A decagon emerges around Saturn's south pole

Saturn is no longer the planet of a single meteorological polygon. Hubble Space Telescope images reveal a meandering, ten-sided band around the south pole, the first regular structure of this scale observed in the planet’s southern hemisphere. The decagon extends across different atmospheric altitudes and behaves differently from the famous hexagon that has encircled the north pole for more than four decades.

Agustín Sánchez-Lavega and colleagues first noticed the undulation in ground-based images submitted by amateur astronomers in 2024. Records from 2025 made the shape clearer; when the team revisited the Hubble archive, it found a subtler version in 2023. The Cassini spacecraft watched Saturn from 2004 to 2017 without seeing a comparable long-lived southern feature. Because the south pole tilted away from Earth in 2012 and returned to view only in 2023, the observations bound—but do not date—the decagon’s formation.

Its scale is planetary: each segment already exceeds 16,700 kilometers, longer than Earth’s diameter. While the northern hexagon remains nearly fixed in longitude, the southern wave migrates eastward at roughly ten kilometers per hour. Hubble observations at different wavelengths, which probe different atmospheric layers, reproduced the outline at several altitudes. That supports the interpretation of a vertically extended wave attached to a jet stream, rather than clouds that happened to draw angles.

The time series also separates the two poles. Voyager 1 and 2 recorded the northern hexagon in 1980 and 1981, and every later look has found it again. The decagon appears more sinuous and variable, with its definition increasing between earlier and recent images. The Outer Planet Atmospheres Legacy program, or OPAL, photographs the giant planets every year. That continuity allowed researchers to recognize a developing pattern instead of mistaking one image for a stable structure.

No mechanism has yet been demonstrated for why the current acquired exactly ten sides. The authors consider whether disturbances from a nearby storm could have triggered the wave, but simulations must test that hypothesis. Saturn is the only known planet with regular polygonal jet-stream outlines. Jupiter arranges polar cyclones in polygons, an atmospheric architecture that offers a comparison but not an established common mechanism.

How long the new shape lasts is now the decisive question. The decagon may settle into a stable configuration like the northern hexagon or vanish as southern seasons and storms evolve. Further observations from Hubble, the James Webb Space Telescope, and ground instruments should measure its speed, vertical profile, and interaction with nearby vortices. If it persists, Saturn will provide two distinct natural experiments in how rapid rotation, jets, and turbulence organize a giant atmosphere.