Chariklo’s strange ring system could be the product of a shepherd moon

The centaur Chariklo has two solid rings that revolve around it. In an attempt to explain their survival, astronomers have proposed a yet-to-be-discovered satellite that they say could keep the rings aligned.

Saturn’s rings have been known for nearly four centuries, but over the past fifty years we have learned that they are only the most dramatic examples of a common phenomenon. The Voyager probes revealed smaller ring systems around the other three gas giants. Between the orbits of Jupiter and Neptune, the bookends of the gas giants are smaller objects known as centaurs.

In 2013, the largest of these, Chariklo, briefly blocked the light of a star (known as an occultation). The star winked at Earth observers when Chariklo eclipsed it, but it also temporarily dimmed four other times, twice before and after each time. These valleys revealed Chariklo’s own pair of rings, with orbits on either side 400 kilometers (240 miles) from the planet’s core.

From the start, there was confusion about how an object with so little mass – Chariklo is only 250 kilometers (150 miles) in diameter and believed to be mostly ice – could retain rings. Since then, a reanalysis of what were once thought to be rings around fellow centaur Chiron indicated that rings are common, without explaining them, before finding that Chiron’s rings are likely an evolving lumpy disk.

Adding to the mystery is that Chariklo’s rings are outside the centaur’s Roche limit, unless their density is much lower than that of Chariklo itself. Beyond the Roche limit, rings should quickly combine into moons.

Subsequent occultations have confirmed the rings’ existence and shown that the innermost one blocks ten times as much light as its companion, allowing astronomers to model what is going on.

The moons Pandora and Prometheus and Saturn’s F ring. These two moons, about 50 miles wide, keep the F ring, which would otherwise be too far away to survive, aligned. Something similar was thought to be going on with Chariklo, but it may only require one moon.

Dr. Amanda Sickafoose of the Planetary Science Institute is among those tackling the problem. “Planetary rings will spread or disperse naturally over time. Chariklo shows two thin rings a few kilometers wide,” she said in a statement. To ensure the rings remain seven to three kilometers wide, there must be “a mechanism to contain the material and prevent it from spreading,” she added.

Sickafoose and co-authors have shown that without such a mechanism, the rings increase in size linearly over time, contradicting the dense, narrow bands indicated by the fairly short dips in brightness. Unless we saw Chariklo almost immediately after the rings formed, there must be something holding the rings tight.

The same models show that a suitably positioned satellite would do this; one about 3 kilometers wide and weighing several tens of billions of tons would be sufficient. We couldn’t see such an object at this distance unless it was perfectly positioned in 2013 to also block the star.

Simulated rings around Chariklo from this study, with a satellite of approximately 3 kilometers radius in an average motion resonance of 6:5.  Ring particles are shown in white.

Simulated rings around Chariklo from this study, with a satellite of approximately 3 kilometers (2 miles) radius in a mean motion resonance of 6:5, producing rings similar to those we observed. Ring particles are shown in white.

Image credit: Sickafoose & Lewis, The Planetary Science Journal 2024 (CC BY 4.0)

Previously it was thought that two shepherd moons might be needed.

Sickafoose noted: “We think the ring particles are mainly composed of water ice, like those on the giant planets. We don’t know the exact characteristics, such as how ‘hard’ or ‘soft’ the ring particles are when they collide, or the particle size distribution.” Rings made of hard particles would be maintained indefinitely by the proposed moon, but beyond the Roche limit, soft particles would be much more likely to become moons themselves.

An alternative explanation is a gravity anomaly on Chariklo’s surface, and a rotation rate that causes the force of this anomaly to keep the rings aligned.

Although Sickafoose described the physics of the two scenarios as similar, such a surface mass concentration seems less consistent with what we know about small planets. On the other hand, it is much easier to imagine that an earlier ring turned into a moon and then kept the remaining moons stable, something that is thought to explain Saturn’s F ring, which is maintained by the small moons Pandora and Prometheus.

Although the idea that Chariklo has a moon seems to solve the mystery, Sickafoose’s recent reevaluation of Chiron’s rings suggests that we shouldn’t be too confident without more observations. If the two centaurs turn out to have completely different environments, it will raise even more questions, such as which provides the better analog for Haumea or Quaoar, and how many other arrangements exist.

The research has been published open access in The Planetary Science Journal.

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