The hook. In the September 8 space digest, a line flashed by about a fresh Hubble image: a "decagon" discovered at Saturn's south pole. A meaningless word, unless you know that at this same planet's north pole, a giant hexagon has lived for forty years — a cloud formation whose sides are longer than Earth's diameter. At first I thought the southern structure was just "another one like it," but checking revealed something far more interesting: the southern polygon was only just born, it didn't exist five years ago, and scientists are observing for the first time in history how a gas giant draws a new geometric figure in real time. And the physics that does this is perfectly reproduced in a tank of water on a rotating table.
In 1980 and 1981, Voyager 1 and Voyager 2 flew past Saturn and photographed its poles. Nobody saw anything special during the flybys: the giant frames were studied for years afterward. Only in 1987 did planetary scientist David Godfrey, stitching together panoramas of the north pole (the paper came out in 1988), notice something that shouldn't exist in nature: a cloud band at the 78th parallel north formed a perfect hexagon — a regular six-sided figure encircling the pole like a collar.
Each side of this hexagon is about 14,500 km. For scale: that's longer than Earth's entire diameter. The figure itself isn't a cloud, but the trajectory of a jet stream: gas rushes along the edges at about 100 m/s, and the structure's depth is at least several tens of kilometers. Inside the hexagon sits a circumpolar cyclone as well.
The most honorable part of this story is proof of permanence. After the Voyagers, Saturn's north pole plunged into a fifteen-year polar night, and seeing the hexagon in visible light was impossible. When the sun returned to the pole in 2009 and Cassini photographed it again, it turned out: the figure stands in exactly the same spot where the Voyagers left it, almost thirty years later. Weather on Earth lives for weeks; this structure hasn't changed longer than a human career.
The first thing to understand: Saturn has nothing that could be a hexagon. No continents, no islands, no underwater ridges that would channel the wind. There's nothing there to stop against. The hexagon isn't an object, it's a shape the flow itself has taken. It's best to think of it as a wave: a jet stream running in a circle around the pole bent six times and froze in that bent state.
The physical underpinning is so-called Rossby waves, planetary waves that are born in any rotating fluid because the planet's rotation speed is "felt" differently at different latitudes. On Earth these waves draw our synoptic charts: those familiar bends in the jet stream over the Atlantic that sometimes bring cyclones to Europe, sometimes lock in heat. Saturn has the same mechanics, but taken to the extreme: the planet is huge, spins rapidly — a day is only about 10.5 hours — and the banded flows there are many times more powerful than Earth's.
The key paradox around which debates revolve: a Rossby wave should move relative to the surface. But Saturn's hexagon stands almost motionless and doesn't shift in longitude like other clouds — its rotation period matches the planet's "radio period," which is used to determine its internal rotation. Hence the main question from forty years ago: why has the wave "parked" and held its position?
The answer was sought in a wonderfully down-to-earth way: they assembled Saturn on a table. At Oxford they took a cylindrical water tank (outer radius 30 centimeters), made the outer ring rotate at one speed and the inner disks at another, and started varying parameters. Between two rotating masses of water a shear layer emerged — a laboratory analog of a narrow fast jet flowing between slow latitudes.
And the water itself started drawing polygons. At some flow speeds the jet writhed into a triangle, at others — a square, pentagon, hexagon, up to an octagon. Most often it came out exactly six. The mechanism, it turned out, is banal in the best sense: a narrow fast jet between slow fluid masses is unstable — this is classic barotropic instability, the same process as smoke jets rolling into eddies. The jet's meanders grow, then stabilize, and a whole number of bends fits around the circle — producing a regular polygon. Along each edge small vortices stand watch, evenly spaced around the perimeter, fixing the geometry.
The scandal is that the number of corners depends simply on the ratio of speeds and latitude: faster and narrower jet — more vertices. No Saturnian exotica required: a regular polygon is the natural state of an unstable circumpolar jet under suitable parameters. These 2010 experiments reproducing the hexagon in a tank became textbook classics, and in 2026 fresh work came out from a Roman group: with modern analysis of particle trajectories confirming that barotropic instability both creates and maintains the hexagonal shape.
True, the laboratory analogy has strict critics: in the tank along the edges you see pronounced vortex "streets" that aren't observed on Saturn, and alternative models draw the hexagon slightly differently — as a standing wave on a deep jet. And a new interpretation from the 2020s takes the roots beneath the clouds entirely: a numerical model of deep convection in a rotating spherical shell spontaneously generates polar cyclones, alternating jets, and polygonal meanders — giant vortices from below literally "squeeze" the jet into a polygon. Data from Cassini's final orbits support this: gravity measurements showed Saturn's zonal winds penetrate to a depth of about 9,000 km below the clouds. So the hexagon's roots may reach down there too.
Back to the news that started it all. While the north demonstrated perfect geometry, the south stubbornly stayed silent for decades: neither the Voyagers nor Cassini, which photographed the entire southern hemisphere in 2004, found any polygon there. Scientists honestly searched for a "mirror" hexagon from the nineties — and found nothing. There was just one oddity: in 2004 an annular jet at the 60th parallel south showed barely noticeable polygonal perturbations that lived for several days and vanished forever — remaining a mysterious stamp on the frames.
Then came a detective-story plot. Since 2017 Saturn has had no active probes, and the south pole itself turned away from Earth: due to the planet's axial tilt it wasn't visible from 2012 and returned to view only in 2023. And now the ground-based amateur network, whose images flow to the Spanish PVOL server, notices something strange in 2024: wavy ripples at the south pole. The key frame came from Trevor Barry, a former miner from Broken Hill, Australia, photographing planets from a homemade observatory in his yard. Spanish planetary scientists requested archival Hubble images — and there, in materials from 2023–2024, they found weak, barely distinguishable vertices of the future figure. In fall 2025 Hubble captured it in full glory: around the south pole at latitude about 58–63° lay a ten-sided wave.
The decagon is the complete opposite of the northern veteran. It doesn't stand still but slowly drifts east, its vertices sway with a period of about 32 days, and most importantly — it changes before our eyes: according to Amy Simon, the northern hexagon "has been there every time we've looked for over 40 years," while this feature "appears to be strengthening" right before observers' eyes. If the hexagon is an antique that survived the polar night, the decagon is a baby we'll now watch over. The authors say it directly: for the first time in history, a giant regular structure is being observed forming and strengthening right now. And why it emerged exactly now — the honest answer is "we don't know": too long a gap in observations ate all the evidence.
The tastiest part of this story is the scale of the phenomenon class. In 2016 Juno entered polar orbit around Jupiter and discovered the same handiwork there, just executed differently: around the north pole eight cyclone-vortices are arranged in an octagon around a ninth central one, at the south — five around a sixth; the stable arrangement has held since August 2016. Each vortex is continent-sized, wind speeds in them 70–90 m/s, and this whole company maintains formation for years. Moreover, the arrangement isn't eternal: in 2019 a new cyclone appeared at the south pole, taking the sixth corner. Jupiter draws polygons with vortices, Saturn — with the jet's shape itself, the mechanisms differ, but the aesthetic is the same.
Earth, of course, also participates in the contest, only its polygons dissolve into weather. The jet stream above us is the same wave in rotating fluid, and when its bends "stick," blockages occur — long periods of anomalous weather, from droughts to floods, determining crop fates. In 2018 elegant work came out showing these traffic jams are described by the same mathematics as automobile traffic: the jet has "throughput capacity for weather traffic," and when exceeded, a jam occurs. Saturn with its perfect figures is simply the cleanest educational demonstration of the physics that draws our synoptic charts.
And finally — small scale, literally domestic. If you rotate a cylinder of water not by a ring but by the bottom, at sufficient speed the water surface also spontaneously breaks from a circle into regular polygons: triangle, square, pentagon, hexagon — depending on water level and rotation speed. This phenomenon was first noticed back in 1990 by hydraulic engineer Georges Vatistas — as a defect in industrial centrifuges — and in 2006 it became a journal cover. The same effect is easily found in videos: a glass of water on a spinning platform draws a polygon on the surface. The shape there is different — free surface, not flow — but the principle "rotation + instability = regular polygon" is the same, from a kitchen bucket to a planetary atmosphere.
There's also quite fresh mathematical beauty. In 2025 work came out where Saturn's hexagon was obtained as an exact solution of equations of motion for a thin atmospheric layer: if air in the jet simultaneously rushes in a circle and oscillates slightly across, then the trajectories of its particles trace a regular polygon with rounded corners — a hypotrochoid, the very curve a spirograph draws. The number of corners is set by latitude and intensity of heating from the planet's interior. That is, in some sense Saturn is a giant pen plotter, and its atmosphere signs its own equations.
The persistence of this shape deserves a separate paragraph. The hexagon isn't a typical "storm" that gas giants have plenty of: the Great Red Spot, hundreds of vortices, seasonal outbursts. It's a boundary, a barrier: Cassini recorded how inside the hexagon over polar years the atmosphere cleared, and with summer's arrival became golden with haze, while the haze outside didn't penetrate the figure. In 2018 it turned out the stratospheric warm vortex above the north pole has hexagonal edges exactly coinciding with the cloud hexagon below: the structure extends hundreds of kilometers vertically, like a column. Eternal form, variable content — a formulation any distributed systems architect would sign.
First. Polygons in atmospheres aren't an anomaly but a pattern: a jet stream in rotating fluid, left without shores and islands, self-organizes into regular figures. Saturn's hexagon long seemed a miracle, but the decagon at the south pole immediately demoted it — from "unique phenomenon" to "first noticed representative of a class." In science this looks like a demotion, but in essence it's a triumph of understanding: miracle became mechanism.
Second. The most beautiful part is unity of scales. The same physics of shear instabilities traces a hexagon in a 30-centimeter tank at Oxford, a decagon in a planet's atmosphere a billion kilometers away, and "traffic jams" in the jet stream above my city. A rare case where laboratory table, numerical model, and cosmos converge in one line — usually at least one step in this triad lies.
Third. The decagon story is a quiet hymn to long observation programs and, more importantly, people without degrees. This wave wasn't found by anyone during Cassini's thirteen orbital years — it was found by ground-based amateurs and Hubble's annual photography, for which it's critical that one person from Broken Hill has been conscientiously photographing the planet from his yard with a homemade dome for twenty years straight. Science isn't always supermachines; sometimes it's a miner whose precision is more valuable than a telescope. And the next question — whether the decagon will stabilize into an eternal figure like the northern one — already awaits its observers. Eyes from Broken Hill, as Barry himself promises, will be there.