“Transmission Strings” mapping reveals 3D shape of exoplanet atmospheres

A joint study by researchers at Cornell University and the University of Bristol, published in Nature Astronomy, presents the first direct spatial measurement of an exoplanet’s asymmetrical atmosphere

By applying a pioneering technique called “transmission strings” to data from NASA’s James Webb Space Telescope (JWST), the researchers mapped the distorted, three-dimensional atmospheric limb of the hot Jupiter WASP-17b, moving beyond traditional models that assume perfectly spherical atmospheric boundaries.

Transmission strings vs. standard transit spectroscopy

Traditional transit spectroscopy measures starlight filtered through an exoplanet’s atmosphere as a single averaged spectrum across the entire outer ring, or limb. While effective for basic chemical identification, this approach obscures spatial variations caused by equatorial jets, temperature gradients, and weather patterns.

The transmission strings methodology, developed by Bristol astrophysicists Dr. David Grant and Dr. Hannah Wakeford, overcomes these limitations:

• Variable radial radius:

  • Instead of fitting a circle of fixed radius, the algorithm measures varying radial distances from the planet’s center to the atmospheric edge at multiple angles across the poles.

• Latitudinal and limb resolution:

  • By tracking how starlight passes through specific angles around the planet’s circumference during transit, the technique separates conditions on the evening limb from those on the morning limb.

3D atmospheric profile of WASP-17b

Located 1,300 light-years from Earth, WASP-17b is a highly inflated gas giant tidally locked to its host star. The transmission-spectrum maps verified several key dynamical features predicted by three-dimensional general circulation models:

• Evening limb asymmetry:

  • The atmosphere on the evening side of the planet’s equator is significantly more extended and inflated than on the morning side.

• Equatorial heat transport:

  • High-speed equatorial wind jets actively sweep superheated material from the permanently illuminated day side around toward the night side, causing the evening atmosphere to expand before it cools along its passage to the morning limb.

• Equator-to-pole gradients:

  • The analysis confirmed steep temperature and structural shifts between the hot equatorial band and cooler polar regions.

Research impact and future applications

Lead author Dr. Lili Alderson (Cornell University) emphasized that this successful demonstration serves as a proof of concept for treating exoplanets as complex, three-dimensional worlds rather than uniform spheres. The transmission strings framework provides a scalable model to map latitudinal weather systems and regional chemical compositions across the broader population of confirmed exoplanets.