James Webb reveals a strange bulge in an alien planet's atmosphere

Astronomers have long depicted distant exoplanets with smooth, circular outlines. But new observations reveal that the atmospheres surrounding some of these scorching alien worlds can have surprisingly uneven shapes. Using NASA's James Webb Space Telescope (JWST), researchers have discovered that the atmosphere of WASP-17b, a giant planet roughly 1,300 light-years from Earth, extends farther outward on one side than the other.

Although its overall outline remains approximately circular, its atmosphere is noticeably asymmetrical. The distortion provides new evidence of how extreme temperatures and powerful winds reshape the atmospheres of distant planets. The discovery, published in Nature Astronomy , comes from a study jointly led by Cornell University in the United States and the University of Bristol in the United Kingdom.

Researchers used an innovative technique called 'transmission strings', developed by Bristol astrophysicists Dr. David Grant and Dr. Hannah Wakeford, to investigate the planet's atmospheric structure in unprecedented detail.

James Webb Reveals the 3D Structure of an Alien Atmosphere WASP-17b belongs to a class of planets known as 'hot Jupiters', enormous gas giants that orbit close to their host stars and experience intense heating. Its unusual atmospheric shape offers a new perspective on how heat moves around these extreme worlds.

Lead author Dr. Lili Alderson, a Klarman Postdoctoral Fellow at Cornell University, began the research while studying at the University of Bristol, where she earned her doctorate in physics. "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.

To move beyond computational exercises and see it for real is a different matter." The observations provide valuable information about WASP-17b's atmospheric chemistry and temperature variations, including differences between its equator and poles and between its permanently illuminated and dark hemispheres.

These results closely match earlier theoretical predictions and computer models. Together, they offer one of the most detailed three-dimensional pictures of an exoplanet's atmosphere yet produced, establishing a useful reference point for studying the more than 6,000 exoplanets confirmed so far.

Co-author Dr. Hannah Wakeford, Associate Professor in Astrophysics at the University of Bristol, said: "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." A New Technique Uncovers Hidden Atmospheric Features James Webb's extraordinary sensitivity has already enabled scientists to investigate alien atmospheres using a method called eclipse mapping.

This approach examines subtle changes in light as a planet moves behind its parent star, helping researchers determine how conditions vary across the planet. The new "transmission strings" technique, also called transit mapping, examines the opposite part of the planet's orbit. As an exoplanet passes in front of its star, some starlight travels through the outer edges of its atmosphere before reaching the telescope.

By analyzing that light from different angles, scientists can investigate the atmosphere's composition and determine how far it extends at different locations around the planet. This outer atmospheric boundary is known as the limb. Traditional methods generally combine light measurements from the entire limb into a single average spectrum.

The new approach instead separates information from different regions, allowing researchers to identify atmospheric variations that would otherwise be concealed. "Rather than assuming your atmosphere is a circle with the same radius all the way around, our technique allows you to pull out different distances from the center to the edge of the planet," Dr.

Alderson said. "We're able to go around the edge. So, we've gone from a ring to understanding where on the ring the gemstones are, so to speak." Powerful Winds Create a Lopsided Atmosphere The new measurements revealed a particularly interesting feature near WASP-17b's equator, precisely where previous models predicted it would appear.

The distortion occurs on the planet's "evening" side, where powerful atmospheric winds carry heat away from the hemisphere that permanently faces its star and toward the hemisphere locked in darkness. This movement of hot gas causes the atmosphere on the evening side to expand farther outward than it does on the cooler "morning" side.

As a result, the planet's atmospheric outline is not perfectly symmetrical. The findings confirm a major prediction about the temperature differences between these regions. The transit mapping measurements also supported other previously identified characteristics of WASP-17b.

Dr. Alderson added: "Theory predicts that the morning and the evening sides should have a strong temperature difference, and WASP-17b fits really nicely in that trend -- pretty much bang-on what we thought it would be." A Breakthrough for Mapping Weather on Distant Planets Successfully applying transmission strings to WASP-17b demonstrates that the technique could be used to study the atmospheres of many other exoplanets.

Until recently, much of what astronomers could determine about these distant atmospheres came from measurements averaged across large regions. The new method offers a way to distinguish conditions at different latitudes, as well as between the day and night sides of a planet. Eventually, this approach could help scientists investigate atmospheric structures on smaller scales, bringing them closer to mapping weather patterns on worlds far beyond our solar system.

Dr. Wakeford said: "Having this very robust mathematical model that we can apply to this state-of-the-art data is incredibly exciting. The results are a really beautiful demonstration of the dynamics that are going on in these multidimensional and multifaceted worlds." The study used observations gathered through JWST's Telescope Scientist Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (TST DREAMS) Guaranteed Time Observations program.

Funding was provided by NASA, the UK Research and Innovation (UKRI) Science and Technology Facilities Council (STFC), and the Klarman Fellowship.