Whay are Uranus' moons in equatorial orbits?

Question

On that program, (which I can't remember the name of), I saw that not only is Uranus' polar axis tilted almost to 90 degrees, but its satellites are fairly nicely arranged on the planet's equatorial plane. I knew about the polar tilt, but not the moons. I figure that somehow the planet got tilted, but why the moons? This would suggest that the whole system formed that way. But why? How? It seems contrary to the expected physics.

That's incredible, Prof. I was aware of tidal locking between planets and their satellites, but I had no *idea* that it could be such an influence that orbits of moons could be altered by 90 degrees, and come out almost planar, at that! I wonder how long it takes and whether our moon keeps up with the earth's precession.

Okay, David, so what you are describing is similar to what is now believed about the earth and the moon. While your explanation is different from Prof's, it is not contradictory to it. Both may be true.

And now that you mention it, that whole batch of small moons in close orbits, (the ones named after the Shakespeare women), would sure seem consistent with the collision hypothesis. The big moons were probably formed naturally with the planet and were pulled around to their present orbits.

Answer 1

It wasn't formed that way.

The rotation of the planet causes it to become oblate, which causes the gravitational field of the planet to be oblate as well, and thus orbits become roughly equatorial over time. Tidal locking forces the rotations of the moons to follow suit. This is a bit of an oversimplification; there's a whole area of study of these kinds of processes. For more information, research "Cassini States." A Cassini State is a stable spin-orbit coupling of an orbiting body.

Answer 2

The axis of spin of Uranus is tipped about 98 degrees to the plane of its orbit. Since everything else in the solar system is pretty much in the same plane and rotates with the poles pointing in the same direction as those of the Sun do, we have to consider that "something" took a normally rotating Uranus and tipped it, or redirected its rotation through the application of a large force. This force could only be another large body. When Uranus and this other body collided, the force of the collision ripped both bodies apart and imparted a spin of the debris along the axis of rotation now seen in Uranus. Most of the gasses and solid debris reformed into the present Uranus. Much of the remaining debris would have orbited the resulting planet and then coalesced into the moons that now orbit Uranus. The debris and remnant rings that now circle Uranus were all aligned along the new equator when they were formed.