The overall outline stays roughly circular. But it is noticeably asymmetrical, and the asymmetry is not noise. Alderson said the temperature difference between morning and evening sides matched theory "pretty much bang-on what we thought it would be."

That deserves a caveat. One planet, one observation campaign, one instrument program. The team's models fit, but models fitting is not the same as models proven across the board. Confirmation on other worlds will need to follow.

How does the technique work?

JWST's sensitivity already lets astronomers use "eclipse mapping," which tracks subtle light changes as a planet slips behind its star. Transmission strings, also called transit mapping, watches the opposite moment: the planet passing in front of the star.

Starlight then filters through the planet's atmospheric edge — the limb — before reaching the telescope. Traditional methods crunch the entire limb into a single averaged spectrum. The new method splits the limb into regions and measures how far the atmosphere extends at each point around the ring.

Alderson described it this way: "We've gone from a ring to understanding where on the ring the gemstones are, so to speak."

The observations also captured details of WASP-17b's chemistry and temperature structure, including differences between equator and poles and between its lit and dark hemispheres.

What does this change for exoplanet science?

Plenty, potentially. More than 6,000 exoplanets are now confirmed, and most atmospheric knowledge comes from measurements smeared across large regions. Transmission strings can separate conditions by latitude and by day-versus-night side.

Wakeford, an associate professor of astrophysics at 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."

Eventually, the team suggests, the approach could resolve atmospheric structures on smaller scales — a step toward mapping actual weather on worlds far beyond our solar system. That word "eventually" is doing honest work. Smaller scales mean fainter signals, and nobody has demonstrated it yet.

Still, the WASP-17b result stands as one of the most detailed three-dimensional pictures of an exoplanet atmosphere produced to date. The data came from JWST's Telescope Scientist Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (TST DREAMS) Guaranteed Time Observations program, funded by NASA, the UKRI Science and Technology Facilities Council, and the Klarman Fellowship.

The prediction held. The bulge is real. The next question — does every hot Jupiter have one? — now has a tool built to answer it.