalable model to map latitudinal weather systems and regional chemical compositions.
Planets are temperamental, their atmospheres complex and chaotic. The fact that all of these numbers came together and showed us this very structured, dynamical picture of this giant world over 1,000 light years away was just really exciting.
Hannah Wakeford, associate professor of astrophysics at the University of Bristol
Co-authored by researchers from the University of Bristol and other institutions, the paper details how the process is beginning to differentiate conditions across exoplanet latitudes. Earlier research led by Cornell utilized the Webb observatory to investigate variations in luminosity when an exoplanet passed behind its host star, whereas this novel approach investigates the reverse orbital phase as the world transits across the stellar face. Together, these complementary observations give astronomers a robust way to test whether atmospheric models describe conditions on distant worlds accurately.
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