It used to be kinda like a sphere (of mostly gas and dust) at one point, or more exactly, everything was moving in vague orbits without preference.
But with movement like that you can imagine that things bump into each other. And when they bump, part of their energy turns into heat and part of it remains as slightly slower movement (when they bounce/glance off and continue in roughly the general direction they were going before the collision). Slower movement means the orbit tightens around whatever thing it’s orbiting.
That cloud of gas/dust must have had the slightest swirl in it. That means things collided slightly less when moving in the direction of the swirl, and more when against or perpendicular to it. Like moving in a crowd, it’s easier to move with the crowd than against or perpendicular to it.
Over eons, the orbits got tighter and tighter because of the collisions and heating up, until the whole movement distilled itself into just the swirl - the only thing they all agreed on. Anything moving otherwise was likely to hit something and lose its movement.
It all formed from the same accretion disk. Very flat, all moving together dust and gas became the whole solar system
But why was that a disk and not a sphere?
It used to be kinda like a sphere (of mostly gas and dust) at one point, or more exactly, everything was moving in vague orbits without preference.
But with movement like that you can imagine that things bump into each other. And when they bump, part of their energy turns into heat and part of it remains as slightly slower movement (when they bounce/glance off and continue in roughly the general direction they were going before the collision). Slower movement means the orbit tightens around whatever thing it’s orbiting.
That cloud of gas/dust must have had the slightest swirl in it. That means things collided slightly less when moving in the direction of the swirl, and more when against or perpendicular to it. Like moving in a crowd, it’s easier to move with the crowd than against or perpendicular to it.
Over eons, the orbits got tighter and tighter because of the collisions and heating up, until the whole movement distilled itself into just the swirl - the only thing they all agreed on. Anything moving otherwise was likely to hit something and lose its movement.