Quaoar and Roche Limit

Description

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Context

Astronomers have spotted a ring around a dwarf planet called Quaoar in the outer reaches of the solar system.
There’s a ring around this dwarf planet. It shouldn’t be there.

Dwarf planet

A dwarf planetis a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto.
Astronomers are in general agreement that at least the nine largest candidatesare dwarf planets: Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus.

Quaoar

The dwarf planet, which is about half the size of Pluto and orbits beyond Neptune, is too distant and the ring too narrow to be seen directly.

Rings and Roche Limit

The observations, by a powerful telescope on La Palma, reveal the ring to be much further away from the planet.
The ring was outside the theoretical maximum for where a ring can survive according to classical theory.
The ring is located at a distance of more than seven planetary radii, twice as far out as what was previously thought to be the maximum radius, known as the Roche limit. Inside the Roche limit, the planet exerts strong tidal forces that prevent debris in the ring amalgamating into a moon.

Roche Limit and the case of Quaoar further decoded

In 1848, Édouard Roche, a French astronomer, calculated what is now known as the Roche limit. Material orbiting closer than this distance would tend to be pulled apart by tidal forces exerted by the parent body. Thus, a ring within the Roche limit would tend to remain a ring, while a ring of debris outside the Roche limit would usually coalesce into a moon.
The rings around the giant planets of the solar system — Jupiter, Saturn, Uranus and Neptune — generally fit within the constraints of the Roche limit. Among the distant smaller worlds, Chariklo’s rings actually lie a bit beyond the Roche limit. The ring around Haumea is within the limit.
Then there is the Quaoar ring. At a distance of 2,500 miles, it is way beyond the Roche limit, which the scientists calculated to be 1,100 miles. At that distance, according to the physics underlying Roche’s calculations, the particles should have coalesced into a moon in 10 to 20 years. The rings shouldn’t be there.

Plausible explanation

A potential explanation for Quaoar’s distant ring is the presence of Weywot. The moon may have created gravitational disturbances that prevented the ring particles from accreting into another moon. At the ultracold temperatures in the outer solar system, icy particles are also bouncier and are less likely to stick together when they collide.

Final Thoughts

Rings around small solar system bodies billions of miles away may seem esoteric, but the clumping — or non-clumping — of the particles is key to understanding the beginnings of the solar system.
It might seem that a small ring around a small object in the distant solar system doesn’t have broad applicability. But actually this process, of how particles accrete, is really the beginning step of planet formation.