Very similar question at Astronomy SE: https://astronomy.stackexchange.com/q/6183/4042
Given a typical protoplanetary disk made up of the usual planetary system stuff, dust and gas and whatnot, orbiting a common center: Material coalesces into planets, clearing the soupy disk in favor of a handful of massive objects.
And this is where my question begins: the conventional wisdom of orbital mechanics is that the smaller the orbit, the faster the motion:
The orbital speed is given by:
$$ v = \sqrt{\frac{GM}{r}} $$
so in the diagram $v_1 > v_2$ i.e. the inner body is moving faster.
So say you've got an asteroid belt that gathers itself up into a proto-Mars. In this case our two bodies aggregate into a single body. If we zoom in to the centre of mass of the two bodies their relative velocities look like:
So the two bodies are revolving clockwise about their centre of mass i.e. in a retrograde direction to the accretion disk as a whole. Doesn't this mean the average angular momentum of the Mars-forming belt will be retrograde with respect to the orbit, given that the inside of the belt moves faster than the outside of it? Why then is the prevailing rotational direction of the planets prograde?
I realize that that assumes circular orbits for all the particles involved, which obviously is unlikely. I suppose in a system of elliptical orbits, a particle or object is probably more likely to be captured by a coalescing mass if it is on the climbing (and therefore slowing) portion of its orbit, which would tend toward an overall prograde contribution of momentum. But is that effect sufficient to explain the overwhelming prevalence of prograde rotation in our solar system?