Rings May Be Common Around a Weird Kind of Asteroid
We know of only one asteroid with rings around it -- but there are likely many, many more.
Chariklo - affectionately dubbed a "strange little object" by one researcher - is an asteroid that orbits in a weird spot. Instead of residing in the usual locations (between Mars and Jupiter, or beyond Neptune) it resides between Saturn and Uranus. What's more, Chariklo has its own ring system. The 2014 discovery is still baffling researchers because the only other rings we know of are around huge planets: Jupiter, Saturn, Uranus and Neptune.
After the discovery, planetary scientist Margaret Pan at the University of Toronto was curious as to how those rings came to be. Recent research she led hints at a solution. Using mathematics and observations of Chariklo, her team suggested a new way the rings could have been created for the asteroid and other space rocks (dubbed Centaurs) in its neighborhood.
Their work suggests that as Chariklo moved out of the Kuiper Belt of icy objects (beyond Neptune) towards its current orbit, the sudden heating would have caused outgassing of carbon monoxide or nitrogen gas. This process would have lifted dust particles off the surface, eventually forming the rings. This theory competes with another idea previously brought out in the literature, which is that Chariklo picked up a small moon while in the Kuiper Belt, a moon that eventually broke apart into a ring when Chariklo passed close to Neptune. Pan said the latter scenario is likely rare; only some Centaurs could get rings that way.
"(Our) scenario predicts that all 100 km-class (62-mile) Centaurs should have some kind of orbiting dusty material. We're eager for future Centaur observations to help distinguish between these formation pathways," Pan wrote in an e-mail to Discovery News.
Why the 100-kilometer limit? Pan says that outgassing on smaller objects wouldn't have enough mass to stop the dusty material from flying away. She adds that Kuiper Belt objects wouldn't be expected to have these rings, because they're too far from the sun and thus too cold to experience this outgassing. Even if a Kuiper Belt object moved close enough to a planet to split a little orbiting moon apart, the trajectory would inevitably change the asteroid into a Centaur.
Observations of Chariklo are limited so far, but astronomers have seen that it has two rings. They are wider on one side of Chariklo than the other, with the second ring only containing about one-tenth the mass of the first ring.
Pan's team suggests that the ring is not quite circular (elliptical). The particles don't form into a circle due to the individual gravitational interactions between particles. The research implies that for this to happen, you would need to have enough material to create a 1-km (0.62-mile) sized ice ball. A typical particle would also have to be a few meters in size.
When comparing Chariklo's rings to other ringed bodies in the solar system, it appears most similar to Uranus despite its diminutive size.
"The geometry of Chariklo's rings, which are fairly dense and narrow, is most like that of the Uranian rings. Interestingly, some of the Uranian rings are also noticeably elliptical, and studies of the Uranian system inspired our treatment of Chariklo's rings," Pan wrote.
Pan plans to next examine how the rings became eccentric in the first place. Some kind of perturbation must have forced them to become eccentric instead of circular, she says. She also is interested (more generally) in seeing if it's possible to predict a ring's eccentricity based on its mass and width.
Pan's research, which has been accepted in the Astrophysical Journal, is available in preprint version on Arxiv.
Artist’s impression of the probable ring system surrounding Chariklo, an asteroid in the solar system.
On Wednesday (March 26), astronomers announced the historic discovery of a ring system around asteroid Chariklo, the largest Centaur asteroid that orbits between Saturn and Uranus in the outer solar system. In this artist's impression, the surface features of the asteroid can be seen with one of the dusty rings blocking the view.
Until now, ring systems have only been seen around Jupiter, Saturn, Uranus and Neptune -- this is the first small celestial body found possessing such a ring system and is indicative of an asteroid collision.
As seen from the asteroid's surface, the ring system would appear pretty dramatic when relecting the weak solar light. The 20 kilometer-wide ring system is 1000 times closer than the moon is to the Earth.
The ring system was discovered as the asteroid drifted in front of a distant star, blocking its light from view. This event is known as an occultation and astronomers got more than they bargained for during this particular celestial dance. As predicted, the asteroid blocked light from the star for a few seconds, but just before and just after the event, there was a slight and unexpected dip in starlight brightness. These extra dips have been interpreted as the presence of a dusty ring system around asteroid Chariklo -- an unprecedented find.
The occultation was predicted and observed by the MPG/ESO 2.2-meter telescope at the ESO's La Silla Observatory. To gain the most detail of the occulation, many South American telescopes participated in the observational campaign.