Saturn's New Decagon: The 100,000 Mile Shape That Appeared While Nobody Was Looking

The Saturn Decagon

Have you ever seen an image and just felt something in the pit of your stomach? That is how I felt when I saw the NASA image of The Saturn Decagon, so down the rabbit hole I went.

In1980, Voyager 1 photographed a hexagon at Saturn's north pole. Six nearly straight sides, fifteen thousand miles across, sitting on a ball of gas. For the next forty five years it was described as unique in the solar system. Scientists said so on the record, repeatedly, in press conferences.

On September 2, 2026, a paper in the journal Science Advances reported a second one. Not a hexagon. A decagon. Ten sides, at the south pole, roughly one hundred four thousand miles across.

The strangest part of the story is not the shape. It is when we found it, and what we were (not) doing during the years it may have been forming.

What is the Saturn decagon?

The location of the decagon

The decagon is a ten sided atmospheric wave circling Saturn's south pole. It was described in "A decagon wave around Saturn's south pole," published in Science Advances on September 2, 2026, by a team led by Agustin Sanchez-Lavega of the University of the Basque Country, with co-authors Amy A. Simon of NASA Goddard, Michael H. Wong of UC Berkeley, Leigh N. Fletcher, Arrate Antunano and Ricardo Hueso.

It is the first large, persistent, regular-sided pattern ever observed in Saturn's southern hemisphere.

Measurement	Value
Latitude	Approximately 58 to 63 degrees south
Width	About 104,250 miles (167,820 km)
Length of each side	About 10,425 miles (16,782 km)
Drift speed	2.5 meters per second eastward
Speed of the jet it sits in	116 meters per second
Full rotation around Saturn	Approximately 800 days
Vertex oscillation period	32 days
Vertex oscillation amplitude	4.6 to 8.4 degrees
Vertical extent	Multiple atmospheric layers

A single side of the decagon is longer than the diameter of Earth.

It is not a surface pattern. Hubble imaged it at multiple wavelengths, which reveals different altitudes in the atmosphere, and the structure appears at every level checked. It extends down into the planet.

The eleven year blind spot

Saturn is tilted on its axis and takes twenty nine and a half years to orbit the Sun. Because of that tilt, the southern hemisphere tilted out of view from Earth in mid-2012 and did not become observable again until 2023.

During part of that window we still had eyes on it. The Cassini spacecraft orbited Saturn from 2004 to 2017 and imaged the south pole extensively. It found a polar cyclone with a hurricane-like eye. It found no long-lived polygonal structure.

On September 15, 2017, NASA deliberately flew Cassini into Saturn's atmosphere to avoid contaminating the moons. There has been no spacecraft at Saturn since.

So the sequence is this. The hemisphere goes dark to Earth in 2012. The only spacecraft there is destroyed in 2017. Visibility returns in 2023. And there is a hundred thousand mile decagon at the bottom of the planet.

Nobody can say when it formed. There is no instrument record covering the gap.

It sat in the archive for two years

Hubble photographed Saturn's south pole in October 2023 with a red filter. In those images there is a faint ten vertex polygon at about sixty three degrees south.

Nobody recognized it at the time.

What actually surfaced the decagon was a hobbyist database. Sanchez-Lavega runs the Planetary Virtual Observatory Laboratory through his university, which collects planetary images from amateur observers worldwide. In 2024, going through that archive, Sanchez-Lavega and amateur astronomers Trevor Barry and Jean-Paul Oger noticed a subtle undulating band near the southern pole.

Ground-based imagery in 2025 strengthened the case. Hubble's Wide Field Camera 3 then imaged Saturn on August 29, 2025 as part of the Outer Planet Atmospheres Legacy program, at 763 nanometers in the red and 889 nanometers in a methane absorption band. The decagon appears in both.

Only then did anyone go back and find it in the 2024 and 2023 images.

Sanchez-Lavega had been searching Hubble images for a southern counterpart to the hexagon since 1990. Thirty three years of looking, and the find came from two amateurs with backyard telescopes.

Why nobody can explain it yet

The wavenumber problem

The standard explanation for Saturn's hexagon comes from a laboratory experiment. In 2010, Ana Claudia Barbosa Aguiar and Peter Read at Oxford published work in the journal Icarus using a thirty liter cylinder of water on a slowly rotating table, with a smaller ring inside spinning faster. The speed difference creates an instability at the boundary, producing an artificial jet stream, which they traced with green fluorescent dye.

Hubble Image of the Decagon

As the ring spun faster, eddies formed along the jet and forced the fluid into a polygon. The key relationship: the greater the rotation difference, the fewer sides the polygon had. Six was the most common outcome, which is why the experiment is treated as the explanation for Saturn's hexagon.

Barbosa Aguiar and Read reported polygons with wavenumbers from two to eight. An earlier rotating annulus experiment by Sommeria and colleagues in 1989 reported wavy jets from three to eight.

Neither produced a ten.

This does not mean a decagon is physically forbidden. Other approaches exist, including deep convection models published in PNAS in 2020 that generate polygonal jets from compressible convection in rotating spherical shells. But the specific body of lab work that made the hexagon feel solved has a demonstrated output range that does not include the shape Saturn just produced.

Three simulations, none of which worked

The research team ran their own shallow water layer model as a first-order test, using fluid depths from five to fifty kilometers. They simulated three scenarios: localized disturbances in the jet, an anticyclone introduced at its observed latitude, and a sinusoidal wave.

None of the three reproduced the observed decagon.

Sanchez-Lavega's own summary, given to Scientific American, is that there is no single model to explain it, and that it may be an instability in the jet stream it sits in or a wave forced from deeper levels of the atmosphere.

The vortex hypothesis that already failed once

There is a suspect. Just north of the decagon, at about fifty five degrees south, sits an anticyclone roughly four thousand kilometers across, which the team informally calls a Red Spot.

The timing is suggestive. The vortex was first observed in 2023, then darkened sharply in 2025, right before the decagon emerged clearly. The spatial correlation supports it too: the decagon is most pronounced near the vortex and least distinct on the opposite side of the planet.

But this exact hypothesis has been run before, on the hexagon, and it failed.

Shortly after the hexagon's discovery, Allison and colleagues proposed that it was a stationary Rossby wave forced by the interaction of the eastward jet with a large anticyclonic vortex visible in Voyager 2 data. When Cassini arrived twenty four years later, the vortex was gone. The hexagon was not. That result is part of what pushed the field toward barotropic instability models in the first place.

Sanchez-Lavega has acknowledged the parallel directly, noting that the same thing happened with the hexagon in 1980, the nearby vortex disappeared, and the hexagon remained.

What is still unknown

• The composition of the aerosols giving the decagon its blue color is unidentified.

• The structure of the wave as it extends into the lower atmosphere is unknown.

• Why the northern hexagon gradually shifted from blue to gold is still debated, after forty five years of continuous observation.

• Whether the decagon will stabilize or break apart is unknown. Unlike the hexagon, whose sides and vertices have stayed uniformly dark for decades, the decagon varies in darkness and some lines appear fuzzy, which Sanchez-Lavega reads as instability or ongoing evolution.

• When it formed cannot be determined from available data.

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Why this actually matters

The takeaway from the lead author is not that the decagon is special. It is the opposite. His stated conclusion is that the discovery suggests the hexagon is not as extraordinary as previously thought, and that conditions in Saturn's atmosphere are such that polygons can form.

Polygons, plural. A property of the planet rather than an accident of it.

Which reframes the question entirely. It was never why does Saturn have a hexagon. It is how many shapes has Saturn made across four and a half billion years, and how many did it make during the eleven years we could not see the bottom of it.

What happens next

Saturn's southern hemisphere is currently moving through spring toward summer, tilting further into the Sun. Sanchez-Lavega has said that if the seasonal insolation cycle plays a role in forming the structure, it may become more pronounced in the coming years, when sunlight at the decagon's latitude peaks around 2032.

For the first time on this question, we are watching one of these structures while it is still forming.

Follow the Fear and Wine Podcast for our upcoming bonus episode on The Saturn Decagon

For further reading:

Sources

Primary research

• Sánchez-Lavega, A., Simon, A.A., Wong, M.H., Fletcher, L.N., Antuñano, A., Hueso, R. "A decagon wave around Saturn's south pole." Science Advances, September 2, 2026. DOI 10.1126/sciadv.aee4251. The paper this episode is built on. Contains the measurements, the shallow water model, and the three simulation scenarios.

• "Deep rotating convection generates the polar hexagon on Saturn." Proceedings of the National Academy of Sciences, 2020. The deep convection alternative to the shallow tank models.

• Preprint of the above (arXiv). Open access. Section 1 contains the full literature review of prior lab experiments and the history of the Allison et al. vortex hypothesis and its failure.

• Sayanagi, K. et al. "Saturn's Polar Atmosphere." arXiv preprint. Comprehensive review of the observational and modeling history, including the critique of the Barbosa Aguiar rotating tank results.

  
  

Institutional releases

• NASA Science. "NASA's Hubble Tracks New Decagon Encircling Saturn's South Pole." Source for the Trevor Barry and Jean-Paul Oger credit and the Sánchez-Lavega quote about searching since 1990.

• NASA Science. "Decagon on Saturn's South Pole (Color)." Image asset page with full caption detail and OPAL program background.

• UC Berkeley Space Sciences Laboratory. "Decagon wave emerges near Saturn's south pole." Source for the unidentified aerosol composition and the 2025 darkening of the anticyclone.

• ScienceDaily. "NASA scientists discover a giant 10-sided pattern on Saturn." Syndicated release with the discovery sequence.

  
  

Reporting with original quotes

• Scientific American. "Scientists discover a mysterious wave shaped like a decagon in Saturn's atmosphere." Source for the "no single model to explain it" quote and the 2032 insolation peak.

• ScienceAlert. "Astronomers Spot an Uncannily Geometric, 10-Sided Structure in Saturn's Atmosphere." The key source. Contains the direct statement that none of the three simulation scenarios reproduced the observed decagon, and Sánchez-Lavega's comparison to the hexagon's vanished vortex.

• Science News. "New Hubble images reveal a bizarre decagon over Saturn's south pole." Source for the Amy Simon quotes on gas giant instability.

• Space.com. "Astronomers discover mysterious 10-sided cloud structure at Saturn's south pole." Source for the dimensions in miles and the 800 day rotation figure.

• BBC Sky at Night Magazine. "Hubble has spotted a 10-sided geometric formation in the planet's clouds." Source for the Planetary Virtual Observatory Laboratory detail.

• Sci.News. "Astronomers Discover Decagonal Wave circling Saturn's South Pole." Source for the Cassini null result and the October 2023 Hubble detection at 63 degrees south.

• Gizmodo. "A Giant Hexagon Has Been Spinning at Saturn's North Pole for Decades. Now, Scientists Found Its Twin." Accessible summary of the shallow water model results.

  
  

The hexagon: laboratory work and historical framing

• Science / AAAS. "Saturn's Strange Hexagon Recreated in the Lab." Coverage of the Barbosa Aguiar and Read rotating tank experiment, including the relationship between rotation difference and number of sides, and the Kevin Baines assessment.

• The Planetary Society. "Saturn's hexagon recreated in the laboratory." Contains images of the tank producing non-hexagonal wavenumbers, including a septagon. This is the visual reference for the wavenumber discussion.

• The Planetary Society. "Creating Saturn's hexagon in the laboratory." Image page with the full experimental setup description.

• NBC News archive, 2007. "Bizarre hexagon spotted in Saturn's clouds." Source for the Kevin Baines and Bob Brown quotes used in the episode cold open, including the "striking differences on opposite ends of Saturn's poles" framing.