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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?


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 '12 at 20:14

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