Hubble discovers a second polar storm on Saturn – a decagon at the south pole

For more than forty years, the hexagonal vortex at Saturn's north pole has remained one of the strangest structures in the solar system. Now astronomers have spotted something similar on the opposite side of the planet.

The Hubble Space Telescope has captured a growing decagon-shaped wave around the south pole, according to a paper published Wednesday in Science Advances.

The structure appears to have formed sometime between 2023 and 2025 and seems to still be changing today. That's a key difference from its northern "twin," which has held its shape for more than four decades.

The hexagon first caught scientists' attention in 1988, when researchers combed through data from the Voyager spacecraft's 1980 flyby of Saturn. The probes' trajectories didn't allow them to image the entire shape, which spans twice the diameter of Earth. A full view of the vortex only became possible once the Cassini spacecraft arrived at the planet.

The hexagonal storm measures roughly 20,000 to 25,000 miles (30,000 to 32,000 kilometers) across, with jet streams reaching about 180 to 200 miles (300 km) above the cloud tops. Given how stable it's been, planetary scientists long expected to find a similar wave in one of the two subpolar jet streams in the southern hemisphere. For decades, searches turned up nothing, whether at the south pole or anywhere else.

Saturn's tilted axis got in the way. Starting in 2012, the planet's south pole simply wasn't visible from Earth, and observations didn't resume until 2023. That October, Hubble images already showed a ten-sided structure, and by August 2024, as the southern hemisphere tilted further into view, the shape became more distinct.

A year later, the telescope captured all ten corners of the shape fully formed. According to the study, three sides of the decagon stand out sharply, four are less pronounced, and the remaining three stay blurred.

The new structure's defining trait is its mobility. The longitudes of the decagon's vertices shift back and forth on a roughly 32-day cycle. That behavior suggests the shape isn't a rigid structure at all, but rather a meandering atmospheric wave held in place by the curved path of a powerful jet stream.

These properties in the movement of the decagon indicate the existence of a complex dynamical behavior, never observed in the hexagon, becoming a subject that deserves future attention.

To understand how it formed, researchers ran shallow-water model simulations to recreate the wave's behavior. The calculations point to two possible scenarios – either the shape arose from a repeating disturbance along the jet stream's peak, or it was driven by a large, counterclockwise-rotating vortex to its north.

If either scenario holds, the decagon would be a temporary, evolving phenomenon. The hexagon, by contrast, remains a stable wave that has held together for decades even amid active storm activity nearby. Comparing the two structures gives planetary scientists fresh material for understanding how storms form and evolve on other worlds.