Before the South-Pole Decagon, Saturn Only Had a Hexagon

A shape hiding in plain sight

The decagon did not announce itself. It accumulated. Hubble's Outer Planet Atmospheres Legacy program, which photographs each of the four giant planets once a year, caught the clearest view, but the first hint came from the ground. In 2024, Sánchez-Lavega and two amateur astronomers, Trevor Barry and Jean-Paul Oger, noticed a subtle wavy band in images of Saturn's south pole. By 2025, more ground data hinted at something stronger. Then the Hubble archive settled it: the ten-sided pattern was already sitting in observations from 2023.

So this is not a brand-new storm. It is a wave we only recently learned to see. Hubble looks at Saturn through different filters that sample different altitudes, and the shape's apparent position shifts slightly depending on the filter, which tells the team the decagon is tall, a vertical structure, not just a smudge at the cloud tops. It sits inside one of Saturn's fast jet streams, and like the northern hexagon, it is a wave trapped in that wind.

I keep returning to how careful that sentence is. Amy Simon runs the OPAL program at NASA Goddard, which means she has spent years watching these planets through Hubble. For her to say she has never seen anything like it is the astronomy equivalent of a forecaster admitting a cloud simply does not fit.

Two poles, two different polygons

The northern hexagon is the elder sibling. Voyager 1 first saw it around 1980, and Cassini confirmed it in 2004, so astronomers have now tracked it for more than forty years. It is enormous. NASA puts it at roughly 20,000 miles (30,000 kilometers) across, with winds near 200 miles per hour (322 kilometers per hour), and says four Earths could fit inside it. At its center spins a hurricane whose eye is about fifty times wider than a typical hurricane eye on Earth. The hexagon barely moves.

The mechanism, as planetary scientists describe it, is a wave trapped on a fast polar jet. The jet's speed and the planet's spin set a preferred wavelength, and around Saturn's north pole that wavelength wraps the pole six times, so the wind folds into six sides rather than a clean circle. The same physics, with a different count, is the best guess for why the south pole now shows ten.

The new southern decagon behaves differently. It appears to be strengthening, and it drifts eastward while the hexagon has sat nearly still for decades. Scientists do not yet know whether it will settle into something stable or keep changing. Here is the part that gets me: Cassini, which orbited Saturn from 2004 to 2017, saw nothing there. The feature probably formed sometime between 2017 and 2023, during a stretch when Saturn's south pole was tipped away from Earth. We missed the birth of a planet's new geometry and only noticed once it was already there.

Why now is the real question

Simon's sharper line is the one every reader lands on. Why would a second giant polygon appear now, after forty years of the first one sitting there in plain view? The honest answer, which I appreciate in a field that loves a tidy press release, is that the researchers do not know. They are running computer models and scheduling more looks with Hubble and the James Webb Space Telescope to work out how the wave formed, how long it might live, and whether it will harden into a stable shape the way the hexagon did.

A second polygon is more than a curiosity. It hands scientists a second data point for a question they have been asking since Voyager: how does a planet's jet stream sculpt a giant geometric shape, and what decides the number of sides? Watching the decagon strengthen, or dissolve, over the coming years is a rare live experiment, because the northern hexagon formed before anyone was looking. Saturn's seasons are slow, a year there lasts about 29 Earth years and each season runs longer than seven of ours, so the south pole has only recently tilted back toward sunlight. Whether that tipped the scales is one of the things the models will test.

The part I like best

What I like best is who helped find it. The first hint came not from a billion-dollar spacecraft but from the ground, where two amateurs, Trevor Barry and Jean-Paul Oger, pointed ordinary telescopes at Saturn and logged images that looked a little off. Their names sit alongside Hubble's in the official account. A discovery about one of the grandest structures in the solar system started, partly, in someone's backyard.

Saturn has always been the planet that rewards a second look. It has the rings, and now it has a hexagon up top and a decagon down below, a planet that cannot seem to settle for a plain circle. The headlines will move on within the week, but the decagon will still be there, and so will the question of how it got there. That is the kind of thing I will still be turning over long after this week's news scrolls past.