Planetary Orbits

[rabbithutch]

If this has been asked and discussed before, please forgive me.

In all the depictions of the planets in relation to the sun that I recall seeing, the planetary orbits are in approximately the same plane, similar to the rings of Saturn being a plane around the planet. (I hope I've described this correctly.)

My question is why aren't the planetary orbits more random like the normal depictions of electrons circling a nucleus? Are there physical/mathematical explanations? Or have my observations been flawed?

[Nexus 6]

Gravity?

[rabbithutch]

Why? Does gravity apply just above an equatorial point? I can't conjure up the nature of the forces at play in my meager mind.

[Nexus 6]

Well I am not sure but thought it could be a possible reason why Bob, but it was nothing more than an educated guess, probably way of the mark!

[inksmithy]

Centrifugal force really.

Alan

[FraserClarke]

Conservation of angular momentum.

Planets form from a disc of material around the proto-sun (ala Saturn's rings - though not quite as thin). The reason you get a disc is conservation of angular momentum. When the sun forms from a collapsing gas cloud, angular momentum means the gas cloud can't collapse evenly, and forms a thin disc in the equatorial plane of the star (roughly, it can collapse along the poles, because it has no angular momentum in this direction, but can't collapse along the equator, because it has lots of angular momentum). The planets form out of this material, and keep that planar shape to this day.

(there are also more subtle dynamical effects which mean it is very hard to have an orbit which is highly inclined to the most massive planets)

[jamespels]

Heh, beat me to it TeaDwarf - was about to say pretty much the same thing!

Also, as you go further out, you start finding behaviour like the OP expected - the Oort Cloud, for instance, is understood to be a largely uniform sphere. Comets are believed to originate in the Oort Cloud and can come in form any direction. The most obviously out-of-plane orbit of any object in the solar system is Pluto (whether or not you accept it as being a planet) - one theory suggests that Pluto is a large Oort Cloud object that was "captured" by Neptune.

Note the words like "believed", "understood" and "theory" - we have a pretty good idea about much of this stuff but very little is known for sure.

[brianb]

My question is why aren't the planetary orbits more random like the normal depictions of electrons circling a nucleus? Are there physical/mathematical explanations?

Yes, angular momentum transferred from random variations in the collapsing protostellar nebula.

And the "normal depiction" of electron "orbits" in an atom bears no relation to reality - at that sort of scale, quantum effects forbid an electron from having a precise location at any given instant, rather they behave as a probability distribution function.

[rabbithutch]

Thank you for the information! I'll do more reading on angular momentum to try and get my brain around the concept.

As for the atomic representation, I'm aware that it does not represent reality but needed an example to point to clarify my question.

Angular momentum! Hmmmmm.

Any other examples of angular momentum to help me understand it? Any way to model it?

Thanks again for the responses.

[tibbs1972]

I've recently seen a video explain Angular Momentum on youtube.

It might help, especially the part with the bike wheel.

[FraserClarke]

Any other examples of angular momentum to help me understand it? Any way to model it?

The classic one is the spinning ice skater... arms out, spin slowly. bring arms in, spin faster. The speed-up is because angular momentum is conserved as the mass (arms) moves in to smaller radius. You can do the same if you have a good low friction spining office chair...

[rabbithutch]

Hello Neil!

Thank you for the YouTube link. That helped me understand the concept of angular momentum and helps to answer my original question.

I have only a vague understanding of even the most elemental concepts of physics. (More than 50 years have passed since I took the class in high school.) I had been expecting that centrifugal force (the force that moves mass to the exterior limits of a thing spinning around an axis, like a weight on a string swung round one's head) would be offered as the explanation. The video now forces me to question whether centrifugal (or is it centripetal?) force and angular momentum are the same thing.

Maybe I'm just too old to understand.