Eli5 on why do planets spin?
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When planets are formed, it is usually due to a collision. When that happens, you have centripetal force as the two or more bodies start to form into a planet. Newton’s first law states that bodies in motion will stay in motion unless acted on by an external force. So these newly formed planets will keep their inertia, in the form of angular momentum, and because space is a vacuum, the planets will keep spinning because there is no friction to stop them. They don’t move in a straight line as the law states because they are bound by gravity.
If a planet is formed by other means, and still spins- then that planet was likely struck by an asteroid, comet, or some other body that imparted its momentum, making the planet spin.
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