Saturn's irregular moons
Saturn's irregular moons are small, distant moons on large, eccentric and steeply inclined orbits, many of them retrograde. They make up the vast majority of Saturn's 274 confirmed moons, and they are thought to be objects captured by Saturn long ago rather than bodies that formed around it. Astronomers sort them into three groups by orbital inclination, named after mythologies: the Inuit and Gallic groups (prograde) and the much larger Norse group (retrograde), which includes Phoebe.
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Regular versus irregular moons
Saturn's moons fall into two broad families. The regular moons, from the ring moons out to Iapetus, orbit close to Saturn's equatorial plane on nearly circular, prograde orbits. They are thought to have formed in a disk around the young planet, or from its rings. The irregular moons are different in every respect: they orbit tens of times farther out, their orbits are elongated and tilted by tens of degrees, and many go around Saturn backward, opposite to the planet's rotation.
These characteristics point to capture. In the early solar system, while the giant planets were still settling into their orbits, countless small bodies passed close to them. Some could be captured if they lost energy, for example through gas drag in the young planet's extended envelope, collisions, or three-body encounters. The same processes seem to have worked around Jupiter, Uranus and Neptune, which also have irregular moon families. The formation and migration of Saturn are therefore linked to the history of these moons.
Three groups, three mythologies
Clusters of irregular moons with similar inclinations are interpreted as collisional families: fragments of a larger captured body that broke apart in an impact. The International Astronomical Union reflects this structure in its naming rules.
The Inuit group
The Inuit group contains prograde moons with inclinations of roughly 40 to 50 degrees. They are named after giants from Inuit mythology: Kiviuq, Ijiraq, Paaliaq, Siarnaq and Tarqeq, among others. Siarnaq, about 40 km across, is the largest.
The Gallic group
The Gallic group also orbits prograde, at inclinations around 35 to 40 degrees. Its moons bear names from Gallic, or Celtic, mythology: Albiorix, Bebhionn, Erriapus and Tarvos. Albiorix, about 30 km across, is the largest.
The Norse group
The Norse group is by far the most numerous and contains the retrograde moons, with inclinations above 90 degrees. Its members are named after giants of Norse mythology, such as Ymir, Skathi, Mundilfari, Suttungr and Thrymr. The largest by far is Phoebe, about 213 km in diameter, discovered by William Pickering in photographs taken in 1898 and announced in 1899. Phoebe is also the source of the vast, faint Phoebe ring discovered in 2009. The second-largest Norse moon, Ymir, is only about 18 km across.
A count that keeps growing
The first wave of discoveries came in 2000, when Brett Gladman and colleagues announced 12 new irregular moons found with ground-based telescopes. Scott Sheppard, David Jewitt and Jan Kleyna added many more between 2004 and 2007 using the Subaru telescope in Hawaii. In 2019 Sheppard's team announced 20 more, and the public was invited to suggest names from the appropriate mythologies.
The largest batches came later. In 2023 Edward Ashton, Brett Gladman and colleagues announced 62 new moons found with the Canada-France-Hawaii Telescope, using a "shift and stack" technique in which many images are shifted along a candidate's expected motion and combined, revealing objects too faint for single images. In March 2025, the same team announced another 128 moons, bringing Saturn's total to 274, more than all the other planets combined. The history of the count is covered on how many moons Saturn has.
Most of the newly discovered moons are only a few kilometers across and many are retrograde. The large number of small retrograde moons, particularly in some orbital clusters, suggests that collisions have continued to break up irregular moons in the relatively recent past, perhaps within the last few hundred million years, although the dating of such events is uncertain.
Naming conventions
Because there are so many, most irregular moons carry only a provisional designation, such as S/2004 S 24, combining the year of discovery and an order number. Once an orbit is well established, a moon receives a Roman numeral and may receive a name. The IAU requires names that fit the group: Inuit giants for the Inuit group, Gallic and Celtic figures for the Gallic group, and Norse giants for the Norse group. A few prograde moons that do not fit neatly into the main groups have also received Norse names. With hundreds of moons, the pool of suitable mythological names is a real constraint, so many of the 2023 and 2025 discoveries are likely to remain numbered for a long time.
How we study them
Irregular moons are too faint and distant to have been studied in detail by Cassini, with the exception of Phoebe, which Cassini flew past in June 2004. For the others, astronomers measure brightness variations to estimate their rotation periods and shapes, and colors to infer their surface composition. Cassini observed several of them from afar, finding rotation periods of hours to days. Comparisons with the irregular moons of Jupiter, and with Kuiper Belt objects, help determine where these small bodies came from. Full tables of moons are in the list of Saturn's moons, and the moons overview puts them in context.
FAQ
What is an irregular moon?
An irregular moon orbits far from its planet on an elongated, tilted, often retrograde orbit. Such moons are thought to have been captured rather than formed around the planet.
How many irregular moons does Saturn have?
About 250 of Saturn's 274 confirmed moons are irregular, the vast majority discovered since 2000 with ground-based telescopes.
What are the Inuit, Gallic and Norse groups?
They are groups of irregular moons with similar inclinations, named after the mythologies used for their names. The Inuit and Gallic groups orbit prograde; the Norse group, the largest, orbits retrograde and includes Phoebe.
Why are most of Saturn's moons retrograde?
The retrograde Norse group contains most of the irregular moons. Retrograde captured orbits may be more stable over long periods, and collisions within this group appear to have produced many small fragments.
Sources
- NASA Science: Saturn's moons
- Minor Planet Center (IAU)
- Gladman, B. et al., "Discovery of 12 satellites of Saturn exhibiting orbital clustering", Nature 412 (2001)
- Jewitt, D. and Haghighipour, N., "Irregular satellites of the planets: products of capture in the early solar system", Annual Review of Astronomy and Astrophysics 45 (2007)