# Complanary systems but why?

1. Jul 4, 2004

### dedaNoe

The planets in the solar system are almost complanary. I wonder why?
I know it has something to do with their common axes of rotation. The planets can have only one common axes of rotation (or do they?).

The galaxies are also almost flat but the atom is not considered this way. So many things to wonder about.

2. Jul 4, 2004

### Janitor

I am not familiar with the word "complanary," but I wonder if it means the same thing as "coplanar." If so, then I think part of the explanation is that the solar system was, early in its history, a spinning disk of gas and dust, with chunks of solid here and there which grew into planets and moons. I know there is a dynamical explanation for why Saturn's rings stay so thin, and I don't doubt there is likewise a dynamical explanation for why the planets have their orbits in planes that are all pretty close to the ecliptic plane. I think there may be a few moons among the outer planets that are pretty far from the equatorial plane of their planet, but I am thinking the explanation for that is that those moons were recently captured and have not had time to be forced into the equatorial plane. I hope somebody here can provide more on this topic.

3. Jul 4, 2004

### Janitor

The distribution of atomic electrons is well understood, though doing the calculations for atoms more complicated than hydrogen requires approximation techniques, which probably amount to finite-difference calculations on a computer. Some electron orbitals have spherical symmetry. The ones that don't have spherical symmetry at least manage to have an axis of cylindrical symmetry, if I remember.

4. Jul 4, 2004

### Entropy

Maybe because the sun was created spinning the way it does, maybe all the planets were influenced to spin the same way.

5. Jul 6, 2004

### Phobos

Staff Emeritus
Welcome to Physics Forums, dedaNoe!

As described above, the solar system formed from a diffuse cloud of material (a nebula). As gravity pulled this asymmetrical cloud together, it has a slight spin. Like an ice skater pulling in his/her arms to spin faster, the nebula material spun faster as it became more compacted. The largest mass of material in the center became the sun and the mass further out became the planets, asteroids, comets, etc. You can see that so far everything is spinning the same way.

With a faster spin, there is more of a "force" to throw matter outward perpendicular to the axis of spin. (Think of yourself on a spinning carnival ride.) So, the combination of gravity pulling matter inward and the spin flinging matter outward caused the material to flatten out into a disk shape. This is why the planets orbit more or less in the same plane and is also why some galaxies (like our own Milky Way) are disk-like. The spare material in the outer part of the disk was swept up by the planets so now the solar system is mostly empty. There are still some leftover building blocks...i.e., the asteroids and comets.

The sweeping up of material is not always a calm experience, but instead can be planet-shattering collisions. Astronomers believe that a Mars-sized planetoid smacked into the Earth about 4 to 4.5 billion years ago and resulted in the formation of the Moon (all the material that was thrown off the Earth and probably the other impactor).

The planets Venus and Uranus rotate on their axis differently than the other planets, perhaps due to similar ancient collisions that tumbled their original axis of rotation (that used to be like the other planets') in a new direction. Venus rotates in the opposite direction. Uranus rotates on its side.

Atoms are a very different thing. In the past, it was often taught that the solar system was a good analogy for electrons orbiting the atomic nucleus. But now that analogy is not too useful except for a very, very basic understanding of electron valence states. Perhaps you could start a new topic in the physics forum about a better, modern analogy.

6. Jul 8, 2004

### dedaNoe

It seems that the centrifugal force is the reason for flatness of our solar system. The centrifugal force according to Newton's second law depends on the linear velosity. I think this was the formulae: $$F_c=m\frac {V^2}{R}$$.

Why would the solar system spin faster then before?

Regarding the similarity between the solar system and the atoms model, I think that the principle must be the same and it should not depend on the sizes.

7. Jul 8, 2004

### turin

The planetary model of the atom is not considered to be physically representative. Back in the '20's, someone (I think it was Bohr), tried to revamp the planetary model to "fix" a decay problem, but that was ultimately unsatisfactory, and, within the same decade, a completely different model took its place.