When a planet crosses the face of its star, astronomers usually treat it as a perfect circle: a disk with a thin, even ring of atmosphere around the edge. That assumption makes the math manageable. A team using the James Webb Space Telescope has now shown how wrong it can be. Their target, the hot Jupiter WASP-17b, has an atmosphere that is lopsided. One side extends farther than the other, and the planet's overall outline is slightly egg-shaped.

The result, published Oct. 14 in Nature Astronomy, comes from a technique the authors call "transmission strings." According to Cornell University, which led the work, it is the first time anyone has resolved the uneven contours of an exoplanet.

A puffy world, locked in place

WASP-17b is an odd planet even by hot-Jupiter standards. Space.com reports that it has about 0.8 times Jupiter's mass but is roughly twice as wide, which puts it in the "super-puff" class and earned it the "marshmallow" nickname. It orbits its star in under four Earth days and is tidally locked, so one hemisphere always faces the star and the other never does. The system is about 1,300 light-years away, according to the University of Bristol.

That arrangement produces a permanent dayside, a permanent nightside, and a boundary ring between them. Seen during a transit, the ring has a "morning" limb and an "evening" limb. Theory has long predicted that the two should differ because winds move heat around the planet. Measuring that difference directly, and turning it into a picture of the limb, is the hard part.

What "transmission strings" does

The authors describe their method as a way to map the atmospheric limb of a transiting planet rather than average it into one number. The paper applies it to WASP-17b using JWST data covering wavelengths from 0.6 to 12 microns. The resulting map captures variations in two directions: east to west, which separates the morning and evening hemispheres, and equator to pole.

The Cornell announcement describes transmission strings as a form of transit mapping and contrasts it with eclipse mapping, a separate approach that Cornell-led research pioneered. Eclipse mapping studies a planet as it passes behind its star, so it mostly sees the dayside. Transit mapping looks at the limb while the planet is in front of the star. Because the two methods sample different geometry, they can check each other.

Bristol says Hannah Wakeford and David Grant developed the technique there. Lili Alderson is the lead author. She is now a Klarman Postdoctoral Fellow at Cornell and earned her PhD at Bristol. Cornell lists 20 co-authors, including Nikole Lewis, Lally and Mullens.

The numbers

The headline figure is the temperature contrast around the limb. The paper puts the morning limb at about 858 K and the evening limb at about 1,522 K, a gap of roughly 650 K.

The error bars are uneven and worth noticing. The morning value carries an uncertainty of +434/-233 K, while the evening value is tighter at +116/-114 K. The cooler side is clearly the harder one to pin down. Still, even at the generous end of its range, the morning limb does not reach the evening limb's central value. The authors also report that the temperature structure agrees with independent measurements of the planet's nightside emission and with eclipse mapping of the dayside. That cross-check matters for a brand-new method.

The chemistry tells a different story. The paper constrains water (H2O) and carbon dioxide (CO2) to constant abundances all the way around the limb. So the temperature changes sharply between the two sides, but these two molecules, at least, do not.

A "gem" on the evening side

The visual result is easy to describe. Bristol says the atmosphere is circular overall but asymmetrical, with one side more extended than the other. A bulge, which the team calls a "gem," sits on the evening side of the equator. That is where wind jets carry heat from the dayside toward the nightside. Space.com describes the atmosphere as a ring with "gems" rising from it.

The physics behind the bulge is intuitive. Hotter gas expands, so a warmer evening side should stand taller than the cooler morning side. A transit with the right resolution would show the planet's silhouette as slightly out of round. The transmission-strings map shows exactly that.

"This is proof of concept that planets are in fact 3D objects," Alderson said in the Bristol release.

Why It Matters

Most exoplanet atmosphere work still treats the limb as uniform, because that is what the data could support. That simplification has a cost. If one side of a planet is hundreds of kelvin hotter and physically puffier than the other, a single averaged spectrum blends two different atmospheres. That can bias the temperatures and compositions that get reported. A method that separates the limb into regions lets researchers test how much this matters, planet by planet.

Bristol describes WASP-17b as a benchmark among the more than 6,000 confirmed exoplanets, and says the technique opens the way to mapping more worlds and eventually to weather-scale patterns. The Cornell framing points to a practical advantage: transit mapping complements eclipse mapping. One looks at the terminator, the other at the dayside, and together they give a fuller three-dimensional picture of how heat moves around a tidally locked planet.

There are caveats. This is one planet, studied with a method just introduced, and the morning-side temperature in particular has wide uncertainties. WASP-17b's extreme puffiness also helps. A thick atmosphere around a large disk is close to the best case for limb mapping. How well transmission strings works on smaller or denser planets is a question for future targets. For now, the result shows that JWST data can resolve some of an exoplanet's shape, not only an average of it.

Sources