Sen—When you think of rings in the Solar System, your first thought is likely the majestic rings of Saturn or those of tilted Uranus. Up until last year, the only bodies in our Solar System known to have persistent rings were the giant planets. However, two discoveries announced over the past year are changing this perspective, hinting that many smaller bodies, known as Centaurs which orbit between Jupiter and Neptune, are harboring secret rings and/or extended arcs of icy particles.
Centaurs are a transitory population of small icy bodies making their way from beyond Neptune in the Kuiper belt, where they likely originated, in towards the inner Solar System.
Centaurs have dynamical lifetimes on the order of tens of millions of years; a blink of an eye compared to the age of the Solar System. This is because the Centaurs are on unstable orbits crossing the giant planets. They rattle about the middle Solar System on temporary orbits that get perturbed as they are gravitationally scattered by Jupiter, Saturn, Uranus and Neptune.
Those Centaurs that survive this planetary pinball machine without being ejected are thought to end up as short period comets like the Jupiter family comets. 67P/Churyumov-Gerasimenko, the comet that ESA's Rosetta mission is currently orbiting and that the Philae lander now calls home, is a Jupiter family comet and likely spent some of its history bouncing around the middle Solar System like present-day Centaurs.
Around this time last year came the announcement of the detection of rings around Chariklo, the largest of the Centaurs. The rings can't be seen directly by ground- or space-based telescopes. Instead they were discovered by Felipe Braga-Ribas and collaborators during a stellar occultation, where Chariklo passed in front of a background star, momentarily blocking the star's light.
Stellar occultations are especially handy for measuring the sizes of small bodies in the Solar System. From the brightness alone, we don't know how big a Solar System body is or the physical properties like how much the surface material reflects and absorbs sunlight matter. The duration of the occultation provides a high precision measurement of the size of the body, and if you have multiple chords of the shadow across the Earth you can probe the exact shape of the planetesimal. If there are rings or arcs of material orbiting the occulting body, for each band you'll get symmetric sharp dips preceding and following the occultation of the parent body. This is exactly how the rings of Uranus were detected back in 1977.
During Chariklo's stellar occultation, the calling card of a ring system was seen. The additional dips in the occultation light curve reveal the presence of two rings: ~7 km wide and 3 km wide, located respectively at a distance of 391 km and 405 km from Chariklo. There's also a 8.7 km gap separating the two rings indicating the likely presence a tiny moonlet, analogous to Saturn's shepparding moons, whose gravitational interactions keep the diffusing ring particles tightly confined to the narrow rings.
Rings around Centaurs come as an unexpected surprise. The ring systems around the giant planets formed out of the icy debris left over from the material surrounding the planet after its rocky/icy core formed and the accretion of its gas envelope. But Chariklo is 470 times smaller than Saturn. Not only are the size regimes drastically different, but so are their formation. Some other mechanism is responsible for producing Chariklo's rings.
A handful of Centaurs have been observed to exhibit cometary activity and outgassing events including the presence of a coma. This release of gas and dust from the interior and surface might be involved in the formation of Chariklo's rings. It's unknown how old the rings are and whether they existed before Chariklo left the Kuiper belt or if they are a relatively recent happening. Dynamically the rings are so closely bound to Chariklo that they would not be disturbed or destroyed during gravitational interactions with the giant planets.
Over the past few months has come the announcement of possible rings/arcs of material around Chiron, the first Centaur discovered and the second largest of the known population. Stellar occultation observations by a group at the Massachusetts Institute of Technology (MIT) had solidly detected additional symmetric dips, consistent with previous past observations that were interpreted as possible arcs of material about Chiron produced by cometary jets.
Chiron in the past has been observed to behave like a comet with the appearance and disappearance of a coma. After combining light curve observations with a reanalysis of previous occultation data, a Spanish group proposes alternatively that Chiron has a ring system like Chariklo. The jury is still out whether Chiron has rings or arcs of material in orbit about it, but future occultations and light curve observations should be able to distinguish these two scenarios.
With the two largest Centaurs having orbiting icy debris this potentially signals that rings and orbiting structures are ubiquitous in the middle Solar System.
The largest bodies in the Kuiper belt where the Centaurs came from don't seem to have rings at least not detected yet by occultation measurements. This may indicate that cometary activity and outgassing associated with the Centaur region has something to do with the formation of the rings/arc structures, but more data is needed. More occultation observations of both small body reservoirs over the coming years should help provide more answers but likely more questions as well.