While Saturn is the uncontested Lord of the Rings, it is by no means the only body in the Solar System to host a ring system.
The flickering effect of stellar occultation – when a planet passes in front of a background star, briefly blocking it – revealed the presence of rings around Uranus in 1977 and Neptune in 1984.
A set of tenuous rings were discovered around Jupiter too, during the Voyager 1 fly-by in 1979.
But only in the last decade or so has it been realised that rings also exist around bodies other than the gas giant planets.
In fact, as Bruno Sicardy at the Observatoire de Paris, France, and his colleagues point out in their research paper, a total of seven objects in the Solar System are now known to have rings.
Rings in the outer Solar System
These newly discovered ring systems exist around small, icy bodies in the outer Solar System, and include Chariklo, a centaur object orbiting between Saturn and Uranus, and the large dwarf planet Haumea, beyond Neptune.
Two rings have been spotted around Quaoar, another large trans-Neptunian object.
Chiron, located between Saturn and Uranus, is now known to be surrounded by a diffuse halo of particles that also contains more dense structures, which may indicate a ring system currently in formation.
A closer look at these strange ring systems
There’s a great deal of diversity in terms of the sizes (relative to their host body) and structures
of these ring systems in the outer Solar System, and they almost certainly originated through different processes.
The rings around Chariklo, for example, probably formed from material ejected from its surface by cometary outbursts during its elliptical orbit.
Haumea is itself a very curious object – it rotates extremely rapidly (taking just four hours) and has been stretched into an elongated ellipsoid shape.
This is thought to be due to a giant collision that knocked Haumea into a frenzied spin and smashed off the material that formed its two moons and rings.
Quaoar's mysterious rings
Quaoar’s two rings are by far the more puzzling.
In contrast to all other rings in the Solar System, Quaoar’s sit well beyond the Roche limit of their parent body.
This means that tidal effects from orbiting the dwarf planet are too weak to break apart larger bodies and the dusty particles should have clumped together into a set of moons.
The discovery of Quaoar’s two rings in 2021 and 2023, therefore, runs contrary to all expectations of how rings can remain stable.
One possible reason for their existence, the authors explain, is that in the super-cold temperatures of trans-Neptunian objects, ice particles tend to bounce off each other rather than accreting.
The researchers end the paper with an intriguing prediction.
Triton, the largest moon of Neptune, is in a slowly decaying orbit, as tidal effects sap its energy.
In about 1.4 billion years, it’s predicted that orbital decay will cause it to dip within the Roche limit of the planet, when it may be ripped apart rather than plunging into the planet’s atmosphere.
If this does happen, the researchers note that Neptune, not Saturn, would then sport the most spectacular rings in the Solar System.
Lewis Dartnellwas reading Origins of Rings in the Solar System by Bruno Sicardy et al. Read it online at: arxiv.org/abs/2412.00853
This article appeared in the March 2025 issue of BBC Sky at Night Magazine.
