Science Forums 2

[CraigD]

Except for special cases involving objects that can stabilize themselves in some manner (eg: a paper airplane, or your example of something sliding across grass), nearly any collection of bodies held together by something (the gas of a star, the rocky material of a planet, held together by gravity, a football held together by leather and thread, etc.) are more likely to spin than not.

This is because there are many more possible initial conditions (eg: infalling gas and solids in a forming solar system) where the individual bodies making up a system have direction other than all toward the system’s center of mass. It is, in principle, possible to throw or kick a ball so that it doesn’t spin, but very difficult to do so. Likewise, a pre-stellar nebula could, in principle, collapse into a star with no rotation, but this would be an incredibly rare occurrence.

There are some astronomical objects that, on some scales, have little spin. Globular clusters are an example. The stars in them don’t orbit their center of gravity in fat ellipses, but in ones so thin they are nearly straight lines. The whole cluster sort of falls in on its own center, contracting into a dense ball, the then falls out again into a tenuous one.

You can run simulations to determine the probability of a particular gas/dust cloud forming a system with particular spin rates. The result (I assume – I’ve not personally done such a sim) shows that most systems behave something like our solar system, or the various other star systems we’ve observed.