Expert: Jayendra Upadhye - 4/6/2005

Question
Could you please tell me why do the planets in our solar sistem have all their orbits (except Pluto) in an almost flat plane? Why aren't orbits in the entire 3 dimensional space around the sun? Why do Neptune and Saturn have flat rings and not a cloud of dust around them ? Also, why do galaxies form flat spirals and not  big round clouds of stars?  Is it just how it is, or is there a reason in all this ?

Answer
Hi Tudor,
The answer lies in plain newtonian physics.
In the orbital plane the bodies tend to fall into stable orbits where the centripetal acceleration due to the parent body , g =  (V*V)/r where r is orbital radius and V is orbital tangential velocity. in a collection of orbiting bodies such as in the rings of saturn, each body satisfies this equation and maintains it unique orbital radius.

bodies with eccentric orbits soo end up colliding with intervening bodies in their own circular orbits. thus what remains is a system of bodies in fairly circular orbits, all tightly whirling around in a spread along the orbital plane, yielding a disc.

Any non coplanar orbits are soon "pulled" into allignment or absorbed by collisions as the pass thru the dense orbital plane. The pulling is possible as there disc as a whole exerts a pull on the non coplanar body and nullifies slowly the velocity component along the 'up-down' axis.

The scheele's limit ensures that no planetary body coalesces out of this debri.

In the case of the solar system, the same holds true.
The planets are formed by an onion like separation of layers in a flattened accretion disc around the proto star.
rather like a flattened onion.

each layer coalesces into an thin dense ring, and then into a planet. the inner and external peripheral velocities of the ring give the planet a spin when it is formed! thus planets in same area have same daylight hours (example mars and earth.)

Of course over billions of years, further collisions alter the spin axes and spin rate of all planets.

In galaxies too, except for the globular clusters that constitute very old red stars, and are evenly distributed in a spherical halo around the galactic center, all other stars "fall into line" or rather plane, due to similar processes.
hope this answers our question.
note:- As galaxies age, spiral galaxies encounter collisions with neighbours and form huge elliptical galaxies and spherical galaxies. The trait being a predominant population of old stars, telling us that the ellipticals are none but frgormed spirals of old!

Jayen