Many bodies in the Solar System have rings—gas giants, dwarf planets, even an asteroid. These examples have allowed us to get a good picture of their physics, leading to models for how rings form and what keeps the material there from falling into the planet or condensing into a moon.
But a discovery described in a paper released today suggests we've gotten something (or maybe more than one something) seriously wrong. A dwarf planet called 50000 Quaoar that orbits beyond Neptune appears to have a ring that shouldn't be there, at 7.4 times more distant than the planet's radius. There are a couple of ideas about why the ring might survive in this location, but nothing definitive at this point.
Ring signals
Quaoar resides in the Kuiper belt, an area beyond the orbit of Neptune. With a low density of icy material and no giant planets around to sweep it up, the Kuiper belt is home to a sparse population of dwarf planets like Pluto. Despite its low density, the Kuiper belt is large enough that there are a lot of bodies out there, and we've only recently developed the telescope hardware necessary to catalog them.
Quaoar, at roughly half the size of Pluto (1,100 km across), was discovered in 2002. Later observations that tracked Quaoar as it passed in front of a distant star indicated it was accompanied by a small moon named Weywot. Despite their size, a number of moons have now been found orbiting Kuiper belt objects, so this wasn't especially surprising. There was also a single observation that hinted that Quaoar might also have a ring—again, something that had been seen in other Kuiper belt objects.
There are two possible configurations for such a ring that are consistent with this data, but one of those configurations places the ring in the same plane as the orbit of Weywot, the moon. Since this is the most typical configuration seen at other planets, the researchers expect that this is the configuration of the Quaoar system. The amount of light blocked by the ring was the same at a large range of wavelengths, suggesting that it was composed primarily of particles that were over 10 micrometers across, but not too much larger, or they would have been driven off by light from the Sun.
Repeated observations suggest that Quaoar's ring is also irregular in shape. This has also been seen in other rings, where small bodies up to a kilometer across orbiting within the ring alter its shape through their gravitational pull. The growth of these bodies is limited by their tendency to run into each other and disintegrate into the smaller particles that are typical of the rest of the ring.
So far, everything about this seems pretty normal. Things got weird, however, when the researchers looked into how distant the ring was from the dwarf planet: 7.4 times the planet's radius, or roughly 4,100 km. That places it outside a key location termed the Roche limit. Inside the limit, the tidal forces generated by the nearby planet are sufficient to limit the size of any body that forms there and tear apart any moons that form elsewhere but wander within the limit.
While none of these processes happen instantly, it's generally thought that the Roche limit sets a simple rule for bodies orbiting a planet: inside the limit, and you form a ring; outside the limit, and rings condense into moons. And in the case of rings condensing into moons, the process seems to only take decades, based on simulations. The chance that we've happened to catch a ring within the few decades that it exists seems pretty remote, so the researchers conclude that all options to explain the ring are pretty bad: "That leaves us with very young or extremely low-density ring particles, or more likely, with the need for revisiting the Roche limit notion."
Their revisit starts with the physics of the condensation process that would convert a ring into a moon. That's based on the idea that the collisions among a ring's particles aren't elastic, in that the particles don't bounce off each other with roughly the same energy they started with. Instead, they smush into each other a bit and lose some of the energy as heat. Over time, this slows them down and allows subsequent collisions to merge particles into ever-growing bodies.
Another possibility is that the small, icy particles collide in a way that's closer to elastic than expected. There's some support for that idea from lab experiments done with actual small bits of ice. But it's very dependent on the composition and surface structure of the particles, and we're not able to determine if those conditions match the ones present in Quaoar's ring.
Consistent with this, the ring at Quaoar and the rings of a couple of other Kuiper belt objects (Chariklo and Haumea) all seem to be positioned near where their orbits would be in a one-third resonance with the spin of the planet. (Although in the other two cases, the rings are also within the Roche limit, so this matters less.) There may be something about this resonance that allows the planet's spin to exert a disruptive force much farther out.
Modeling could potentially address that, but it's clear from their paper that the team decided to publish while the models were still in development. And the team hasn't ruled out Weywot as an influence yet, though the moon's properties aren't well constrained given that it can't be directly imaged.
This may not seem like such a big deal to a casual reader, but it's hard to overstate how influential the Roche limit has been on our thinking about complicated ring systems like Saturn's. Finding out that we need to revisit it and add a few caveats is pretty unexpected. The biologist Thomas Henry Huxley once joked about "the great tragedy of science—the slaying of a beautiful hypothesis by an ugly fact." Quaoar's ring might be beautiful to look at in person (we're unlikely to find out any time soon), but it also seems to be one of those ugly facts.
Nature, 2023. DOI: 10.1038/s41586-022-05629-6 (About DOIs).