Emily LakdawallaJan 27, 2015

A second ringed centaur? Centaurs with rings could be common

Planetary rings are in the news this week because of a press release concerning a gargantuan ring system around a possible exoplanet or brown dwarf. But I have a different ring story to share, one that's much closer to home. A tip on unmannedspaceflight.com led me to this paper by Jose Ortiz and a pile of coauthors, which suggests that Chiron, which is both a centaur and a comet, may also have rings.

If the rings are real, Chiron would be the second known centaur to have them; Chariklo was the first. Centaurs are icy worlds that probably originated in the Kuiper belt before their orbits were perturbed to send them among the giant planets. Alex Parker wrote about centaurs at length in his guest blog on the discovery of the ring system at Chariklo. In both cases, the bodies are too far away and potential rings too dim to be directly observed; what we actually detect is the dimming of background stars during occultations. Chiron and Chariklo are the largest centaurs and the only two for which stellar occultations have been successfully observed.

Ringed Chariklo
Ringed Chariklo Chariklo is the largest Centaur, an icy world in the realm between asteroids and comets. In 2013, it was discovered to have not one, but two, rings. Chariklo is about 250 km or so in diameter. Its inner ring is at 391 kilometers from the center of Chariklo and is 7 kilometers wide. Its outer ring is at 405 kilometers and is 3 kilometers wide. The gap between the rings is 8 kilometers wide.Image: © Don Davis

Ortiz and coauthors note that during past stellar occultations of Chiron, observers noted dimming of the stars before and after the occultation, and attributed them to comet-like jets. But given the discovery of rings at Chariklo, it made sense to look at those Chiron occultations again and see if they might have been due to rings. I can't critique the quality of the arguments in the paper, but a ring around Chiron seems at least as plausible as symmetric jets of comet-like material as an explanation for secondary occultations, and the paper makes some interesting points about things planetary astronomers should think about, if rings turn out to be common in the outer solar system.

The dynamics of planetary rings dictate that they are pretty much circular and pretty much flat. Their orientation in space stays constant over time, and if that orientation is tilted with respect to the ecliptic, we viewers on Earth will observe the ring system more open and better lit at some times, and more closed and less lit at other times. Uranus provides a nice illustration:

Keck's Changing View of Uranus (2007)
Keck's Changing View of Uranus (2007) From 2001 to 2007, Uranus's motion around the Sun has brought spring to its northern hemisphere. These images taken through Keck II's K prime filter. The images also show how the Adaptive Optics system has improved over time. In the final image, the planet seems brighter because Earth (and Keck) are on the dark side of Uranus' rings; when the image was brightened to make the rings more easily visible, the planet was also brightened.Image: Imke de Pater, Seran Gibbard, Heidi Hammel / W. M. Keck Observatory

But we can't get such clear pictures of Chariklo and Chiron; they're too small. And they're the biggest of all the centaurs. When we look at their light, all we can see is a point whose brightness and color contains all the information we have about what's going on at those worlds. If they have rings, changes in ring opening angle can make the brightness of an object seem to change over time. Such a change in brightness has, in fact, been observed with Chiron, but it was previously attributed to cometary activity. If the stuff that makes up the rings has a distinctly different composition from the stuff that makes up the surface of the frozen world, then observing it at different times can make spectral features (particularly those of water) appear and disappear.

At the end, Ortiz et all propose an interesting idea:

An intriguing possibility is whether the color bimodality observed in the color distribution of centaurs (Peixinho et al. 2003, 2012) is due to centaurs with rings and centaurs without rings. Centaurs with rings would have a bluer or a more neutral color than the other centaurs because of the spectral contribution of the rings, which is different to that of the main body. Indeed, the centaur color distribution shows a group with red colors and a group of more neutral colors (among which Chiron and Chariklo are). If the bi-modality is caused by the presence of rings, rings would be very common in centaurs.

More study is needed!

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