Saturn Grew a Decagon at Its South Pole While Nobody Watched

Saturn has had a hexagon at its north pole for as long as we have been able to look at it properly. As of this week it has a decagon at its south pole, and the strange part is that nobody saw it arrive.

NASA published Hubble tracking on 3 September confirming a giant, evolving ten-sided atmospheric wave encircling Saturn's south pole. It is the first large regular-sided jet pattern ever recorded in the planet's southern hemisphere.

What the Saturn decagon at the south pole actually is

It is a jet stream forced into a standing wave. A fast band of wind circling the pole develops a fixed number of bends, and because the atmosphere beneath it is deep and largely frictionless, those bends hold their shape for years instead of smearing out. The northern hexagon has kept six sides since Voyager photographed it in 1981.

Ten sides rather than six is a difference in wind speed and latitude, not in physics. The number of bends depends on how fast the jet runs relative to the rotation of the planet, which is how the two poles can carry different shapes at the same time.

The gap where it formed

Astronomers first noticed a subtle undulating band in 2024. Ground-based imagery in 2025 strengthened the case, and a trawl back through the Hubble archive showed the pattern was already there in 2023.

Cassini is what makes the dating possible. It orbited Saturn from 2004 to 2017 and photographed the planet more thoroughly than anything before or since, and it saw nothing like this at the south pole. So the decagon formed somewhere between 2017 and 2023, in the window after the orbiter was gone and before anyone from Earth thought to look closely.

A feature the size of a continent grew on another planet, and the only reason we can put a date range on it is that a telescope in Earth orbit happened to be pointing that way. Both Hubble and JWST have further observations planned.

Watching Saturn from the ground

None of this is visible from a back garden. The polar shapes need a spacecraft or a space telescope, and an amateur instrument at best shows the disc, the rings and a handful of moons.

That is still worth doing. Saturn is well placed this autumn, and a steady pair of binoculars such as the Celestron Cometron 7x50 will show it as a distinctly non-round point of light, roughly what Galileo saw in 1610 before anyone worked out what the rings were. A small telescope resolves them properly.

For the view no telescope gives you, our piece on Webb's sharpest image of the Lion Nebula covers what happens when two instruments see the same object at different wavelengths, and the Roman Space Telescope is the survey instrument built to catch exactly the kind of change nobody was watching for.

What to watch

The decagon is not necessarily stable. The northern hexagon has held for four decades, but a wave that appeared inside a six-year window could lose or gain sides inside one. Hubble now has a baseline, so the next few years of imaging will show whether Saturn's south pole settled into a shape or is still working one out.

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