Astronomers have discovered a ring system surrounding an asteroid named Chariklo. The finding is a complete surprise to planetary scientists, who are yet unsure exactly how such rings could have formed.
Chariklo orbits in a region between Saturn and Uranus. At 155 miles across, or about the length of Massachusetts, Chariklo is the largest known asteroid in its neighborhood.
Astronomers noticed something odd as Chariklo passed in front on a distant star in June 2013, the star’s light flickered just a bit immediately before and after Chariklo’s pass. The asteroid’s two dense rings briefly blocked the starlight.
The thicker inner ring is about four miles wide, while the thinner outer ring is a little less than two miles. Spectroscopic analysis of the starlight also revealed that the rings are composed partially of water ice.
There are only four other known ring systems in our solar system — around Jupiter, Uranus, Neptune and, most dramatically, Saturn — and all the other ones have formed around planets.
Astronomers aren’t yet sure if Chariklo’s ring system makes it unique among asteroids.
Thin rings around an object will tend to spread out, making their edges fuzzy. Chariklo’s rings appear to have very sharp edges, which might indicate that they contain itty-bitty shepherd moons — just a few miles wide — whose gravitational influence pries open the lane between the two rings and confines their edges. Such shepherd moon are how planets like Uranus keep their thin rings so sharp. But many-ringed Saturn also has a few very thin rings that aren’t maintained by shepherd moons, suggesting that this explanation could turn out to be wrong.
Perhaps studying Chariklo’s simple ring system further could help astronomers understand the behavior of larger rings in the solar system.