QUESTION:
Why do planets spin,  and why do planets move in orbits?

ANSWER from Howard Bushouse on March 22, 1996:
Both of these motions have to do with the fact that the big cloud
of gas and dust that the sun and planets originally formed out of
must have been rotating a little bit to begin with. Almost nothing
in the Universe is sitting in space absolutely still. It would only
take a little bit of rotation of the original cloud because of a
law of physics known as "conservation of angular momentum". What
that means is that the amount of angular momentum that the cloud
had at the start has to stay the same after it collapsed to form
the solar system.

From watching something spinning - like an ice skater for example -
you can see that angular momentum depends on both the speed of
rotation and the size of the thing rotating (or more specifically
the distance from the outer edge to the center of rotation). So for
the example of the ice skater, as the person pulls their legs or arms
in towards their body, their speed of rotation increases (so that
they end up with the same amount of angular momentum).

The same thing had to happen with the cloud that formed our solar
system. As it collapsed (due to the force of gravity of all the
material inside the cloud) the speed of any rotation that it had to
begin with had to increase to conserve angular momentum. And once
the planets formed out of the cloud material they just kept moving
around the center of the cloud (where the sun is now) and kept
spinning on their axes.