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Why are our planets in the solar system all on the same disc/plane/layer?

After watching this video I realized that the orbits of the planets in our solar system basically reside along a similar 'flat' plane:

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Meaning that the angles of orbit are no greater than 20 degrees (if you still count Pluto as a planetary object). Why are there none of the planets at a 45 or even 90 degree orbit (in relation to the earth)? Is this due specifically to the gravitational pull from the Sun?

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The shortest answer is that systems "like" to be in low energy states. If we conserve the total angular momentum of the planets, then a flat disc has the least total kinetic energy.

Consider if a a new planet enters the solar system at a 90 degree angle to the others. The solar system will continue to dissapate energy in collisions and gravitational effects until all planets are roughly in line. This obviously takes a while, but out solar system has had 6? billion to do so.

You might also observe that galaxies are flat discs, while one could imagine stars clumping into a sphere instead. This occurs for the same reason. A flat disc has less kintetic energy than a sphere of stars orbiting in random planes.

Just like how a ball always rolls down a hill, solar systems and galaxies move towards the lowest energy state.

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    $\begingroup$ You've got a lot of the right bits in here, but a mature systems, like ours is so nearly empty that high inclination orbits would mostly be expected to be stable $\endgroup$
    – dmckee --- ex-moderator kitten
    Oct 29, 2012 at 20:14

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