A New Shape in the Clouds
In early September 2026, astronomers announced a stunning discovery: a massive, 10-sided pattern, or decagon, encircling Saturn’s south pole. This colossal atmospheric wave was spotted in images from the Hubble Space Telescope, which has been monitoring
the outer planets for over a decade as part of its Outer Planet Atmospheres Legacy (OPAL) program. While Hubble provided the sharpest views, the first hints actually came from the ground. In 2024, a team led by researcher Agustín Sánchez-Lavega, along with amateur astronomers, noticed a strange, undulating band in images of the planet's southern hemisphere. By piecing together observations, scientists traced the decagon's faint origins back to 2023, noting that it has become progressively more distinct since. Unlike a fleeting storm, this appears to be a large-scale, vertically deep structure, extending through multiple layers of the atmosphere.
Not Saturn's First Polygon
This discovery is particularly exciting because Saturn is already famous for a similar, yet distinct, geometric feature. Since the Voyager spacecraft flew by in the 1980s, we have known about a persistent, six-sided hexagon at its north pole. That northern hexagon is a remarkably stable jet stream, so large that multiple Earths could fit inside it. For over 40 years, it has remained a fixed and puzzling feature, leading many scientists to believe it might be a unique fluke in the solar system. The Cassini spacecraft, which orbited Saturn from 2004 to 2017, studied the hexagon extensively but found no similar long-lived structure at the south pole. The emergence of the southern decagon proves that such polygonal waves are not a one-off anomaly, suggesting the conditions that create them may be fundamental to Saturn's atmosphere.
How an Atmosphere Makes Shapes
How can a planet’s weather system form a near-perfect polygon? The answer lies in what gas giants like Saturn don’t have: a solid surface. On Earth, jet streams are disrupted by mountains and the varied temperatures of oceans and continents, creating chaotic and unpredictable weather. Saturn, by contrast, is a massive, spinning ball of gas with a much more uniform composition. This allows its powerful jet streams—rivers of air moving at hundreds of kilometres per hour—to flow in more regular, symmetric patterns. Under the right conditions, disturbances in these jet streams can settle into stable, standing waves. Laboratory experiments have shown that spinning a circular tank of liquid at different speeds can spontaneously create hexagons, squares, and other polygons, mimicking the physics at play in Saturn's atmosphere.
Theories on the Decagon's Birth
While scientists are confident the decagon is a jet stream wave, they don't know exactly what triggered it. One leading hypothesis points to a nearby storm. Just north of the decagon's location, astronomers have been tracking a powerful anticyclone—a high-pressure vortex similar to a smaller, temporary version of Jupiter's Great Red Spot. This vortex intensified right before the decagon became more prominent, leading some researchers to believe it may have provided the initial disturbance that kicked the jet stream into its 10-sided shape. Computer simulations support this idea, showing that a periodic disturbance can indeed create such a wave. However, scientists are cautious. A similar vortex was once near the northern hexagon but later vanished, leaving the hexagon behind.
An Unfolding Mystery
The southern decagon is not simply a mirror image of its northern counterpart. Its 10 sides, compared to the hexagon’s six, is the most obvious difference. But it is also located at a different latitude, being less polar than the hexagon. Perhaps most intriguingly, the decagon appears to be moving. While the northern hexagon is almost stationary relative to the planet's rotation, this new southern feature is slowly drifting eastward. Because it appears to have formed so recently, scientists have a rare and precious opportunity to watch a giant planetary pattern develop in real time. Is it a temporary feature, or will it settle into a stable state like the hexagon? Why did it form now, and why with 10 sides? These are the questions astronomers are now rushing to answer.
