Saturn's Famous Hexagon
For over forty years, Saturn has been showing off a party trick unlike any other in the solar system: a colossal, six-sided jet stream at its north pole. First spotted in images from the Voyager flybys in the early 1980s, the hexagon is a stunningly persistent
weather pattern. Each of its sides is wider than our own planet, and the entire structure churns with winds moving at over 300 kilometres per hour. Yet, despite this speed, the geometric shape remains remarkably stable, rotating in sync with the planet's interior. For years, planetary scientists believed this feature was utterly unique. While probes like the Cassini spacecraft observed it changing colour with the seasons—shifting from blue to gold as sunlight created new haze—the six-sided shape itself remained a constant, perplexing feature. It stood alone as a testament to the strange physics governing the atmospheres of gas giants.
A New Shape Joins the Family
In early September 2026, the story of Saturn's lonely hexagon was completely rewritten. Using the Hubble Space Telescope, astronomers discovered a second, even larger polygon: a ten-sided 'decagon' swirling around the planet's south pole. This came as a major surprise. The Cassini spacecraft, which orbited Saturn from 2004 to 2017, never saw such a structure. The discovery was only possible recently because Saturn's southern hemisphere, which was tilted away from Earth's view for over a decade, has finally turned back toward us. Combing through recent Hubble images, scientists were able to trace the decagon's emergence back to 2023, suggesting it's a relatively new and evolving feature. Its discovery was aided by a network of amateur astronomers who helped monitor the planet, filling a crucial observation gap left after the Cassini mission ended.
The Science of Planetary Polygons
So, how does a planet known for its rings start drawing geometric shapes in its clouds? The answer lies in its jet streams. Scientists believe these polygons are a type of 'standing wave' or 'Rossby wave'—essentially, a giant meander in a fast-moving river of air. A jet stream flowing at a specific speed can get 'pinched' by surrounding air currents and eddies, forcing it into a stable, repeating pattern. The number of sides—six for the hexagon, ten for the decagon—is determined by the specific conditions of the jet stream and the atmospheric layers it flows through. This phenomenon has even been recreated in laboratories on Earth using rotating tanks of fluid, which can produce squares, hexagons, and other polygons depending on the speed of rotation. While the exact triggers are still being investigated, the underlying physics suggests that under the right conditions, these bizarre shapes are a natural outcome of fluid dynamics on a planetary scale.
Similar, But Not Identical Twins
While both poles now host a polygon, they are far from identical. The northern hexagon is a seasoned veteran; it has been stable for at least 45 years and appears to be a deeply embedded, rigid structure that extends hundreds of kilometres into the atmosphere. The southern decagon, by contrast, is the new kid on the block. It appears to be more dynamic and less defined, with some of its ten sides appearing sharper than others. Observations suggest the decagon is not a rigid structure but a meandering wave, with its vertices shifting back and forth over a cycle of about 32 days. This suggests scientists may be watching the formation of a polar vortex in near-real-time, an unprecedented opportunity. One theory is that a nearby vortex, a smaller high-pressure storm, may have provided the initial 'push' that kicked off the wave, which then settled into a ten-sided pattern.
