Saturn's north pole has long been home to a mysterious six-sided atmospheric structure known as the hexagon, first spotted by Voyager probes in 1980 and later imaged by Hubble and Cassini. Now, astronomers led by Agustín Sánchez-Lavega of the University of the Basque Country have discovered a second polygonal feature—this time a ten-sided decagon circling Saturn's south pole.

The decagon was first spotted in amateur observations from 2024 by Trevor Barry of Broken Hill Observatory in Australia and Jean-Paul Oger of the French Astronomy Association. Hubble data confirmed the structure and traced its origins back to 2023, revealing a vast atmospheric wave moving at roughly 10 kilometers per hour along an eastward jet stream traveling at 420 kilometers per hour.

Like the hexagon, the decagon is not merely a surface pattern. Multi-wavelength observations show it extends across altitudes and latitudes, indicating a vertically layered structure. However, the two polygons are not mirror images—the decagon sits at a less polar latitude and may be less stable.

Sánchez-Lavega noted that the discovery suggests polygonal waves can form in both hemispheres under Saturn's atmospheric conditions, making the hexagon less unique than previously believed.