Why planet's orbit is not perpendicular or random ? It always seems each planet is revolving on the same geometric-plane around the star.

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Short answer: conservation of angular momentum.

Long answer:

The origin of almost any planetary system is a sparse cloud. That cloud starts to contract due (typically) to a pressure wave crossing it.

The cloud fragments as it contracts, and each fragment is what we know as a pre-star cloud.

Since almost always there is some movement in the matter in each cloud, the cloud as a whole starts to rotate, very slowly. Contraction helps because, due to angular momentum conservation law, when the cloud contracts, the rotation accelerates.

Soon we get a protostar with the most contracted matter, surrounded by a protoplanetary disk composed with the less contracted matter. The rotation of the whole system is in the same plane, due to conservation of angular momentum.

The protostar becomes a star, and the protoplanetary disk becomes a bunch of planets. Each planet, in turn, orbits the star and rotates on itself, all in the same direction, based on which point of the protoplanetary disk started accreting mass.

Later on, interaction among massive bodies disturbs this shiny process by changing some orbits and by having some collisions, but that's minor.

  • $\begingroup$ I always found the short answer somewhat confusing. A globular cluster, where the orbits are much more random, also conserves angular momentum. $\endgroup$ – chris May 10 '14 at 9:17
  • $\begingroup$ The orbits of stars on a globullar cluster are seemingly random due to the enormous amount of interactions they have had among the stars, including ejections of some individuals that effectively change the angular momentum of the cluster. $\endgroup$ – Envite May 12 '14 at 9:12
  • $\begingroup$ how would you explain elliptical galaxies then:-) $\endgroup$ – chris May 12 '14 at 9:57
  • $\begingroup$ The intercation among stars on galaxies is very tiny. Globular clusters have a density of 10^5 to 10^6 stars in a very small sphere (on the magnitude of 1pc, almost never reaching 10pc), while galaxies have a density of 10^11 or 10^12 stars in an enormous disk (on the magnitude of 10^4 or 10^5pc) $\endgroup$ – Envite May 12 '14 at 10:12
  • $\begingroup$ so elliptical galaxies conserve the angular momentum of each star but non the less are not flat $\endgroup$ – chris May 12 '14 at 10:17

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