Why do all the planets orbit in the same plane? (Intermediate)

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.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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