Minor planet ring that should not exist - What prevents Quaoar’s ring from accreting to a moon?

An international team has discovered a narrow ring encircling a minor planet at a distance of seven planetary radii, much further away than any previously known ring of macroscopic particles.  This discovery proves wrong the standard view according to which rings should rapidly accrete into a moon beyond the classical Roche limit,  due to gravitational forces between the ring particles. The surprising new results are published in the 9 February 2023 issue of the science journal Nature, and were obtained in a large observing campaign coordinated by Paris Observatory. The proposed theoretical explanation for the prevention of accretion was developed by professor Heikki Salo, University of Oulu.
Comparison of Saturn and Quaoar.
Cassini image of Saturn, together with artist’s view of Quaoar and its ring. Dashed circle indicates the Roche limit. Figure: Heikki Salo / University of Oulu. Read more about the figure below.

Quaoar resides far in the solar system beyond Neptune’s orbit and has a diameter of only half of Pluto’s. It appears as a mere point of light even in the largest telescopes, including the James Webb Space Telescope. The size and shape of Quaoar has been measured indirectly via stellar occultation measurements. The existence of rings was revealed by the smaller dimming events observed besides the main eclipse caused by the central body. Quaoar’s rings are the third ring system found around a solar system small body, all with a similar method.

French astronomer Edouard Roche showed in 1850 that inside a certain distance from a planet, its tidal force wins over the mutual gravity of orbiting particles. A moon falling inside this so-called classical Roche distance, located typically at 2 - 3 planet radii depending on the densities of the planet and the moon, is torn to pieces, and its fragments spread into a ring around the planet.  According to standard textbooks, the same mechanism works also in the other direction, so that outside Roche limit a ring always accretes to a moon. This view was originally inspired by the good match of the outer edge of Saturn’s rings with the Roche distance for icy particles. Also, other later-found rings have supported this view. The newly-discovered Quaoar’s ring makes a striking exception: in order to place it inside the Roche limit, its constituent particles should be unrealistically fluffy with densities much smaller than that of water ice. The explanation that we have caught the ring just before it has accreted is equally implausible.

The solution to the existence of  Quaoar’s ring, offered in the article, is based on detailed modeling of the ring particle collision dynamics. Quaoar orbits much further from Sun than Saturn, and therefore its rings are much colder. According to laboratory measurements made in Lick Observatory in 1980’s, ice particles are more elastic at lower temperatures. The new computer simulations made in Oulu showed that very elastic collisions between particles do no lead to gravitational accretion even if Quaoar’s ring resides outside the Roche limit and its particles have normal ice densities.

Heikki Salo thought about the possible solution immediately when he learned about the new observations: ”Professor Bruno Sicardy, who coordinated the observations, showed me the occultation curve with sharp secondary minima, corresponding to a ring far beyond any plausible Roche limit. I remembered the old measurements of the elasticity of ice at low temperatures, as well as recalled some computer simulation experiments I had conducted for fun with such extreme elasticity models. The large distance of Quaoar made the pieces of the puzzle to fit together, as confirmed by new extensive computer simulations during the next weeks, exploring a wide range of ice elasticity models.

The joint research efforts of Paris Observatory and Oulu continue, with the goal to explain which mechanism keeps the rings of Quaoar, as well as those of other minor bodies, so narrow.  A clue is offered by the location of the rings at a distance where the particles’ orbital period equals three central body rotations. ”The theoretical calculation made by Sicardy indicate that the non-spherical shape of the central body induces perturbations which accumulate orbit-by-orbit, in a similar manner by which a swing gains speed by tiny synchronized pushes. According to my computer simulations such resonant perturbations modify the influence of collisions, so that they tend to confine the ring, instead of dispersing it radially”, Salo tells.

Modeling planetary rings has a long history in Oulu astronomy research group, initiated by Professor Antero Hämeen-Anttila in 1960’s. Results of their N-body simulations have been published several times in Nature (1992, 2007) and Science (1998, 2011), related for example to gravitational wakes in Saturn’s rings and to the structure of Neptune rings arcs.

The article  “A dense ring of the trans-Neptunian object Quaoar outside its Roche Limit” is published in the February 9th  2023 issue of Nature. Link to the article online: Morgado et al.

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Image caption

Cassini image of Saturn, together with artist’s view of Quaoar and its ring (sizes exaggarated by a factor of hundred compared to Saturn). Dashed circle indicates the Roche limit for icy particles, which before the current discovery was considered as an outer limit beyond which particle rings can not exist.  
Credit: Saturn image by NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic; the illustration: Heikki Salo / University of Oulu

Last updated: 8.2.2023