# Eli5: why do planetary rings congregate around the equator?

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I understand there’s quite a few forces that prompt the disk shape of rings: particles in toroidal orbits interacting and cancelling their momentums; magnetic field lines being weakest farthest from the poles; corriolis effect drawing particles towards the path of least rotation; etc…

By why specifically the equator? Why don’t rings align with the poles, or some other angle? And why is this also the case for black hole accretion disks?

In: Physics

Because all of the effects you cited are specific applications of the true effect responsible for all of this:

The conservation of angular momentum.

We live in three spatial dimensions. One of the consequences of that fact is that you cannot have two non-parallel planes that do not intersect (specifically, they must have a line of intersection). Angular momentum is described by a rotation axis, a vector; this vector points perpendicular to the plane in which the rotation occurs. Because we live in 3-space, we can know *for sure* that any collection of rotating objects has one, and *only* one, axis of net rotation. If we lived in 4-space or higher, this wouldn’t be true and you wouldn’t see rings (except in *fantastically* rare coincidences.)

As a result of this conservation, massed object will continue to orbit circularly, but will also feel a perturbation force, which always points toward the rotation plane (read: equator) of the object. The object also (usually) is a slightly oblate spheroid as a result of its rotation, meaning gravity is *very slightly* stronger along that plane and *very slightly* weaker elsewhere, holding things better along the plane of rotation.

You can actually see a tug of war going on with a real object: the Moon. See, in this era of our solar system, the Sun actually contributes more gravity to keeping the Moon where it is than the Earth does (it’s always accelerating more toward the Sun than the Earth). As a result, the *Earth’s* perturbations are a force pushing it toward an equatorial orbit relative to Earth, while the *Sun’s* perturbations are a force pushing it toward an *ecliptic* orbit, in line with the planets.

Rings will never align with the poles, nor be anything more than a tiny amount off from being equatorial, so long as they are (a) bound by the gravity of the planet only, and (b) the planet has enough angular momentum to be the dominant contributor to the net rotation of objects in its vicinity.

It’s the only arrangement that survives long enough to form real rings.

If it’s not a ring then particles in different orbits keep colliding with each other, eventually forming a ring.

Rotating planets are a bit wider at the equator so their gravitational field isn’t perfectly symmetric. Orbits not aligned with the equator precess: They change their orientation in space over time. Different orbits precess at a different rate. A ring that’s not aligned with the equator would become a cloud of of countless different intersecting rings over time, which then has collisions again, making a ring around the equator.