[AntonioLao]

about 4.5 billions years ago, when the planets were still being formed out the many planetesimals collisions, the incremental angular momenta from all the planetesimals were added together creating a point called the center of mass. So that the total angular momentum is the vector sum of all these elemental momenta. But once the change in mass is stabilized without anymore further collisions, the total angular momentum became conserved and has been for the past 4.5 billions years. The rotational rate might be slowing down due to constant accumulation of micrometeorites, increasing the mass of the earth gradually and very, very slowly in the millions of years.

Not only planets rotate on an axis, but the sun, neutron stars, black holes, galaxies, comets, asteroids, and even subatomic particles all possess angular momentum. In the case of elementary particles, the angular momentum is called spin and this property is quantized. It has only two values, say spin-up or spin-down.