March 27, 2014
Planetary rings aren't reserved exclusively for the likes of Saturn, Jupiter, Uranus and Neptune. A team of astronomers found a centaur – a minor planet with characteristics of both asteroids and comets – in our solar system that has two rings.
Artist's impression of Chariklo with it's two rings. Credit: ESO/L. Calçada/M. Kornmesser/Nick Risinger
The centaur in question is Chariklo, discovered in 1997. It is currently the largest known centaur with a diameter of 250 kilometres. Part of its orbit lies between Saturn and Uranus.
As Chariklo passed in front of a distant star in June 2013, seven telescopes located mostly in South America observed a dip in star's luminosity as the centaur passed in front of it and blocked its light. Only one of them had a camera sensitive enough to notice two additional, shorter dips in brightness. European Southern Observatory's La Silla telescope noticed one dip before and one after the longer dip which obviously represented Chariklo.
After comparing data from all telescopes, lead author of a study published in yesterday's edition of Nature magazine, Felipe Braga-Ribas of the National Observatory in Rio de Janeiro, Brazil, and his team figured out that two shorter dips must come from two rings only 3 and 7 kilometres wide. This was surprising, considering the size of Chariklo.
"We thought that maybe having rings was linked with the mass of the object. So finding them on a small object was very unexpected," Braga-Ribas says.
Previous observations have shown the composition of Chariklo changing from time to time, which was puzzling. However, if the rings made of ice were one time seen head-on and other side-on, it could explain those inconsistent observations.
How were planetary rings formed and remain stable is a mystery. It is possible the material spun off with time as Chariklo rotated. If there was a collision involved, it must have been a small and slow one, because a centaur with the diameter of 250 kilometer doesn't have gravitational pull strong enough to keep fast-moving objects in its orbit.
As far as the stability of rings is concerned, astronomers haven't found any yet, but one or more small moons could be orbiting Chariklo among the rings and keeping together the material rings are made from. Further studies are sure to follow and when we find out what those rings are made of, we will probably have more answers about their origin and how they remain stable.
"The shepherd mechanism seems to be universal from the giant planets to the small minor planet," Braga-Ribas said. "This mechanism may be acting in other kinds of debris discs, such as proto-planetary nebulae and galaxies."
Although this observation comes as a surprise, other small objects in the Kuiper belt and between orbits of gas giants, like Chariklo, could have rings. They are small and dark and thus not easy to study, so it's a small wonder we only now found one with rings. Inner planets don't have rings because solar winds are too strong and blow away small pieces of dust and ice before they can form rings, but all four outer gas giants have rings, most famous of which are those around Saturn and so could centaurs and dwarf planets, despite their low mass.