University of Leicester observations help reveal mysterious shape in Saturn’s clouds
Saturn in the mid-infrared from the European Southern Observatory’s Very Large Telescope (VLT), using the VISIR instrument on August 16th 2025. The images show thermal emission from the planet, with brighter colours indicating warmer temperatures. The 17.6-µm image is sensitive to Saturn’s troposphere, revealing the pattern of warm and cool bands. The 12.3-µm image is sensitive to Saturn’s stratosphere, revealing the seasonal asymmetry between the warm autumn hemisphere and cooler springtime hemisphere. Credit: ESO/VLT/L.N. Fletcher/University of Leicester.
An international team of scientists has discovered a mysterious ten-sided structure, known as a decagon, in the atmosphere of the ringed planet Saturn.
By using the Hubble Space Telescope, images from amateur astronomers and observations by a University of Leicester scientist, they have revealed the presence of a wave around the planet’s South Pole that is a near twin of a long-lived hexagonal structure at its North Pole.
The details of the discovery have been published in the journal Science Advances today (2 September) and add to the continuing mystery of atmospheric phenomena on the gas giants of our Solar System.
When the Voyager 1 and Voyager 2 spacecraft flew past Saturn in 1980 and 1981, one of their many discoveries was the presence, within the planet’s clouds, of a hexagonal wave encircling its north pole. The hexagon has persisted to this day, for more than one Saturnian year (which is equivalent to about 30 Earth years). Despite the symmetry in the distribution of winds and atmospheric dynamics across the planet, in all the years since then, no similar structure had been observed in the southern hemisphere.
But recently, as the planet’s southern hemisphere became more visible from Earth between 2023 and 2025, images taken by amateur astronomers and the Hubble Space Telescope (HST) revealed not a hexagon but a decagonal structure (a 10-sided polygon) around the planet’s south polar region. Images subsequently obtained by the HST in 2024 and 2025, as well as those taken with ground-based telescopes, confirmed its existence.
Infrared (IR) images taken by the Very Large Telescope (VLT) at the European Southern Observatory (ESO) by Professor Leigh Fletcher from the University of Leicester allowed the team to study the thermal structure in the decagon region and obtain wind measurements that show changes in wind speed with altitude. Leicester has been leading a programme of mid-IR Saturn observations for many years using the VISIR instrument, tracking the slow seasonal progression of the planet’s thermal emission, providing the highest-resolution thermal images of Saturn that we can get from Earth. VISIR was retired from the Very Large Telescope in 2026, so this study took advantage of the last Saturn imaging from this facility.
Co-author Professor Leigh Fletcher from the School of Physics and Astronomy at the University of Leicester said: “Saturn still has the ability to surprise us - given the season, Saturn’s southern hemisphere is emerging from more than a decade in winter darkness. Something has clearly changed during that long winter when we weren’t looking, presenting us with this stunning new feature around the South Pole.
“VISIR was essential in providing the thermal structure of the atmosphere in the vicinity of the new decagon. These temperatures allow us to determine the stability of the atmosphere, and how the winds change with height - both vital ingredients to understand how the new decagon appears stable over the 2023-25 period.”
Recent NASA Hubble Space Telescope images show the gas giant Saturn and its southern pole, where astronomers have discovered a 10-sided atmospheric wave. Observations show the decagon extends through multiple layers of Saturn’s atmosphere. Image: NASA, ESA, STScI, Agustín Sánchez-Lavega (University of the Basque Country), Amy Simon (NASA-GSFC), Michael Wong (UC Berkeley); Image Processing: Alyssa Pagan (STScI)
At the same time, HST images taken in different spectral ranges reveal vertical stratification in Saturn’s upper clouds and hazes that make up the decagon. The images show that some vertices of the decagon are less distinct than others, suggesting that this is an evolving phenomenon, perhaps less stable, for the time being, than the northern hexagon.
To track the decagon’s evolution, the research has drawn on an extensive network of amateur astronomers around the world that, using their telescopes, were able to capture images of the decagon’s vertices throughout 2025, thereby enabling its precise movement to be tracked. Like the hexagon, the decagon is embedded in a jet stream with a maximum speed of around 400 kilometres per hour, although the decagon itself, as a whole, moves slowly relative to the planet’s rotation at around 10 kilometres per hour.
Professor Fletcher adds: “What’s really exciting about this work is the involvement of amateur observers with their backyard telescopes, sharing their images online, and searching Saturn’s clouds for new discoveries. It was their data that first hinted something exciting was going on.
“The hexagon itself is now hidden from view, as the North Pole disappears into autumn and winter. Who knows whether that will still be there next time we can look? And how long this new decagon will hang around for?”
Lead author Agustín Sánchez-Lavega of Universidad del País Vasco EHU said: “This research raises many fundamental questions about atmospheric phenomena on gas giants. Why is Saturn the only planet to form this type of polygonal waves? Is the phenomenon in any way related to the 5- and 8-sided polygonal arrangement of the cyclones surrounding Jupiter’s poles? Further observations over the coming years, as visibility of the planet’s southern hemisphere gradually improves, will help to characterise it and develop advanced models that will enable us to solve this intriguing atmospheric mystery.”
- A decagon wave around Saturn’s south pole is published in Science Advances, DOI: 10.1126/sciadv.aee4251