Why do all major planets orbit the Sun in nearly the same direction as the original nebula spun?
Answer
Because the disk was flattened by increasing rotational speed during collapse.
One of the key regularities explained by the nebular hypothesis is the uniformity of planetary motion. As the initial diffuse, rotating nebula contracted under gravity, the conservation of angular momentum dictated that its spin rate increased. This rapid spinning forced the cloud to flatten into a rotating accretion disk. Since the planets formed from the material within this flattened disk, their resulting orbital planes naturally align nearly coplanar with each other, and they all orbit in the direction of the original nebula's spin (counterclockwise when viewed from above the Sun's north pole).
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