Eli5 on why do planets spin?
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If you throw a bunch of stuff together randomly then it is very unlikely to end up with exactly zero rotation. Initially the average rotation will be slow, but as the stuff collapses and forms smaller objects (like stars and planets) the rotation rate increases. You can see the same effect with ice dancers or if you have a rotating chair, spin with extended arms and then pull in your arms.
Mostly, it’s due to how they form. Have you ever been on a roundabout in a playground? If so, you’ll probably know about how reducing the distance of your mass for the center increases the rate of rotation, or in other words, focusing all the mass from a wide spread to a small spread increases angular momentum!
Well, this is what happens when all things form in space. You start with a cloud of random dust and gas and whatnot that is drawn together over huge amounts of time by gravity. Except all of the little bits would have been travelling in some direction to begin with. So, as the cloud forms, the sum total of different particles’ momentum creates a very slight rotation. As the cloud condenses, that very slight rotation increases due to the focusing of mass towards the center of rotation. And then hey presto, bingo bongo, you eventually have a rotating planet!
Then you ask, but what about the moon? We only ever see one side of that
The moon is actually rotating, just perfectly in time, its orbit around the world! Amazing really
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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.
Stuff moves perpetually in the vacuum of space.
You are underestimating that planets move so slowly that it takes literally all day for them to spin once. 🤣
Think of a sheet that’s pulled tight by it’s corners. Throws some marbles on the sheet of significant weight to pull the other marbles in on each other. As the marbles collide, they impart a bit of their energy on the other marbles. They move to a center position and circle around a bit before eventually stopping. NOW, remove the sheet and imagine this marbles in space, with no friction. Instead of eventually stopping, the marbles keep moving, eventually all spinning at the center of gravity. Now imagine the marbles are atoms and molecules that make up a planet.
When all the particles start to converge, they impart their momentum on each other, colliding and orbiting until they all start to press up against one another to form a single shape, a planet. AND because there’s no friction in space, there’s nothing to stop the total momentum of the system from continuing forward. Thus, spin.
Here’s another way to think about the question.
If you have a planet going around the Sun, and it didn’t “spin” at all (fixed orientation with regard to the galaxy), it would be rotating from the point of view of the Sun, wouldn’t it?
And a planet going around the Sun, always facing the Sun with the same face (not “spinning” relative to the Sun) would be spinning in relation to other objects in the solar system / galaxy.
So spin one way or another is going to happen. But perhaps you mean, why do planets spin so quickly relative to their orbit around the Sun? That’s answered by others well enough — when matter condenses together, it has a tendency to have some angular momentum to it. Just as planets orbit the Sun, so does matter “orbit” or spin when formed into a planet.
Conservation of angular momentum and a near frictionless environment.
Planets started out as swirling clouds of dust.
When everything was a dust cloud as it condensed into a planet it acts like water going down a drain in that it naturally starts to spin this keeps going as the planet forms and the planet keeps spinning.
I remember learning this as a kid at science world I think, hopefully I remember it right.
There’s a big circular funnel ,the bottom of the funnel represents a massive object that is bending space. ( So think of space as like a sheet of rubber and the sun is a big heavy ball that sinks it down forming a funnel).
Now when you put an object like a ball into the funnel, it doesn’t take a beeline to the center of the funnel,it starts to spin around the edges getting closer and closer to the center of the funnel and will take quite awhile to reach the center.
This is basically how gravity and orbit works but I don’t actually know why the object takes a circular spiraling path instead of just going straight towards the center.
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