Related: Do/Can Ringed Stars Exist?

Is there any particular difference, in behavior or properties, between a planetary ring system and a circumstellar disk? Or is the only real difference a matter of what's in the middle?

This question came up as a matter in the related discussion, particularly because the (game-simulated) object that inspired the question is a brown dwarf - something between a star and a gas giant. What I observed in-game appeared to be more similar to the rings around Saturn, than to the description and image provided of a circumstellar disk.

So, are there properties of the rings/disks themselves which can differentiate one type from another? Or is the type of central body the only thing that defines them?

If the central body is the only factor, what should we call a ring around a brown dwarf, assuming one could exist in nature?


From Physics SE:

A circumstellar disk is a set of objects that orbit about a star-like object and that collectively look disk-like in some way.

From Wikipedia:

A ring system is a disc or ring orbiting an astronomical object that is composed of solid material

So, a circumstellar disk may have gas, dust, asteroids, etc., but it must orbit a star-like object. A ring system must have something solid--dust or moonlets--but it may orbit any astronomical object.

Other than that, you specifically asked about a planetary ring. That would mean a ring system that specifically had a planemo as the central body.

In other words, a disk around a brown dwarf could be described as a circumstellar disk (a brown dwarf is a star-like object) or a ring system (as long as the disk contained solid material). It could not be called a planetary ring, because a brown dwarf is not a planet.

  • $\begingroup$ So, it seems accurate to say that all circumstellar disks made up of solid material are also ring systems - but a ring system is only also a circumstellar disk when it is around a star? $\endgroup$ – Iszi Aug 11 '16 at 15:20
  • $\begingroup$ Based on the currently available definitions, yes, but in practice I would suspect that professional astronomers rarely refer to circumstellar disks as ring systems. $\endgroup$ – called2voyage Aug 11 '16 at 15:21

Besides all the obvious properties (size, distance, central body, etc.), there are a few other differences between the two.

Firstly, planetary rings usually have a clearly defined boundary. This is because rings usually have shepherd moons — small moons that orbit near the inner or outer edges of rings or within gaps in the rings. The gravity of shepherd moons maintains a sharply defined edge to the ring; material that drifts closer to the shepherd moon's orbit is either deflected back into the body of the ring, ejected from the system, or accreted onto the moon itself.

Circumstellar disc boundaries can be more complicated. Usually, planets play a similar role to shepherd moons in ring systems, so we can define an inner and outer edge for the asteroid belt. However, discs that lie beyond the planets are trickier. The Kuiper belt's outer edge is approximately 47.8 AU from the Sun, at the 1:2 resonance. However, some objects have been detected up to 55 AU from the Sun. Meanwhile, scattered disc objects have extreme orbits and the disc's outer edge is not clearly defined. Distant SDOs have been proposed as Hills cloud objects, causing a vague ambiguity between the two.

In addition, rings are much denser than circumstellar discs. As a result, collisions are relatively common between ring debris, while they are very rare in circumstellar discs.

  • $\begingroup$ In other words, to quote Jane Goodall: "It's a very wuzzy line". $\endgroup$ – called2voyage Aug 10 '16 at 19:10

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