Eli5 on why do planets spin?
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The real question is what could make them not spin. When an object is flying passed a mass (protoplanet) in space, it keeps whatever momentum it had as it approached the mass. The gravity of the mass might just bend the trajectory of the object as it whizzes by, but the object may be close enough and slow enough to be “captured” by the gravity well of the larger mass, causing it’s trajectory to be bent so much it goes into orbit around the mass, still maintaining it’s momentum (and thereby in turn “pulling” on the mass to make it spin). Or, more likely, the orbit is short-lived, and “decays” as the object circles around the mass, eventually crashing into it. The object isn’t really “falling”, it’s just getting closer to the mass as it continues “forward” under its original momentum. Every part of every planet started this way, as clouds of dust coalesced around their center of mass and crashed into each other to form a single massive object we call a planet. So planets start out spinning, and there’s nothing that can stop them.
This explanation ignores tidal forces, too complicated to explain and not large enough to matter in this context. The only real confounding issue is the randomness of the original momentum of the object, which theoretically could oppose rather than add to the rotation of the planetary mass. But solar systems form the same way as planets (coalescing clouds of material) so the whole system is “rotating”, meaning that the planets are orbiting the star (center of mass). Since the planet is moving through space as well as spinning, any object that is captured will add slightly more spin if it’s trajectory matches up with the planet’s orbital motion and will subtract slightly less spin if it’s trajectory is opposite or orthogonal, so statistically the spin of the mass will be maintained for essentially as long as the planet exists.
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