Put a Ring on It

Chariklo, a Minor Planet Nicknamed a “Centaur,” Discovered to Have Rings

Chariklo was discovered in 1997, but astronomers recently discovered its two rings. It’s also likely to have one or more moons.

L. Calçada/Nick Risinger/ESO

Saturn’s rings make the planet one of the most beautiful objects in our Solar System. Though the rings look solid, they consist of a huge number of icy particles that reflect sunlight back. Thanks to telescopes and space probes, we also know that other worlds have less showy ring systems: the giant planets Jupiter, Uranus, and Neptune have them, along with the asteroid Chariklo(roughly pronounced “KAReekloe”), which was recently revealed to have two narrow rings made of water ice.

If that last one surprises you, it’s no wonder. Jupiter and Saturn are bright enough to be seen even in light-polluted cities; Chariklo is so small that you have to know where it is to go looking for it. We didn’t even know it existed until 1997, and it had no particular reason for fame until now.

Chariklo definitely is in the “smaller” objects category. No telescope is good enough to measure its shape, but it appears to be about 250 kilometers (155 miles) across, less than the distance between New York City and Boston. (For comparison, Pluto is roughly 2,300 kilometers across, while Earth’s Moon is about 3,500 kilometers in diameter.) I consulted with my astronomer friend David Dickinson, who told me anything short of a super villain’s backyard observatory wouldn’t see it.

Beyond size, another challenge is Chariklo’s location between Saturn and Uranus. It orbits in a long ellipse, ranging from 13 to nearly 19 times farther from the Sun than Earth. This position, along with its composition of rock and ice, marks Chariklo as a “centaur.” Just like mythological centaurs are half human and half horse, astronomical centaurs combine features of asteroids and comets. (Centaurs would grow comet-like tails if they fell closer toward the Sun.) Tens of thousands of centaurs may lurk among the giant planets, though most of those are much smaller than Chariklo, the largest known centaur.

If Chariklo is so small and distant, how did astronomers find its rings? The answer: by watching the asteroid directly pass in front of a star, which is called occultation. (“Occult” literally means “hidden from view,” which is why we use it both in astronomy and to refer to secret knowledge.) The length of time of the star’s disappearance reveals how large the asteroid is.

For Chariklo, astronomers at eight different observatories in Brazil, Argentina, Uruguay, and Chile measured the occultation of a faint star. In addition to the eclipse caused by the asteroid, they found two extra, much shorter blips, evenly spaced on opposite sides of the centaur. Comparing the observatories’ data, the result was clear: Chariklo has two rings, one about 7 kilometers thick, the other about 3 kilometers. (Occultation is also how the rings of Uranus were discovered. And yeah, make your Uranus jokes. I’ve heard ‘em all.)

In fact, the rings help explain a mystery: starting in 1997, Chariklo dimmed abruptly, meaning that something was absorbing some of the light it reflects from the Sun. The nature of the absorbed light indicated water ice was responsible, but researchers couldn’t be sure what was going on. Now the picture is clear: Chariklo’s icy rings were in between the asteroid and Earth during the dim time, absorbing some of the light.

What’s even cooler: these rings are too narrow to exist by themselves. Saturn’s and Uranus’ rings are kept in order by gravity from tiny satellites known as “shepherd moons.” That means Chariklo likely has one or more tiny shepherd moons itself, making the centaur a miniature solar system.

It’s not far-fetched: many asteroids have moons, including Ida with its tiny satellite Dactyl and the near-Earth asteroid 1998 QE2 (nicknamed for the Queen Elizabeth 2 cruise ship). Chariklo’s shepherd moonlets are probably less than kilometer across and therefore hard to spot, but the neatness of the rings makes their existence likely.

So where did these rings come from? The giant planets’ rings—especially Saturn’s bright rings—are probably destroyed moonlets or other small objects that strayed too close. That’s unlikely for a centaur because its gravity is too weak. Instead, something may have collided with Chariklo, knocking debris loose, which was then shepherded into rings. That’s similar to the Moon’s birth: something large struck Earth shortly after it formed, breaking off a big chunk of material that made a ball instead of a ring.

Chariklo’s rings may seem surprising, but if the collision hypothesis is correct, then it’s possible other asteroids and small objects could have them too. In particular, Pluto could be ringed: we know it has at least five moons, which, other than Charon, are tiny. Their orbits are consistent with something big punching Pluto and breaking stuff off; a ring or two wouldn’t be out of the question. Scientists working with the New Horizons mission to Pluto are concerned about that idea, since the probe’s trajectory could take it through the rings—a potential disaster.

We’ll know for sure when New Horizons arrives at Pluto next year. The prognosis is good: no Plutonian ring so far. In the meantime Chariklo showed us it’s possible for tiny objects to be ring-bearers. This centaur is also a hobbit.