Space Forum / Space Science / April 2005

correct me if I'm wrong but I thought it was because of Jupiters'
massive gravitational influence preventing this.

R.

-> Well, acording to gravity, the largest asteroid should attract the smallest  
-> ones, building up larger and larger with increasing gravity and attracting  
-> more and more objects until is uses up all the small bodies. If it gets  
-> large enough, intenal pressures would liquify the center producing internal  
-> heat, volcanos and an atomsphere. It should have happened a few billions  
-> years ago. If two large planets collided and broke up, then they should be  
-> coming together again.
 
When bodies collide, there are competing tendencies. They may coalesce,
or they may smash each other into even smaller fragments. Gentle
collisions tend to produce coalescence, so small bodies build up into
bigger ones. Violent collisions cause fragmentation.
 
Planets or asteroids can collide gently enough to coalesce only if
their initial orbits are very similar. But, in the asteroid belt,
everything gets stirred up by Jupiter's gravity, so when asteroids
collide they tend to do so rather fast. The result is that they often
smash each other apart, so they don't all join together to form a
single "planet". (Actually, if they did, it would be a very small body
by planetary standards - maybe as big as Pluto, but smaller than all
the other planets.)
 
                     dow