There are a few things with rings in our solar system; five to be exact, including Jupiter, Saturn, Uranus, Neptune…and now the asteroid Charilko. Similar to the old game, ‘one of these things is not like the others,’ Charilko is the only one of the five that is not a very large planet. The discovery has surprised astronomers who have expressed uncertainty as to how so small an object could have retained such rings.
Granted, Charilko is the largest of the asteroids in the belt between Jupiter and Saturn, a class of asteroids called “Centaurs.” But it is still thousands of miles smaller than even the smallest of the gas giants previously thought to be the only “ring-bearers” of this solar system. To give the comparison a sense of scale, Neptune, the smallest entrant previous to now, is 30,599 miles in diameter; and Earth, which of course was before now thought to be too small to retain such rings, is 7,918 miles across.
The scientists who discovered the rings were stunned, since they were not even looking for such evidence in the first place. The discovery was in effect an accident.
Astronomers at the European Southern Observatory in Chile were actually using the Danish telescopes housed there to search for exoplanets, i.e. planets from outside this solar system. They were observing a distant star called UCAC4 248-108672 when they noticed flickering right before the asteroid crossed in between their telescopes and the star, and once again right afterward. This occultation of the star’s light before and after the body of the asteroid eclipsed the star indicated that something else surrounding the asteroid had come between observers and the star before the asteroid itself did.
Seven other observatories in South America noted the phenomenon first seen at ESO in Chile, as the occultation could only be observed from that continent. Further study of this asteroid and its possible rings yielded a great deal more information.
Using data collected from all sightings, astronomers were able to determine that the rings around this asteroid are made of ice and rock, and are a double set. One ring is three kilometers wide, another seven kilometers. The two rings are separated by nine kilometers of space between them, and they are very dense for their thin size. It is now considered an astronomical fact that the asteroid Charilko is to be included in the category of things with rings in this solar system.
In ring systems around the gas giants, separations between rings are usually caused by moons called shepherd moons which orbit within the rings. It is likely that there are some larger bodies like tiny moons orbiting the asteroid within the ring system and causing this division. On the other hand, Saturn has many rings without shepherd moons, which makes the study of Charilko’s rings important to furthering our understanding of how ring systems work.
Despite searches over the decades, no rocky planetoid has been found to have rings before now, causing scientists to believe that only the huge gas giants could form ring systems. Charilko is the first ever object not a gas giant to have been found to have such rings, causing astronomers to revise their theories about how such rings could have been formed.
One theory is that impacts on Charilko’s surface could have thrown up dust that became caught in the large asteroid’s gravity to become rings. Another is that a nearby Centaur-type asteroid might have been destroyed in an impact, with Charilko’s gravity capturing the pieces to form rings and a tiny moon or two.
The Centaurs are so named because they are a rare mix of comet and asteroid, as the centaur of Greek mythology was a mixture of man and horse. As such, the Centaur bodies, 250 of which are known to exist in our solar system, are composed of ice and rock as opposed to being simply rocky bodies like so many other asteroids.
As the largest of the known Centaurs, Charilko is about 155 miles across, which is about the length of Massachusetts. It is amazing to consider the size of Charilko in comparison with the other massive bodies in our solar system to have ring systems, including Earth’s largest planetary neighbors. There is a very short list of things with rings in this solar system; but now that list must include an asteroid, making Charilko a small brother in very large company.
By Kat Turner