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.
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