Saturn's north pole has hosted a hexagonal jet stream for more than forty years, unchanged since the Voyager flybys first spotted it. Straight edges in a planetary atmosphere seemed like a northern-hemisphere quirk — Cassini orbited Saturn for thirteen years and never detected any polygon in the south. On September 2, *Science Advances* announced otherwise: at 63 degrees south latitude, a ten-cornered wave is growing around Saturn's south pole.
How It Was Found
Saturn's axial tilt kept the southern hemisphere facing away from Earth for over a decade. The turning point came in October 2023, when Hubble captured a faint ten-vertex structure in a jet stream at 63°S. By August 2024 the outline had sharpened. On August 29, 2025, Hubble imaged it at 763 nanometers, and ground-based telescopes confirmed all ten vertices fully formed, with uneven brightness flickering around the circle. NASA's OPAL program, which takes annual portraits of the outer planets, provided more than a decade of archival data that proved decisive. This feature is genuinely new.
- 1980s — Voyager discovers the northern hexagon
- 1990 onward — Astronomers begin searching for a southern counterpart
- 2004–2017 — Cassini's thirteen-year mission finds nothing in the south
- October 2023 — Hubble images faint ten-vertex structure at 63°S
- August 2024 — Outline becomes clearer
- August–September 2025 — All ten vertices fully formed
- September 2, 2026 — Paper published in *Science Advances*
The Newcomer and the Old-Timer Are Not Alike
The paper's corresponding author, Agustín Sánchez-Lavega of the University of the Basque Country, has searched old images for a southern counterpart since 1990, reasoning that Saturn's northern and southern jet streams look symmetric — what exists in the north should exist in the south. Cassini's images were exhaustively examined. Nothing. What makes the decagon so striking is its age: Hubble's data traces it back no earlier than 2023. It was not there before; it was recently born. Less than three years old.
| | North hexagon | South decagon | |---|---|---| | Corners | 6 | 10 | | Age | 40+ years observed | Under 3 years | | Behavior | Completely stable | Still developing |
Amy Simon of NASA's Goddard Space Flight Center, OPAL's lead, was近乎 astonished in the press release: we had never seen anything like this in the southern hemisphere. The northern hexagon is there every time we look. What fascinates her most is why it formed now.
Why a Gas Planet Draws Polygons
Saturn has no coastlines, no mountains, nothing to block the wind — its atmosphere is nearly unconstrained. In such a world, stable geometry can only emerge from the jet stream itself: when the velocity difference across a polar jet crosses a threshold, the flow becomes unstable and waves arrange themselves along latitude circles — six peaks make a hexagon, ten peaks make a decagon. This explanation has been refined over decades for the northern hexagon, with many competing models. But this time nobody rushed to apply them, because the newcomer is vertical: the wave structure penetrates several atmospheric layers, not just a skin at the cloud tops.
How it penetrates, why it formed now, and how long it will last — the paper leaves a string of open questions for Hubble and JWST to pursue.
By comparison: the northern hexagon has 46 years of observations; the southern decagon is at most three years old — an age gap of over ten times, with four extra corners. A new experiment in planetary atmospheres has just opened, and we hold tickets from opening night. The next time someone tells you gas giant atmospheres are boring, show them this decagon — it is a planet, with a 100,000-kilometer-wide mouth, slowly learning a new shape.