February 9, 2023 | David F. Coppedge

Ring Around the Theory

We all fall down, say theorists about a ring
around a minor planet where it shouldn’t exist


Quaoar is a minor planet beyond Neptune that has been known since 2002. Classed as a Trans-Neptunian Object (TNO), it is the seventh largest minor planet (Pluto is #1). Astronomers, using occultation techniques, inferred a dense ring orbiting Quaoar. Problem: it shouldn’t be there.

Satellites around small bodies are not as surprising, but rings? According to theory, rings form when objects move into a radius from a body called the Roche Limit and break up. What has long been accepted is that tidal forces break apart objects within the Roche limit; then, the disrupted debris becomes a ring. Another TNO (Haumea) has a ring, and a space rock around beyond Saturn has one. Those rings fit the Roche theory. Finding a ring outside Quaoar’s Roche limit was not only a surprise, but a threat to long-standing ideas in textbooks. Here’s what news reports say about this:

A dwarf planet beyond Neptune has a mysterious ring that astronomers can’t explain (Space.com, 8 Feb 2023). “The ring is so far from the dwarf planet’s surface that its material should have coalesced into a moon,” writes Tereza Pultarova. “But somehow, it didn’t.”

“What is so intriguing about this discovery around Quaoar is that the ring of material is much farther out than the Roche limit,” Giovanni Bruno, an astronomer at Italy’s National Institute for Astrophysics (INAF) and one of the authors of the paper, said in a European Space Agency (ESA) statement. “As a result of our observations, the classical notion that dense rings survive only inside the Roche limit of a planetary body must be thoroughly revised.

A new ring system discovered in our Solar System (University of Sheffield, 8 Feb 2023). This press release by authors of the paper about Quaoar’s ring in Nature says, “The ring system orbits much further out than is typical for other ring systems, calling into question current theories of how ring systems are formed.”

What makes the ring system around Quaoar remarkable is that it lies at a distance of over seven planetary radii – twice as far out as what was previously thought to be the maximum radius according to the so-called `Roche limit’, which is the outer limit of where ring systems were thought to be able to survive. For comparison, the main rings around Saturn lie within three planetary radii. This discovery has therefore forced a rethink on theories of ring formation.

Comparison of the moons of the solar system.

A planetary ring in a surprising place (Nature News and Views, 8 Feb 2023). Matthew Hedman’s review of the paper states, “An object in the distant Solar System has been shown to have a ring that is unusually far from its host — prompting speculation about how the ring material has avoided clumping together to form moons.”

A dense ring of the trans-Neptunian object Quaoar outside its Roche limit (Morgado et al., Nature, 8 Feb 2023). This is the research paper that describes the problem.

Up to now, all known dense rings were located close enough to their parent bodies, being inside the Roche limit, where tidal forces prevent material with reasonable densities from aggregating into a satellite. Here we report observations of an inhomogeneous ring around the trans-Neptunian body (50000) Quaoar. This trans-Neptunian object has an estimated radius of 555 km and possesses a roughly 80-km satellite5 (Weywot) that orbits at 24 Quaoar radii. The detected ring orbits at 7.4 radii from the central body, which is well outside Quaoar’s classical Roche limit, thus indicating that this limit does not always determine where ring material can survive.

The authors try out a new theory that elastic collisions and resonances can maintain a ring:

Our local collisional simulations show that elastic collisions, based on laboratory experiments, can maintain a ring far away from the body. Moreover, Quaoar’s ring orbits close to the 1/3 spin–orbit resonance with Quaoar, a property shared by Chariklo’s and Haumea’s rings, suggesting that this resonance plays a key role in ring confinement for small bodies.

It remains to be seen if the new theory can hold a ring. Even if that is true, none of the popular articles say if physicists can explain how a tenuous feature like a ring can be maintained for billions of years.

The Roche limit has always been taught as gospel truth, and seems to make physical sense. This is a severe anomaly for the theory. We’ll have to see if the theory can be patched up with auxiliary hypotheses or not, and how plausible those are. Too many epicycles can destroy a favored theory.

The idea of “elastic collisions” maintaining a ring seems implausible. Solar system rocks are not like rubber handballs; they don’t bounce and keep on bouncing numerous times. Most likely they would fragment quickly into dust. The ephemeral rings we know of (Jupiter’s Ring, and some of Saturn’s rings) contain fine dust, some like smoke particles. Those were already problematic for ring theories.

The part about particles condensing into a moon is much more speculative. Particles in collision do not accrete; they bounce. One  should insist on better evidence that accretion is possible.

The ring around Quaoar is a somewhat minor anomaly for old-agers to explain, but it’s another among many anomalies throughout the solar system: the Enceladus geysers, Io’s volcanoes and Titan’s atmosphere, and many more. How can a ring around a low-gravity object (Quaoar is only 697 miles wide) be maintained for billions of years? Can they think we are just lucky to see a recently-formed ring now after billions of years have transpired? That would be special pleading.

Once again, the assumption of deep time is a problem, not a benefit. A ring like this might last for thousands of years, but not billions.

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