Re: How did the Solar system form

Gravity is indeed the weakest of the four fundamental forces.  It is not important when 
considering interactions between atoms or molecules, but it is the dominant force for large-scale 
structures.  Look here for a good comparison of the strengths of the fundamental forces and 
the distances to which they are effective.  Both gravitational and electromagnetic forces have 
infinite ranges (they are present at all distances), and since electromagnetic forces are stronger 
than gravitational forces (by tens of orders of magnitude), electromagnetic forces should dominate 
at any distance.  We know, however, that gravitational forces of macroscopic objects (including 
very large objects like the Earth) are important, whereas they do not exert strong electromagnetic 
forces.  This is because most macroscopic objects are electrically neutral (they have roughly the 
same number of protons and electrons).  Gravity operates on all matter, regardless of electric 
charge, making it a universal attractive force.  So, although the gravitational force between two 
particles is small, it is larger than the other three fundamental forces.

The solar system began to form around 4.6 billion years ago from an interstellar cloud that 
became dense enough to shrink under its own gravitational forces.  A central area formed which 
would become the sun.  This area was surrounded by the solar nebula: a disk of gas and later 
grains of dust that condensed as the nebula cooled.

These dust particles would have undergone low-velocity collisions with one another.  These 
particles stuck together due to gravitational forces (though electrostatic attraction may have 
occurred as well).  Ultimately, gravity caused these small planetesimals to accrete into larger 
objects, possibly becoming multi-kilometer objects on the order of only a few thousand years.  
Since the larger objects were more massive, they had stronger gravities and grew faster than the 
smaller objects.