The upstart asteroid who showed rings are for everybody

By Joshua Sokol

As far as anyone knew, the club of astronomical objects with rings only took guests of a certain calibre. Picture Saturn floating majestically over a soundtrack of strings and softly clinking silverware. Apart from Saturn, only the solar system’s other giant planets qualified.

Then a weird little asteroid named Chariklo crashed the party. It pulled the tablecloth out from under theories of ring physics, forcing astronomers to ask: if this misfit thing can have rings, what else could?

The discovery came in June 2013, when observers in Brazil, Chile and Argentina all trained their telescopes on Chariklo, a lumpy rock some 240 kilometres across that orbits the sun between Saturn and Uranus. As Chariklo crossed in front of a background star, they hoped to analyse its shape and maybe find evidence of comet-like jets of ice escaping its surface.

But within a day of the data arriving in the lab of Bruno Sicardy at the Paris Observatory, it started to look as if Chariklo had two narrow, sharply defined rings encircling its main body.

Maryame El Moutamid of Cornell University in New York, Sicardy’s graduate student at the time, remembers him sharing the news in a conspiratorial whisper. Sicardy had also co-discovered Neptune’s rings in the 1980s – could this new find be just wishful thinking?

“I was like, ‘what did you smoke? You see rings everywhere’,” she says. But the analysis kept showing rings, and so have additional observations.

It’s still hard to convey just how strange that is, says Mark Showalter of the SETI Institute in California. It made sense that giant planets could be ringed: their gravitational fields are smooth, which should help rings stay stable.

But we had never seen rings around an atmosphere-free object, not to mention a misshapen lump like Chariklo with a correspondingly bumpy gravitational field. Some theorists doubted it was even possible. “It still boggles my mind,” Showalter says.

Tight and neat

Since then, the questions have only multiplied. Any rings around Chariklo should fall apart after just a few thousand years – so either we spotted them in an especially lucky cosmic instant, or some poorly understood process is keeping them together. And the rings aren’t sloppy, either. They are especially tight and neat, similar to those of Uranus.

One way to keep them tidy is if they are flanked by as yet undiscovered shepherd moons only about a kilometre in size, their gravity tugging stray material back in line.

Since Chariklo’s position puts it in front of the dense star fields at the Milky Way’s centre, there should be plenty of chances to learn more as it crosses background stars. But even as the mystery of Chariklo’s rings lingers, the discovery has shown us that rings can exist in a far wider array of environments than we knew.

Apart from Chariklo, there are hints of rings around the neighbouring asteroid Chiron, Saturn’s moons Rhea and Iapetus, Mars, Pluto, and even the enormous exoplanet J1407b. And many more bodies are now under scrutiny: Sicardy’s team is looking at other asteroids, and the New Horizons probe is searching for ring systems in the farther reaches of the solar system.

In the meantime, the list of confirmed ring-bearers is still small enough to count on one hand. There’s Jupiter, Saturn, Uranus, Neptune – and then Chariklo with a foot in the door, ushering in untold others.