If you’ve taken physics, you know what angular momentum is. When small particles or objects as big as planets collide, their angular momentum must be conserved, according to the law of conservation of angular momentum.

When the hot ball of gas that used to be our solar system got pulled together by gravity, things started to smash together. And even though objects are free to collide and bounce off into any direction, the up and down motions cancel out, leaving us with a 2 dimensional axis that the planets, stars, and asteroids rotate about.

Good question! The answer is because angular momentum is a vector quantity. While it was forming, the solar system was a diffuse cloud of dust and plasma, interacting with itself through fluid dynamics. This means that the matter in the solar system shared its angular momentum with the rest of the matter in the solar system, with the result that nearly everything in the solar system has largely coaligned angular momentum vectors. Because orbits are perpendicular to these vectors, the orbits of the coaligned vectors will wind up in the same plane.