why do planets rotate on the same plane

I am a novice astronomer and have been enjoying learning about the stars and the local universe. I have noticed that the orbits of the planets are relativly planer (with the exception of Pluto). I'm curious why this is. Are the orbits planer because of the gravitational pull the planets have on each other (I envision the creation of the solar system with eratic orbits all dampening to the center of mass). I don't know if there is a "proven" answer to the question so I am interested in some of the prevailing theories. The orbits of the planets are coplanar because during the Solar System's formation, the planets formed out of a disk of dust which surrounded the Sun. Because that disk of dust was a disk, all in a plane, all of the planets formed in a plane as well. Rings and disks are common in astronomy. When a cloud collapses, the conservation of angular momentum amplifies any initial tiny spin of the cloud.

As the cloud spins faster and faster, it collapses into a disk, which is the maximal balance between gravitational collapse and centrifugal force created by rapid spin. The result is the coplanar planets, the thin disks of spiral galaxies, and the accretion disks around black holes. This page was last updated on July 18, 2015.
George Spagna, chair of the physics department at Randolph-Macon College, explains. Stars and planets form in the collapse of huge clouds of interstellar gas and dust. The material in these clouds is in constant motion, and the clouds themselves are in motion, orbiting in the aggregate gravity of the galaxy. As a result of this movement, the cloud will most likely have some slight rotation as seen from a point near its center.

This rotation can be described as angular momentum, a conserved measure of its motion that cannot change. Conservation of angular momentum explains why an ice skater spins more rapidly as she pulls her arms in. As her arms come closer to her axis of rotation, her speed increases and her angular momentum remains the same. Similarly, her rotation slows when she extends her arms at the conclusion of the spin. As an interstellar cloud collapses, it fragments into smaller pieces, each collapsing independently and each carrying part of the original angular momentum. The rotating clouds flatten into protostellar disks, out of which individual stars and their planets form. By a mechanism not fully understood, but believed to be associated with the strong magnetic fields associated with a young star, most of the angular momentum is transferred into the remnant accretion disk.

Planets form from material in this disk, through accretion of smaller particles. In our solar system, the giant gas planets (Jupiter, Saturn, Uranus, and Neptune) spin more rapidly on their axes than the inner planets do and possess most of the system's angular momentum. The sun itself rotates slowly, only once a month. The planets all revolve around the sun in the same direction and in virtually the same plane. In addition, they all rotate in the same general direction, with the exceptions of Venus and Uranus. These differences are believed to stem from collisions that occurred late in the planets' formation. (A similar collision is believed to have led to the formation of our moon. )

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