|Artist's rendition of Chariklo, Courtesy of ESO|
This means that most of these objects are too far away for us to ever learn much about them if we can even detect them in the first place. But sometimes we get lucky. If one of these objects passes in front of (and consequently dims the light) of another star, we can use that information to determine how big the object is.
When astronomers observed an asteroid named Chariklo passing in front of a star (not the sun) last summer, they were able to calculate not only its size (250 kilometers in diameter), but something far more amazing. They found that Chariklo has rings. To be exact, it has two rings, one thin one and one thicker one. This makes the asteroid only the fifth object in our solar system to have rings, after Jupiter, Saturn, Uranus and Neptune.
How did the cosmologists come this conclusion? As Chariklo traveled across the path of that distant star, the light from that star was dimmed. But that slight fading didn’t happen just once, as it would if a single body were crossing in front of the star. Instead, there were five light dips: a small one (A), a bigger one (B), a much bigger one (C), then one exactly the same size as B, and finally one the exact same size as A. The regularity of the dips strongly suggests that A and B are rings.
It’s very likely that Chariklo is not the only asteroid with rings. After all, we once thought Saturn was the only planet with rings, and we now know that all the gas giants have them. It may be all the more amazing if relatively tiny asteroids can have rings since none of the rocky planets, which are far larger than asteroids, have rings. Even Mercury is nearly 20 times larger than Chariklo and it doesn't have rings.
You can read more about this at Bad Astronomy.