Combination of several factors actually, but gravity is a primary one, otherwise more systems would be more “round”, especially ones that formed earliest and had matter being pulled in from all around. Gravity, and the perceived centripetal forces it and inertia combine to create, pull matter in and “flatten” the disc despite the angular momentum of the smaller particles of matter that might come in from perpendicular angles.
Conservation of angular momentum does explain why it’s likely that bodies that are at wider orbital angles likely formed outside of the formation of the system or suffered collisions with larger objects very late in or after the formation of the system since once the matter is flattened into a disc by gravitational and inertial forces, it’s unlikely they would deviate from that angle of orbit without major external forces.
Due to conservation of angular momentum, not gravity.
Combination of several factors actually, but gravity is a primary one, otherwise more systems would be more “round”, especially ones that formed earliest and had matter being pulled in from all around. Gravity, and the perceived centripetal forces it and inertia combine to create, pull matter in and “flatten” the disc despite the angular momentum of the smaller particles of matter that might come in from perpendicular angles.
Conservation of angular momentum does explain why it’s likely that bodies that are at wider orbital angles likely formed outside of the formation of the system or suffered collisions with larger objects very late in or after the formation of the system since once the matter is flattened into a disc by gravitational and inertial forces, it’s unlikely they would deviate from that angle of orbit without major external forces.