Eli5 on why do planets spin?
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Moons, planets, stars, solar systems, galaxies all spin for basically the same reason.
If you have a cloud of gas full of particles moving in random directions, then they will all attract each other due to gravity acting on their mass. If nothing else is near this cloud, then the common gravity will be the dominant force on all of the particles. They will all be attracted to the common centre of mass and tend to orbit that centre.
Imagine looking at this cloud from a side. Some of the particles will be moving downwards, some up. Some will be moving left, some right. All will be accelerating towards the centre of mass, and so will move in a circle (well, ellipse) around it.
Looking from the side, some of the particles will be moving left, others right. Some up, some down. All though will be moving around the centre of mass. A lot of the left momentum will be cancelled out with the right momentum, either due to the particles attracting each other or from occasional collisions. The same will happen for the ‘up’ and ‘down’.
If you tilt your head a bit, changing the angle you are looking at the cloud from, you can find an angle where ALL of the up momentum equals ALL of the down momentum. Eventually, either through collisions or just constant slow gravitational pulling, all of the up and down momentum will cancel out, and your cloud of particles will end up as a flat pancake of particles, in the plane where the up and down cancelled each other.
If you now look down on your pancake, some of the particles will be moving clockwise around the centre of mass, some anti-clockwise. Through collisions and gravitational pulling, the clockwise and anticlockwise will tend to cancel out. There will almost certainly be an imbalance of particles going clockwise and anti-clockwise, and so after the cancelling out, the remaining system will either be spinning clockwise or the other.
In practice, both of the up/down cancellations and the clockwise/anti-cw ones happen at the same time. Every cloud of particles will collapse into a rotating plane in the absence of any other gravitational force, and that plane will be rotating in one direction. This concept is the conservation of the angular momentum of the original cloud.
The result is that gigantic clouds of dust collapse into galaxies that form flat discs. Within the galaxies there will be local clouds of dust where the attraction from the local cloud dominates that from the galaxy, and so that local gas will collapse into a disc itself as it rotates around the galactic core. Most of this disc will gather in the centre and form a star. Within that collapsing star disc there will be regions that have a bit more dust, and these will attract to themselves contract to a smaller disc orbiting its star. The centre of this disc will collapse into a planet. Within the planetary disc, further outlying concentrations of particles will form into discs that eventually make moons, orbiting the planet.
All of the mass collapses into flat discs that preserve the original angular momentum of the cloud. Their movement will cancel out in some up/down direction, and then in some clockwise/anitclockwise direction around the flat plane. The result is that the original mass will clump together into a star/planet/moon, but it will retain all of the original rotation of the cloud of particles from which it formed.
Hence planets spin.
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