Eli5 on why do planets spin?
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It’s leftover energy from when it was formed. A basic practical demonstration you can do in a spinning desk chair is to hold your arms straight out, preferably with something heavy in your hands like a big book or even a dumbbell. Start spinning. Then bring your arms in so your hands and whatever they’re holding are against your chest You should notice you spin faster now. Stretch your arms back out and you slow down again. Play around with this for a bit.
Now planets are formed from random bits of space debris in a big solar system sized cloud that collapsed under gravity. If there was any spin at all before gravity pulled it all together, that spin will be magnified as it collapses and shrinks. The effect is much greater because of the vast distances involved here, where debris from millions of miles around collapses down to something just a few thousand miles in size.
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