Origin and stability

It is unlikely that the stability of such a system is long term. Contrary to Saturn’s rings, Chariklo’s can only have been there for some tens of thousands of years at most. “Which is very little on the solar system’s scale”, relativises Emmanuël Jehin. “Their discovery is a godsend. At the same time, we’ve only been able to observe occultations by these objects in the past few years, and we’ve already found a system such as this! Either we’re very lucky, or this phenomenon is more common than we think and can be observed for other asteroids. Well, before, we didn’t know that this could exist so we weren’t looking for it. Now, we shall try to unearth them and if it’s relatively common, we should be able to observe others.”

The origin of these rings is unknown. The most probable cause was a collision between Chariklo and a smaller object that ripped off surface material. This material, hurled out during the collision, but still subject to the asteroid’s attraction, went into orbit. “These rings are made of small pieces of quite dirty ice, and are similar to those around Uranus. But the scale is very different. Saturn’s rings have a diameter of 140,000 kilometres compared with only 800 for Chariklo’s."

This difference in scale raises questions about this system’s stability in time. The study of Saturn’s rings led to the conclusion that they were unstable, with a tendency to become distorted. Following numerous calculations, researchers at the time imagined the presence of small satellites orbiting between the rings, confining them around their planet. “These satellites are called shepherd satellites. It was just a hypothesis in the beginning. But when the Voyager space probes approached the giant planets, we discovered a whole series of small moons barely a few kilometres in diameter. Therefore, we can also imagine that these shepherd satellites with a diameter of about one kilometre could maintain the two rings around Chariklo.”

A surprising loss of brightness

Chariklo was observed for the first time in 1997. A surprising fact, its brightness dimmed by almost 40 % until 2008 before rising again. “It was an observation that we were unable to explain”, the researcher reveals. But the presence of rings shed light on this grey area. “At the time, the rings were visible face on and, although they weren’t clearly discernable, they strongly contributed to Chariklo’s brightness. Their angle then changed with respect to us. We saw them gradually from the side. There was no longer the level of brightness they provided when the asteroid was discovered.”

In proportion to these variations, the researchers also noted the gradual disappearance of the content of water ice detected in the Chariklo's spectrum. At first, it was thought that this ice was on Chariklo’s surface. But these traces are now visible again, which leads the researchers to believe the rings are producing this signature and that they are therefore mainly composed of ice. “In fact", the researcher reminds us, “beyond Jupiter, and more precisely beyond what we call the ‘ice line’, the solar system abounds with water in the form of ice. Between this line and the sun, the temperature is too high so the ice sublimated and disappeared, but apart from that, this molecule is very present. For instance, Jupiter’s satellites, as well as Saturn’s rings, are mainly made up of ice, which explains their relatively low density and why they sometimes shine very brightly.”

Chariklo, between a comet and an asteroid

Chariklo is a rather unusual object; it belongs to the family of Centaurs. It is a hybrid object, between a comet and an asteroid, and like its fellow kind, it is very distant in the solar system, between Saturn and Uranus, and contains a large proportion of ice. Asteroids, on the other hand are found, mainly between Mars and Jupiter, and have none or very little water. This is a property of comets that come from the outskirts of the solar system, in the coldest regions where ice has survived. But comets are smaller. The biggest known comets have a nucleus of about 30 kilometres in diameter.