Why Are All Planets in the Same Plane?

[aetherguy881]

I've been wondering why in our solar system, all of the planets are in a single plane rotating around the sun. Why are there no intersecting planes? Also, would there be any solar systems with such planes?

I'm not asking for any minor angles. For example, I'm asking if there's any solar systems that have a planar axis such that they would intersect at ~30-90 degrees.

Thanks

 

[lzkelley]

Good question.
First i should point out that all of the planets are only close to the same plane. Usually we define a reference frame defined by the orbit of the earth, then refer to the angle of other planets' orbits relative to that. For instance mercury (with the largest angle of inclination) is about 7 degrees off (i think).

Anyway, as you suggested those are very minor angles.
The reason they are all so close to planar is due to their formation. When a nebula (collection of tiny tiny tiny particles) starts to coalesce into a planetary system - a nucleus has to form that becomes the sun. As particles "accrete" together, increasing the mass, a lot of angular momentum has to be conserved (think of spinning on a swing and pulling your legs in - you speed up), so the whole system starts to spin and flatten out into a disk. Once you have the disk, the center keeps increasing in mass, and smaller bodies start to form further out (which become planets). And that's the in-a-nutshell story of our creation, and why we're so close to a plane.

You can't have an accreting sphere (as apposed to disk), because the rotating particles in different planes will collide and flatten into a disk again - so planets can't form drastically out of the plane.
But it should by possible for a system to capture a larger body out of the plane. If this happened it wouldn't be stable for very long however... so overall the chances of seeing something like this are very slim in a single sun system.

In binary and trinary systems, some much more complex dynamics can come into play... maybe its possible there? (i think it would still be rare)

 

[LURCH]

It is also worth noting that Oort Cloud objects (otuside the orbits of Neptune and Mercury) appear to orbit the Sun at fairly random angles.

 

[aetherguy881]

So even with a drastically different orbit radius it would still flatten out?

Say the orbit of Mercury vs, the orbit of orbit of Neptune? What if orbits that different in size were in separate planes. There would be no way that they could collide. Would it still flatten?

Thanks for answering the initial question.

 

[russ_watters]

I think the previous explanation glosses over one issue a little. If the cloud is rotating about a single axis, but evenly distributed (say, in a uniform sphere), objects near the poles are not in orbit around the center of the cloud, just like the circle New York traces around the Earth as it rotates doesn't go around the center of the earth. So an object at high latitude in our rotating cloud has to either be "swept-up" by the disk or it would fall into the sun. I've never seen a computer simulation of this, but there is probably a latitude above which all objects fall into the sun and below which get pulled into the disk.

 

[Redbelly98]

So even with a drastically different orbit radius it would still flatten out?

Say the orbit of Mercury vs, the orbit of orbit of Neptune? What if orbits that different in size were in separate planes. There would be no way that they could collide. Would it still flatten?

Thanks for answering the initial question.

The collision and flattening process happens way before the planets have formed, when it's all dust or smaller particles.

 

[lzkelley]

Redbelly has answered why they wouldn't form that way.

But drastically different orbital radii would be required for the situation to arise another way (how i have no idea - it may not be possible).
If the radii were similar, even if the bodies didn't physically collide - their mutual attraction would (i think) certainly destabilize the orbits.

Another thing i just thought of: although the effect of individual planets on the sun is almost negligible, they do have slight effects - the sun actually orbits a point within its own radius.
say for instance that mercury orbits at a right angle to mars' orbit - and there are no other planets. The 2 always perpendicular forces on the sun could very possibly develop resonant effects -> magnifying and instabilities... another reason this would be hard to occur.

 

[russ_watters]

Turns out that a simulation was pretty easy to find: http://astronomyonline.org/SolarSystem/SolarSystemFormation.asp

 

[aetherguy881]

Ok I guess that answers my questions guy, thanks.