Hubble spots ten-sided storm at Saturn's south pole, mirroring famous northern hexagon

Saturn has revealed a second geometric puzzle. Astronomers using the Hubble Space Telescope have identified a ten-sided storm encircling the planet's south pole, a structure roughly 104,000 miles across with each side stretching approximately 10,000 miles. The discovery, led by Agustín Sánchez-Lavega of the University of the Basque Country in Bilbao and reported in Science Advances in early September, suggests that the formation of polygonal jets may be a regular feature of Saturn's atmosphere rather than a singular oddity confined to the north.

The finding arrives as a counterpart to Saturn's famous northern hexagon, a six-sided jet that has circled the planet's north pole since Voyager first spotted it over forty years ago. Yet this southern decagon is not simply a mirror image. The difference between six sides and ten carries physical weight: the number of lobes in such a wave reflects the width, speed, and shear of the jet that carries it, meaning the southern jet operates under different conditions than its northern twin. The decagon sits centered near 63 degrees south latitude, riding a fast circumpolar current, and appears to be strengthening rather than weakening.

Part of why this feature went unnoticed for so long comes down to Saturn's geometry. The planet's axial tilt and its long orbital period mean each pole disappears from useful view for years at a time. The south pole was effectively invisible from Earth between roughly 2012 and 2023. The Cassini spacecraft, which orbited Saturn from 2004 to 2017 and had good southern coverage in its early years, recorded a warm polar vortex and a cyclone at the south pole but found no long-lived polygonal jet to match the hexagon. Whatever this decagon is, it does not appear to have been a permanent southern fixture waiting for discovery.

The trail began with amateur astronomers. Ground-based observers including Trevor Barry in Australia and Jean-Paul Oger, working through the Planetary Virtual Observatory Laboratory, captured suggestive structure at the south pole across 2024 and 2025—hints that seemed easy to dismiss in isolation but impossible to ignore in sequence. When researchers went back through Hubble's archive, they found the pattern faintly present in 2023 imagery and increasingly pronounced in subsequent years. Those images came from OPAL, the Outer Planet Atmospheres Legacy program led by Amy Simon of NASA's Goddard Space Flight Center. OPAL was designed precisely for this work: a yearly, consistently calibrated portrait of the outer planets taken with the same filters, year after year, so that atmospheric changes reveal themselves rather than hiding inside differences of technique.

The decagon is not a surface marking but the visible expression of a wave locked into a powerful eastward jet stream. The team found it extends vertically through multiple layers of Saturn's atmosphere—evidence that emerges from a subtle but telling observation. When the same feature is imaged through different filters, its apparent position shifts slightly. Each filter samples a different depth in the hazes and clouds, so a shape that leans with altitude will appear to move as wavelength changes. The polygon is a column, not a decal, a structural echo of the northern hexagon, which Cassini showed to be similarly deep-rooted, its corners persisting through changing seasons and atmospheric color shifts from blue to gold.

The core implication is that Saturn's hexagon may not be a one-off anomaly. If a jet at the opposite pole can organize itself into a regular polygon, then polygon-making is something Saturn's atmosphere does under the right conditions, rather than something that happened once and nowhere else. Laboratory experiments with spinning fluids have produced similar polygons for decades, and the underlying physics is thought to involve instabilities in strong circumpolar flows. But no one is claiming this specific decagon is fully explained.

The open questions are the ones that matter most. Why did the pattern emerge in the 2020s, when Cassini's earlier southern survey found nothing comparable? Is it seasonal, tied to the slow swing of sunlight across Saturn's poles over a 29-year orbit? Will it settle into decades-long permanence like the hexagon, drift, change its number of sides, or dissolve entirely? Answering these questions requires exactly what OPAL was designed to provide: more years of consistent measurement. Continued Hubble monitoring can track whether the sides sharpen or blur and whether the whole pattern drifts in longitude. The James Webb Space Telescope, working at infrared wavelengths that probe temperature and deeper structure, can test how far down the feature reaches. Numerical models of Saturn's jets will have to reproduce both a hexagon and a decagon in opposite hemispheres simultaneously.

For now, the accounting is simple and incomplete: a ten-sided wave, roughly 104,000 miles across, in the southern jet of a planet that has apparently been doing this in more than one place all along.