To properly understand the difference between a comet and an asteroid, we have to take a small step back in time. When the solar system was created, a large nebula of gas and dust contracted to form a gigantic disk around the sun, the protoplanetary disk. This dust stuck together and formed planetesimals, asteroids ranging from a few metres to several hundred kilometres in diameter. By continuing to collide, these asteroids ended up forming the planets we know today. “What remains of this period is the asteroid belt, which contains millions of them that haven’t stuck together, orbiting in relatively stable orbits between Mars and Jupiter. They are mainly made up of rock. Astronomers have discovered around 400,000 with a diameter larger than 100 metres, and we believe there must be more than a million of them. But many more of them are smaller."

The Kuiper belt, an area that lies beyond Neptune, was also formed at this time in much colder regions. It contains icy asteroids that are called ‘trans-neptunian objects’. “At this distance, we can only observe the biggest specimens. Astronomers have discovered about a thousand of them since 1992 thanks to increasingly powerful telescopes. All of them have a diameter greater than 100 kilometres and we believe that there are also millions of smaller comets but they are too small for us to see them directly at such distances. From time to time, as a result of some gravitationnal instability, one of them leaves  its orbit and passes into our sky. Its sudden proximity with the sun sublimates the ice on its surface, giving it its tail. Beyond Kuiper’s belt, at about one light year away, is the Oort cloud, a spherical reservoir centred on the sun that is made up of billions of comets that orbited between the giant planets a very long time ago.”

And what if something else was behind the rings’ origin?

The small solar system bodies are indeed residues from the formation of the solar system. “And”, explains the astrophysicist, “the difference between comets and asteroids is becoming increasingly nuanced. Astronomers have discovered comets in the asteroid belt, while others assert that some asteroids still contain water; there is probably a continuum between the two types of object.”

And to make the link, there are the Centaurs, such as Chariklo, half-asteroid, half-comet. Orbiting between the giant planets and constantly perturbed by their attraction, these objects generally have a more unstable orbit, and will probably be ejected from the trajectory in several thousand of years. Which still leaves to the researchers plenty of time to study them !

“In the meantime, the hybrid nature of Chariklo has led me to imagine another hypothesis to explain the origin of its rings”, Emmanuël Jehin continues. “Some Centaurs present in the same region display comet-like behaviour and, occasionally, a tail similar to that of comets. This has never been observed on Chariklo. But I wonder whether there hasn’t been comet-like activity in the past, with the ejection of material in space. Part of this material could have been trapped and kept in orbit. Chariklo is indeed quite a big asteroid.”