I'm trying to see this. OK we have a Big Bang. Particles flow out in a straight line from the central explosion. There are no large bodies with gravity YET and so nothing to slow the particles down from moving in a straight line. The particles could continue to move in this straight line for infinity ,but somehow they start to move in orbits ?????
Dan
You're understanding of the Big Bang is far too close to a conventional explosion. In reality, it wasn't similar at all. The Big Bang is less about particles flying away from a single point and more about the actual
space expanding. Imagine a bunch of dots on a deflated balloon. Now, inflate the balloon. The dots will spread apart, but it's not the same as all of them simply flying away from each other.
Gravity casues the early matter to be held together despite the expansion of space. The result is essentially a multitude of clusters of matter, which are the precursors to modern galaxies.
This is still very much an oversimplification, so please bear with me.
Unfortunately, the Big bang doesn;t really have anything whatsoever to do with the formation of a solar system - it's a completely different topic (there were no heavy elements, for example, in the first eons of the universe's existence).
Planetary formation is very different.
Given a nebula, which is basically just a large amount of gas in a localized area, the gravitational attraction of the gas will cause it to concentrate itself and squeeze towards the center of mass. If there is a sufficient amount of gas, the force of gravity will be sufficient to induce nuclear fusion, and birth a star.
The remaining material, still circling the star in what's essentially a whirlpool into the new star's gravity well, will have some clusters of debris at the appropriate distance and speed to have a stable orbit with the star. The material in between these orbits will either be flung into space, accrete to the orbiting bodies due to their gravitational attraction, or fall into the new star.
Once this process is complete, a solar system will have formed. Occasionally a stray asteroid will be captured by a planet's gravity and become a new satellite rather than being drawn down into a collision (as seems to be the case with the two diminutive moons of Mars, for instance). Some moons will form from the debris left over from the planet's formation, happening to be in a stable orbit, exactly as the planets formed around the star.
The important things to remember:
1) The Big bang wasn't an "explosion" in the way we think of a stick of dynamite. Space expanded, matter was not being propelled away from itself.
2) Gravitational attraction causes matter to clump up, which prevents the even distribution and decreasing density that you're envisioning.
Every time a fundy breaks the laws of thermodynamics, Schroedinger probably kills his cat.