Eli5 on why do planets spin?
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Inside the vacuum of space, before a solar system resembles a solar system, it is just a massive cloud of dust. This dust has one force acting upon it, gravity. Gravity pulls this dust towards itself, as it does, the whole thing starts to spin.
A reasonable way to think of this is with the plastic sheet demonstration that is used to show how mass bends spacetime.
Stretch a sheet out by all 4 corners, this sheet represents the fabric of space. Now throw a bunch of marbles at it, this represents your gas cloud. Now there are other forces here as the earth’s gravity has an effect, as well as the friction from the sheet, but when you toss a bunch of marbles at it, they first follow the curvature made by other masses on the sheet. This curvature causes the marbles to spin around each other. On the sheet, it stops quickly due to friction, but in space where friction is basically zero, the gas keeps rotating. This eventually gathers enough matter to form spinning rocks and spinning planets.
Basically, any slight force applied to something in space will continue for a very long time until acted upon with equal (or greater) force in the opposite direction. During the formation of a planet, gravity is the force acting upon the dust cloud and the effect of gravity causes the spin. This spin just carries on, in practical terms, indefinitely.
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