Final answer:
The solar nebula flattened into a disk due to the conservation of angular momentum as the nebula collapsed and spun faster, with material at slower spinning poles falling toward the center and faster spinning equatorial material forming a disk. This process led to the formation of a star at the center and a flat disk around it, from which planets formed and continue to orbit in the same direction.
Step-by-step explanation:
Why did the solar nebula flatten into a disk?
The solar nebula flattened into a disk as a result of its own rotation and the conservation of angular momentum, similar to how a figure skater spins faster by pulling in their arms. When the nebula started to collapse under the force of its own gravity, it spun more quickly, causing material at the poles, where the spin was slower, to fall directly into the center. Material moving faster closer to the equator, due to higher angular momentum, collapsed into a flat disk around this central concentration. This rotation caused the nebula to flatten into a disk with most material concentrated at the center, forming a star - the Sun. The planets formed from the remaining disk material, orbiting in the same direction due to this common origin from a rotating disk.
The concept is reinforced by observing other stars with circumstellar disks, also formed from nebular material that cooled, leading to the formation of planetesimals, which are the building blocks of planets and moons.