WASP-17b: The Egg-Shaped Planet That's Lighter Than a Marshmallow

For more than thirty years, astronomers have done something a little embarrassing: every exoplanet they studied — no matter how strange — was treated in their mathematical models as a perfect circle. That shortcut just ended. A team led by Cornell University has, for the first time, measured the true shape of an alien planet's atmosphere. It isn't round. It's lopsided — slightly egg-shaped, with one side of its atmosphere puffed out further than the other. The study was published on October 14, 2026, in the journal Nature Astronomy, using data from the James Webb Space Telescope.

The planet that shouldn't hold together

The planet in question is WASP-17b — officially named Ditsö̀, a name chosen in the NameExoWorlds campaign by Costa Rica from Talamancan mythology. It is a "hot Jupiter," a gas giant orbiting its star 1,300 light-years away in the constellation Scorpius. And it is one of the most extreme worlds ever found.

WASP-17b has less than half the mass of Jupiter, yet it's nearly twice as wide — giving it a volume more than seven times Jupiter's. Its average density sits around 0.08 to 0.19 grams per cubic centimeter. For comparison, Jupiter is about 1.33, water is 1.0, and a marshmallow is about 0.21. Yes — this planet is less dense than a marshmallow.

It orbits its star at a distance of just 0.051 AU — roughly one-eighth the distance between Mercury and the Sun — completing a full year in 3.7 Earth days. Temperatures range from around 1,000 kelvins (725°C) on the nightside to nearly 2,000 kelvins (1,725°C) on the permanent dayside. The planet is tidally locked — one face forever staring at its star — and orbits backwards, counter to its star's rotation, the first planet ever found to do so when it was discovered in 2009.

How do you measure the shape of a planet you can't see?

No telescope can photograph WASP-17b directly. Astronomers study it through a trick called transmission spectroscopy: when the planet passes in front of its star, starlight filters through the thin ring of atmosphere around the planet's edge — its "limb" — picking up the chemical fingerprints of gases at different altitudes.

The problem: the old method collapsed all of that information into a single spectrum averaged around the entire limb, effectively assuming the planet was a perfect circle with the same radius all the way around. Everything — morning and evening, equator and poles — got mushed into one blurry measurement.

The new technique, called "transmission strings," developed at the University of Bristol by astrophysicists Dr. David Grant and Dr. Hannah Wakeford, works differently. It tracks the planet's progress across the star's disk during a transit and extracts readings from different points around the atmosphere's edge as the transit unfolds — then mathematically reconstructs how the distance from the planet's center to its atmospheric edge varies from point to point. Instead of one averaged snapshot, it builds a map.

One side bulging, winds doing the work

Applied to WASP-17b, transmission strings revealed an extended, inflated region on the planet's "evening" limb near its equator — the side where winds have just carried blazing heat around from the permanent dayside. The "morning" limb, where gas has spent the night cooling, sits lower and tighter.

Behind this shape sits a broad banded jet racing around the equator, with vast wind speeds that transport material and heat from the irradiated dayside to the shadowed nightside. The hot side puffs up; the cooled side contracts. Exactly what the models predicted — now confirmed in real data.

"This is proof of concept that planets are in fact 3D objects," said lead author Dr. Lili Alderson, a Klarman postdoctoral fellow at Cornell who earned her doctorate at the University of Bristol, "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 weirdest atmosphere you've never heard of

The shape discovery is only the newest layer on one of the strangest atmospheres in the exoplanet catalog. WASP-17b's sky is mostly hydrogen and helium, laced with water vapor and hints of carbon dioxide. And in 2023, the Webb telescope found something no one expected: clouds made of quartz — nanocrystals of SiO₂, the same stuff as beach sand and window glass — drifting through the hot gas giant's atmosphere. It was the first time silicon dioxide had ever been detected in an exoplanet's atmosphere.

Put it all together — a marshmallow-light giant with a quartz-cloud atmosphere, glass crystals floating over a 1,725°C dayside — and WASP-17b now has one of the most complete three-dimensional portraits of any exoplanet among the more than 6,000 confirmed to date. It has become the benchmark other worlds will be measured against.

Why this changes everything

Until now, every picture of an exoplanet you've ever seen was drawn from an assumption: a circle. Transmission strings sweeps that assumption aside. The researchers say the demonstration on WASP-17b opens the door to applying the technique to other exoplanets — moving beyond simple day-versus-night comparisons and beginning to map weather-like patterns on distant worlds.

Think about that for a moment. We went from dots of light, to planets, to atmospheres — and now to weather maps of alien skies. A planet 1,300 light-years away, shaped like a deformed egg, floating like a marshmallow, with glass clouds racing around its equator. The universe, it turns out, was never interested in perfect circles.

Sources & further reading

• Alderson, L. et al., "Transmission strings" study of WASP-17b, Nature Astronomy, October 14, 2026
• Cornell Chronicle: "Pulling strings: New method maps exoplanet's true shape," October 14, 2026
• NASA Science: Exoplanet WASP-17 b artist's concept (MIRI data)
• Space.com: "Bizarre super-puffy exoplanet hosts rare thermometer molecule"
• Wikipedia: WASP-17b