"Accretion disk". Licensed under Public Domain via Wikimedia Commons
Accretion disks crop up a lot in astronomy, around black holes, neutron stars, protostars, and in the creation model for a solar system like ours. In general, orbiting planets, dust particles, etc., seem to always arrange themselves in a plane perpendicular to the axis of rotation of the main body. The reason for this is fairly straightforward. Any sufficiently massive celestial body will form a sphere due to gravity. If, however, the body is spinning, then the body will be squashed in a polar direction. This is true for the earth and the sun. In fact, it is necessarily true for anybody other than a black hole (because of their Born rigidity). For a uniform non-rotating spherical body, we can assume that the mass acts as though it were all at the centre. This is no longer true for these squashed shapes (oblate spheroids is the more technical term). A correction needs to be applied to any orbits that are calculated. The effect gives rise to an annual precession of the axis of the obit. The precession will eventually bring the axes of all orbiting bodies into line and then the precession stops. Consequently, given enough time, any orbiting body will eventually orient itself with the main axis of rotation. Of course, some bodies will not have had time to become aligned. The moon is an example of this.
For a black hole, the mechanism is similar but is due to the Kerr geometry, that makes any off-axis orbit precesses until it is aligned, with the main axis, just as with other bodies.
These explanations are not new, but I give it here because it is largely ignored by astrophysicists, and other more spurious explanations offered in the readily available literature.
To give some feel for the sort of explanation offered, try this:
"There are two basic reasons given that accretion disks can be flat:
- The material that is feeding the accretion disk usually comes from gas that is orbiting the black hole and therefore already in a flat plane, and
- The accretion disk is able to cool down efficiently, so it doesn't have a lot of excess heat that can cause the gas to expand out of the flat plane that it starts in." - Dave Rothstein on the "Ask a Scientist" website - that is, it is flat if it starts out flat!