If I talk to you about rings you will think of Sauron, a jewelry store or Saturn. Let’s leave the first two and focus on the planet. Saturn is famous for its rings and holds a special place in the collective imagination for how impressive it looks in photos. However, it is not, by far, the only planet, nor the only body in the solar system, with this type of structure. Other planets have rings but also at least one dwarf planet and one asteroid.
But before we talk about them, let’s see how rings form around a planet . This mainly occurs when a relatively large object, such as an asteroid or a moon, passes the so-called Roche limit could the Moon. This limit is the point at which the gravity of the planet equals the gravity of the asteroid or moon itself.
Imagine, for example, the case of the Earth and the Moon. Gravity on the lunar surface is one-sixth that of Earth’s surface, so there will be a certain distance from Earth where the planet’s gravity equals that of the lunar surface. If we were to somehow make the Moon fall this far, being on the satellite you would feel the same gravitational pull towards the Moon as towards the Earth. At that moment you would begin to float. Therefore the Moon (or any body that is held together due to its gravity), would begin to disassemble, because its parts would be more and more attracted to the Earth. In addition, the part near the Earth would enter the Roche limit before the far part, accentuating this breakup process. A smaller meteorite or rocket won’t feel this effect because what holds them together and in one piece are chemical forces, electrical forces that are vastly stronger than gravity.
The Roche limit would serve not only to create rings from massive objects that enter it, but also to prevent clouds of material from forming a satellite too close to the planet. Being so close to the planet, any satellite that gets large enough will eventually break back into its various components.
Another option is of course to have a collision so violent in the vicinity of the planet, or against the planet itself, that it fills its neighborhood with dust and debris from the collision, as to form a ring around it. However this was more likely in the early solar system (and this is how we think our Moon formed). At present we think that the most usual thing will be that a moon or asteroid enters the Roche limit and breaks up due to the gravity of the planet.
Both mechanisms will have been key in the formation of the different rings that we can observe in the solar system. Everything indicates that the densest and most visible rings of Saturn were formed as a direct consequence of the aforementioned Roche limit. Either a moon got too close to Saturn or the material present during the planet’s formation couldn’t form a large new moon at that distance. However, its outermost ring could have been formed (and is still being replenished today) from particles ejected by its moons, mainly by geysers on Enceladus. Still, the rate at which Saturn’s rings are losing material seems to suggest a relatively recent origin , a few hundred million years.
Jupiter’s rings, by contrast, would be made up solely of material ejected from its innermost moons. Material that was ejected during successive asteroid collisions with these moons. Being so close to the gas giant, these satellites could act as gatekeepers, putting themselves in the path of asteroids attracted to the planet. This greatly limits the amount of material these rings can hold, making them incredibly dim. In fact, they weren’t discovered until the Voyager 1 probe’s visit in 1979.
The rings of Uranus are more complex and brighter than those of Jupiter and that is why they could be discovered from Earth in 1977. They are also relatively young and have a distribution in very fine and differentiated bands, which suggests the presence of several moons in the intermediate gaps, which would concentrate the dust particles in these bands.
Neptune’s rings, which are quite faint like Jupiter’s, have the peculiarity that they do not seem to complete a circumference around the planet, but instead have gaps between different arcs.
But as I said at the beginning, these planets are not the only ones to have rings. In 2013 two separate rings were discovered around 10199 Chariclo , the largest of the centaurs. A centaur, in astronomical contexts, is an asteroid that inhabits the region of the solar system between the orbits of Jupiter and Neptune. Chariklo in particular has an equivalent diameter of about 250 km (because it is not spherical) and two rings located less than 100 km above its surface. These were the first rings discovered around an object other than a planet. This discovery surprised the scientific community , due to its unexpectedness. At present we do not have enough information to determine the origin of these rings, but they are also very young, with a lifespan of a few million years, or they are constantly replenished by an unknown mechanism.
Another body with a similar ring is Haumea , a dwarf planet that orbits beyond Neptune and whose ring was discovered in 2017. Haumea is larger than Chariklo, hence the different classification, although it is believed that its ring could have similar characteristics. Haumea’s ring is within its Roche limit , which would prevent it from regrouping to form a satellite, like the two satellites that do orbit the dwarf planet at more than 10 times the distance of the ring.
No rings have been discovered around any exoplanets at present, but the presence of these structures on all four gaseous planets in the solar system and even on smaller bodies indicates that they should be relatively frequent, although quite difficult to detect with current instruments.
REFERENCES:
J.L. Ortiz et al, 2017, The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation, Nature, https://doi.org/10.48550/arXiv.2006.03113
