<p>Short answer: conservation of angular momentum.</p>

<p>Long answer:</p>

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

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

<p>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.</p>

<p>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.</p>

<p>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.</p>

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

CC BY-SA 3.0
2014-05-10T09:17:39.680Z
I always found the short answer somewhat confusing. A globular cluster, where the orbits are much more random, also conserves angular momentum.
chris
CC BY-SA 3.0
2014-05-12T09:12:24.237Z
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.
Envite
CC BY-SA 3.0
2014-05-12T09:57:21.903Z
how would you explain elliptical galaxies then:-)
chris
CC BY-SA 3.0
2014-05-12T10:12:35.700Z
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)
Envite
CC BY-SA 3.0
2014-05-12T10:17:39.433Z
so elliptical galaxies conserve the angular momentum of each star but non the less are not flat
chris
CC BY-SA 3.0
2014-05-12T10:24:00.067Z
Elliptical galaxies are dwarf galaxies, sort of giant globullar clusters, so they have had the same processes as globullar clusters, including huge amounts of interactions and ejections. Each star, indeed, keeps its own Angular Momentum, and the system as a whole (elliptical galaxy plus ejected stars) does as well, but the galaxy not counting the ejections, which is the observable body, does not.
Envite
CC BY-SA 3.0
2014-05-18T13:21:15.103Z
The typical deflection time due to encounters in elliptical galaxies (which would change the angular momentum of a given star) is larger than the age of the universe. Hence Ellipticals are essentially collision-less and each stars in Ellipticals conserve angular momentum. Nonetheless they are not flat. Hence angular momentum conservation cannot be the only reason why a disc is a disc.
chris
CC BY-SA 3.0
2014-05-19T07:03:31.083Z
Typical deflection time does not mean that all deflections take place at that moment. There are deflections way before that. Think in typical height for men in your country. Are there men higher than that? And shorter?
Envite
CC BY-SA 3.0
2014-05-19T13:02:16.010Z
sure; but not many men are higher than 1 km :-)
chris
CC BY-SA 3.0
2014-05-07T11:17:56.493Z
2014-05-07T11:17:56.493Z
Envite
<p>Why planet's orbit is not perpendicular or random ? It always seems each planet is revolving on the same geometric-plane around the star.</p>

<p><img src="https://i.sstatic.net/lyIL6.png" alt="enter image description here"></p>

<star><orbit><planet><orbital-elements>
Why planet's orbit is not perpendicular or random ?
AmitG
# Why planet's orbit is not perpendicular or random ?

(Asked by: username_0 on 2014-05-07)

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

![enter image description here](https://i.sstatic.net/lyIL6.png)

**Tags:** star, orbit, planet, orbital-elements

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## 1 answers

### Answer by username_1 on 2014-05-07. Score: 3 | Accepted answer

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.

> **Comments:**
> **username_2**: I always found the short answer somewhat confusing. A globular cluster, where the orbits are much more random, also conserves angular momentum.  
> **username_1**: 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.  
> **username_2**: how would you explain elliptical galaxies then:-)  
> **username_1**: 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)  
> **username_2**: so elliptical galaxies conserve the angular momentum of each star but non the less are not flat  
> **username_1**: Elliptical galaxies are dwarf galaxies, sort of giant globullar clusters, so they have had the same processes as globullar clusters, including huge amounts of interactions and ejections. Each star, indeed, keeps its own Angular Momentum, and the system as a whole (elliptical galaxy plus ejected stars) does as well, but the galaxy not counting the ejections, which is the observable body, does not.  
> **username_2**: The typical deflection time due to encounters in elliptical galaxies (which would change the angular momentum of a given star) is larger than the age of the universe. Hence Ellipticals are essentially collision-less and each stars in Ellipticals conserve angular momentum. Nonetheless they are not flat. Hence angular momentum conservation cannot be the only reason why a disc is a disc.  
> **username_1**: Typical deflection time does not mean that all deflections take place at that moment. There are deflections way before that. Think in typical height for men in your country. Are there men higher than that? And shorter?  
> **username_2**: sure; but not many men are higher than 1 km :-)  

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