Rings from Close Encounters

We’ve recently discovered narrow sets of rings around two minor planets orbiting in our solar system. How did these rings form? A new study shows that they could be a result of close encounters between the minor planets and giants like Jupiter or Neptune.

Unexpected Ring Systems

Positions of the centaurs in our solar system (green). Giant planets (red), Jupiter trojans (grey), scattered disk objects (tan) and Kuiper belt objects (blue) are also shown. [WilyD]

Positions of the centaurs in our solar system (green). Giant planets (red), Jupiter trojans (grey), scattered disk objects (tan) and Kuiper belt objects (blue) are also shown. [WilyD]

Centaurs are minor planets in our solar system that orbit between Jupiter and Neptune. These bodies — of which there are roughly 44,000 with diameters larger than 1 km — have dynamically unstable orbits that cross paths with those of one or more giant planets.

Recent occultation observations of two centaurs, 10199 Chariklo and 2060 Chiron, revealed that these bodies both host narrow ring systems. Besides our four giant planets, Chariklo and Chiron are the only other bodies in the solar system known to have rings. But how did these rings form?

Scientists have proposed several models, implicating collisions, disruption of a primordial satellite, or dusty outgassing. But a team of scientists led by Ryuki Hyodo (Paris Institute of Earth Physics, Kobe University) has recently proposed an alternative scenario: what if the rings were formed from partial disruption of the centaur itself, after it crossed just a little too close to a giant planet?

Tidal Forces from a Giant

Hyodo and collaborators first used past studies of centaur orbits to estimate that roughly 10% of centaurs experience close encounters (passing within a distance of ~2x the planetary radius) with a giant planet during their million-year lifetime. The team then performed a series of simulations of close encounters between a giant planet and a differentiated centaur — a body in which the rocky material has sunk to form a dense silicate core, surrounded by an icy mantle.

simulation outcomes

Some snapshots of simulation outcomes (click for a closer look!) for different initial states of the centaur internal structure, its spin, and the distance of closest approach of the centaur to the giant planet. Blue and red represent icy and silicate material, respectively. [Hyodo et al. 2016]

The outcomes of the close encounters are diverse, depending strongly on the internal structure and spin of the minor planet and the geometry of the encounter. But the team finds that, in many scenarios, the centaur is only partially destroyed by tidal forces from the giant as it passes close by.

In these cases the icy mantle and even some of the centaur’s core can be ripped away and scattered, becoming gravitationally bound to the largest remaining clump of the core. The particles travel in highly eccentric orbits, gradually damping as they collide with each other and forming a disk around the remaining core. Further dynamical evolution of this disk could easily shape the rings that we observe today around Chariklo and Chiron.

If Hyodo and collaborators’ scenario is correct, then Chariklo and Chiron are differentiated bodies with dense silicate cores, and their rings are either pure water ice, or a mixture of water ice and a small amount of silicate. Future observations of these minor planets will help to test this model — and observations of other centaurs may discover yet more ring systems hiding in our solar system!

Bonus

Check out this awesome animation from ESO showing an artist’s impression of the ring system around Chariklo! [ESO/L. Calçada/M. Kornmesser]

Citation

Ryuki Hyodo et al 2016 ApJ 828 L8. doi:10.3847/2041-8205/828/1/L8

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