Rings form around many planets – but usually only at a certain distance. There are physical reasons for this. However, the dwarf planet Quaoar seems to break this rule.
We know that rings form around planets – this phenomenon can be seen particularly spectacularly with Saturn, one of the large gas planets in our solar system. But Jupiter, Uranus and Neptune also have ring systems, which are less easy to recognize. The rings form from ice or rock particles, dust and rocks. It is believed that sometimes the remains of moons that fell towards the planet and were torn apart by gravity also form the material of such rings.
One discovery, however, is puzzling science: The dwarf planet Quaoar, a so-called “trans-Neptunian object” located far out at the edge of our solar system and orbiting the Sun in a near-perfect circle, has a ring. However, he shouldn’t actually have it – because this ring seems to contradict the previous physical assumptions.
Quaoar shouldn’t have his ring
Roughly speaking, rings around objects in space only form in a certain proximity to the respective object. When ice, rock and dust are outside of this zone, known as the Roche limit, the prevailing forces compress the material into a compact lump, forming a satellite, or moon, which then orbits the larger object. Within the Roche limit, the tidal forces act from planets too strong for moons to form – they’ll be torn apart right away. This applies to all planets or asteroids with rings that have been studied so far.
Only Quaoar is a confusing exception. The team led by Bruno Morgado from the National Observatory in Rio de Janeiro made the discovery: the researchers found out almost by accident that the dwarf planet has a narrow, rather irregular-looking ring. “We saw decreases in brightness that were not caused by Quaoar itself, but indicated the presence of material in a circular orbit around it,” Morgado said. “Unlike all other known denser rings, Quaoar’s ring is well outside the classical Roche limit.”
Science puzzles over causes
The ring is a good 4100 kilometers from the surface of Quaoar, but its Roche limit would already be 1780 kilometers. “Material so far outside the Roche limit should actually accumulate within a few decades,” say Bruno Morgado and his team. There are three ways of explaining this. What remains is “either a very young or extremely thin ring or we have to revise the assumptions about the Roche limit.”
The extreme cold of the region where Quaoar orbits may also have something to do with his mysterious ring. The scientists suspect that material clumps less well under these circumstances. Alas, gravitational interactions triggered by the so-called 1/3 spin orbital resonance could have something to do with the phenomenon. However, one thing is already clear: Quaoar has disproved that rings around planets or other objects can only occur within their Roche limit.
Sources: “Scinexx”, University of Central Florida