Saturn Grew a Second Geometric Shape Nobody Anticipated

Keypoints:

• Hubble discovered a 10-sided atmospheric wave, called a decagon, circling Saturn's south pole
• It's the first large, regular-sided jet pattern ever observed in Saturn's southern hemisphere
• The feature has strengthened year over year in Hubble images dating back to 2023
• Each side of the decagon spans more than 10,000 miles
• Findings were published September 2 in Science Advances

For more than four decades, Saturn's calling card has been a single geometric oddity, a six-sided hexagon circling its north pole, first spotted by Voyager in 1980 and mapped in exhaustive detail during Cassini's 13-year orbital mission. Astronomers just found the planet has a second one entirely, a 10-sided wave, or decagon, forming around its south pole instead, a shape nothing in Cassini's own archive ever hinted at.

Led by Agustín Sánchez-Lavega of the University of the Basque Country in Bilbao, Spain, the team pieced together several years of Hubble observations and found the feature already present, faintly, in 2023, growing more sharply defined in each successive year. 

Each side of the shape stretches more than 10,000 miles, wider than the continental United States, and the wave drifts slowly eastward at roughly 6 miles per hour while extending through multiple layers of Saturn's atmosphere. That vertical depth is what convinced researchers this is a genuine, deep atmospheric structure, sitting inside one of Saturn's powerful jet streams, rather than a shallow cloud-level pattern that might dissolve within weeks.

Nobody caught this earlier for a simple, almost mundane reason. Saturn's long seasonal cycle had turned its south pole away from Earth's view for years, and no spacecraft has orbited the planet since Cassini's mission ended in 2017, leaving ground-based telescopes and Hubble as the only instruments capable of watching once the pole finally rotated back into view. 

Amy Simon, principal investigator of Hubble's Outer Planet Atmospheres Legacy program, said the team has never seen anything like it in Saturn's southern hemisphere. "The northern hexagon has been there every time we've looked for more than 40 years," she said. "This feature is different, it appears to be strengthening, giving us the rare opportunity to watch a giant atmospheric pattern develop."

The finding reshapes a longstanding assumption in planetary science. Researchers had spent forty years treating the northern hexagon as a one-of-a-kind fluke, a strange enough shape that nobody expected to find its twin anywhere else on the planet, let alone on the opposite pole. Sánchez-Lavega told Science News the discovery suggests otherwise. 

"This discovery suggests that the hexagon is not as extraordinary as previously thought," he said. That reframing raises a genuinely open question for astronomers studying Jupiter, Uranus, and Neptune next: if Saturn can quietly grow a second polygon on the pole nobody was watching, other gas giants may be hiding similar patterns that simply haven't been caught mid-formation yet.