It Turns Out Asteroids Can Have Rings. But Why?

Rings are one of the most striking—and uncommon—features of our Solar System. Saturn, of course, is the planet associated with rings, but Jupiter, Neptune, and Uranus also sport them. For a long time, these were considered to be the only ringed bodies in our cosmic backyard. Until now. Observations made at multiple sites in South America have revealed the existence of rings around an asteroid named Chariklo.

The discovery came as a surprise.

“We weren’t looking for a ring and didn’t think small bodies like Chariklo had them at all, so the discovery—and the amazing amount of detail we saw in the system—came as a complete surprise!" says Felipe Braga-Ribas from the Observatório Nacional/MCTI in Rio de Janeiro, Brazil.

The rings were spotted by chance, as the distant asteroid passed in front of a star, thanks to a new camera on the Danish telescope at ESO's La Silla Observatory in Chile. "The camera was specially developed at the Niels Bohr Institute and has a stunningly high resolution, which we especially exploit to look for exoplanets. But when the area where we are looking for exoplanets isn't 'up' in the sky, we use our observation time for other projects and so we followed Chariklo, which just passed in front of a star," explains Uffe Gråe Jørgensen, Associate professor of Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen in Denmark.

Looking at the backlit body, astronomers noticed something odd. A few seconds before, and a few seconds after the asteroid passed in front of the star, its brightness dipped ever so slightly. Comparing observations of these subtle dips from different sites, the team was able to determine not only the shape of the body, but the width, orientation, and other properties of a surrounding ring system.

Astronomers were able to discern two distinct rings around Chariklo, which have been provisionally nicknamed Oiapoque and Chuí, after two rivers near the northern and southern extremes of Brazil.

The rings are pretty small; one measures just 4.4 miles and the other 1.86 miles across. They are separated by a very clear 5.6-mile gap. It’s the asteroid’s small that makes all this so surprising: It measures just 155 miles across and it resides just under 1.3 billion miles from the sun between the orbits of Saturn and Uranus.

Jørgensen put the system into an interesting perspective: "I try to imagine how it would be to stand on the surface of this icy object—small enough that a fast sports car could reach escape velocity and drive off into space—and stare up at a [12.5 mile] wide ring system 1000 times closer than the Moon.”

Finding the rings is one thing, but the discovery has raised far more questions than it has answered. Namely, where did these rings come from? Astronomers suspect that the system formed from debris left over after a collision with another body. The two defined narrow rings suggest the presence of two small shepherd moons like those that keep Saturn’s rings in shape (the existence of which has yet to be confirmed).

Chariklo’s origin is another interesting part of the story. It was once a Kuiper Belt object sitting among the thousands of dwarf planets and comets that orbit beyond Neptune. At some point, it was cast out of this belt into its present orbit where it sits with a collection known as the Centaur objects. Chariklo is the largest of these bodies, and it has been known to astronomers for years, but there was never any evidence to suggest the existence of rings.

Another question is whether the rings will eventually coalesce into a moon for the asteroid. It’s a compelling idea, and if it turns out to be the case, we could apply this chain of events to other bodies, possibly explaining the formation of other satellites around planets, including our own moon. There's still a lot of observation and research left to be done, but the ringed asteroid could once again call into question what we think we know about the solar system.