October 18, 2026
One side of this planet is 650°C hotter than the other — and Webb just mapped it in 3D
Illustration by The Discovery Wire — artist's concept of WASP-17b's uneven atmosphere (hot extended evening limb in red, cooler morning limb in blue)
Every planet you've ever seen drawn is a perfect circle. A Cornell-led team just proved that, for at least one planet, the circle was a lie.
Using the James Webb Space Telescope and a brand-new analysis method called "transmission strings" (also called transit mapping), astronomers have mapped the true shape of an exoplanet's atmosphere for the first time. The result: WASP-17b — a puffed-up "hot Jupiter" about 1,300 light-years away — is subtly egg-shaped, with one side hundreds of degrees hotter than the other. The study, led by Dr. Lili Alderson of Cornell University with colleagues at the University of Bristol, was published October 14, 2026 in Nature Astronomy (DOI: 10.1038/s41550-026-02986-4).
From a ring to the gemstones on it
For decades, studying an exoplanet's atmosphere during a transit meant blending all the starlight filtered through the planet's edge into one single spectrum — a uniform ring, averaging together the morning and evening sides, the equator and the poles. That approach could identify molecules, but it washed out exactly the spatial variation that models of scorched planets predict should be there.
Transmission strings abandons the circular assumption. Instead of fitting one radius, the technique — developed by Bristol astrophysicists Dr. David Grant and Dr. Hannah Wakeford, and run through the open-source light-curve fitting software Harmonica — lets the fitted radius change continuously with angle. As different parts of the planet enter and leave the stellar disk, each slice of the atmosphere gets its own spectrum.
"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 centre to the edge of the planet," Alderson said. "So, we've gone from a ring to understanding where on the ring the gemstones are, so to speak."
A 650-degree divide
The team applied the method to WASP-17b, an almost ideal test case: about half the mass of Jupiter but roughly 1.9 times its radius, swollen by heat, with an equilibrium temperature near 1,760 kelvin. The analysis combined Webb observations from three instruments — NIRISS, NIRSpec, and MIRI (0.6 to 12 micrometers), plus Hubble optical data — from the TST DREAMS Guaranteed Time program.
The numbers: roughly 850 kelvin at the morning limb and 1,500 kelvin at the evening limb — a divide of about 650 kelvin (the same numerical difference in °C). Because hotter gas rises higher, the evening side's atmosphere puffs up to nearly 1.8 times the scale height of the morning side, giving it deeper, larger absorption features. The two halves of the limb are, the team showed, "genuinely distinct worlds" — fitting one terminator's spectrum with the other's model fails decisively.
The mechanism is winds. An eastward equatorial jet carries heat from the permanently lit dayside toward the night side, inflating the atmosphere where the gas arrives — the evening side. Water vapor and carbon dioxide, the most reliably constrained molecules, turned out to be broadly consistent all the way around the limb: the chemistry is even, the weather is not.
"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," Alderson said. "To move beyond computational exercises and see it for real is a different matter."
Why it matters
This is the first step toward weather maps of distant worlds. The technique is beginning to differentiate conditions across latitudes — not just day versus night — and the WASP-17b results now give the more than 6,000 confirmed exoplanets a benchmark for comparison. Apply transmission strings to enough targets, and astronomers can start sorting which atmospheric patterns are universal and which belong to each planet's own circumstances.
The team was also careful about the limits: the cooler morning region is less tightly constrained, clouds and hazes couldn't be reliably mapped, and the analysis assumed the northern and southern hemispheres behave the same. This isn't a finished weather chart — it's the first sketch of one.
For now, the picture is striking enough: a planet 1,300 light-years away, drawn for the first time not as a circle, but as it is — lopsided, windswept, and hundreds of degrees divided against itself.
Sources: Cornell Chronicle / University of Bristol press releases (Oct 14, 2026); Alderson et al., "A Spatial Map of the Atmospheric Limb of a Hot Jupiter," Nature Astronomy (2026), DOI: 10.1038/s41550-026-02986-4. Illustration (press kit): Engine House and Hannah Wakeford.
Source: Cornell Chronicle — read the original report
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