# Eli5: why do planetary bodies rotates around axis?

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Why do planetary bodies rotates around axis. Which force is causing this rotation and why. Also are the stars rotating as well and do planets orbit them because they are rotating or is there some other reason.

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Once something is rotating (or just moving for that matter), it won’t stop unless a force slows it down.

All of the debris that came together to form the planets was, on average, rotating, so the planets are still rotating as nothing’s stopped them.

This is also why planets are orbiting. They were moving sideways to begin with and nothing has slowed them down.

The force is called ‘conserved angular momentum’ and its a leftover force from when the planets first formed from a huge cloud of gas and dust. The cloud was spinning already and so that momentum carried over into the planets.

Stars are all spinning for the same reason planets do. Planets orbit them because they formed from the same gas/dust cloud the star did.

Imagine a big magnet dangling from a thin strand of wire.

Surrounding by the dangling magnet is a crowd of people, where each person has a handful of paper clips. Everyone is throwing their paper clips one at a time at the magnet from all random directions, with varying levels of aim. Every paperclip that hits the magnet sticks to the magnet (or every paperclip that hits the growing wad of paperclips will stick to the wad).

If a paperclip strikes the magnet directly dead-on, it will simply stick to it and have no extra effect. But if the clip lands a glancing blow off to one side, it will stick to the magnet and cause it to start spinning that way slightly.

There is pretty much zero chance that every single paperclip hitting the manget will hit it dead-on in this chaotic setup. And there’s basically zero chance that all of the glancing blow paperclips will just happen to exactly cancel out all the rotation they cause. So in pretty much all cases, once the crowd of people run out of clips to throw, the resulting wad of paperclips stuck to the magnet will be spinning.

All objects in the universe formed in a manner similar to this, just instead of magnets it was (usually) gravity, and instead of paperclips it was… well, whatever random crap happened to be floating around.

In the real world, the dangling magnet will eventually slow down and stop spinning due to forces in the wire holding it up and friction with the air in the room. But in space, there is neither of those things, so things that have started spinning will more or less spin forever. There is no “force” needed to *keep* them spinning. The force of stuff crashing into them long, long ago *set* them spinning, and nothing has ever stopped them.

There are many more states for a mass that include rotation than don’t. There is only one non-rotating energy state while there are an infinite number of rotating energy states. It would be strange for something NOT to rotate. Any external not perfectly balanced force would want to cause a rotation and there isn’t anything in space that provides drag like air or water in the atmosphere so all the tiny little pushes add up over the millions of year. For planets in general it is how they come together as they condense from the original stuff that created the solar system. The conservation of momentum means that as they all aggregate into a planet they add up to rotation.

it’s a collection of particles that all had their own energies and directions of motions before they collided and stuck together. it’s overwhelming unlikely that gazillions of them forming a planetary body all coalesce in a perfect way that no net rotation forms as a result. like two cue balls crashing and coming to a complete stop with no glancing upon contact, except imagine a gazillion of them. sitting on a swivel chair, stretch out your arms and see if you can pull them in quickly without ending up with some rotation in the chair.

after the last particle joins the system there is no force that keeps it rotating per se. any forces that can be observed in that rotating system does not result in any movement in the direction of that force. so no energy is exchanged, no work is done, thus that rotation continues forever.

I say per se because there is a force that exists holding those particles together and it is a combination of electric and gravitational forces. if for any reason those are overcome, then the planet breaks up and all particles leave in straight lines away from what was once the planet. but of course that means there is no planet anymore which is a different question.