[nudephil (OP)]

We got this question sent in from David:

What causes a planet to rotate (Earth's rotation being 24 hours) and why is that period different for other planets in the solar system?

Can anyone answer?

[Halc]

What causes a planet to rotate (Earth's rotation being 24 hours) and why is that period different for other planets in the solar system?

The rotation of any planet is initially due to the angular momentum of all the material that originally accreted into the young planet. This same angular momentum (of the entire cloud forming the solar system) also manifests in the orbital velocity of each of the planets, which therefore tend to have the same plane of rotation.  The material making up the various planets can come in from random angles and might result in a planet with abnormal (heavily tilted) rotation like Venus and Uranus.  The current spin of Earth is probably mostly determined not by the small particles that originally formed it, but rather by the angle at which it was struck by Theia, forming the current Earth-Moon system as we know it.

Anyway, that rotation tends to slow over time as tidal friction removes energy from the system and transfers angular momentum from one object to another.  In Earth's case, its spin rate has decreased from a few hours to today's nearly 24 hours. That momentum has mostly been transferred to the moon's orbital momentum, and some of it to Earth's orbital momentum.

Each planet has a different essentially random spin rate which is continuously changing due to the magnitude of the friction torque exerted by it.  Jupiter spins fast because its moons are just too small to put significant braking on it.

Mercury is currently locked into its spin rate since the torque forces on it are exactly balanced.

[evan_au]

Uranus orbits on its side, possibly because it got whacked by something much larger than the hypothetical Theia.