First, dust in the early solar nebula settles to the mid-plane of the nebula and accretes together 
to form small solid bodies called planetesimals. This is currently the least-understood phase of
 planet formation, with several competing theories. The first is that planetesimals form as the 
result of gravitational instability in the solar nebula, in which solids are sufficiently concentrated
 that planetesimals are able to form purely by self-gravity (e.g., Goldreich and Ward, 1973,
 Ward, 2000 and Youdin and Shu, 2002). The second is that planetesimals form by the direct 
collisional accretion between colliding particles (e.g., Weidenschilling, 1980, Weidenschilling and Cuzzi, 1993 and Wurm et al., 2001). 
While this phase of planet formation still remains to be fully quantified, the key point is that 
once these planetesimals reach a size where they can gravitationally perturb each other, generally 
on the order of a few km, their orbits begin to cross. 
In the second stage, which is much better understood, collisions between planetesimals on 
crossing orbits lead to the growth of larger planetary embryos. The growth of planetary embryos
 initially proceeds by a process called runaway accretion (e.g., Greenberg et al., 1978, Wetherill and Stewart, 1989, Kokubo and Ida, 1996 
and Weidenschilling et al., 1997). In a swarm of planetesimals, the relative velocity vrel is governed by their frequent encount
ers with one another, and given their small gravity, is kept low. When one body starts to grow larger than the others, 
geometric cross section as well as its gravitational field strengthens, allowing it to sweep up more and more planetesimals. 
Runaway growth is stalled somewhat when the planetary embryos grow large enough that their gravitational perturba
tions on the planetesimals become the dominant influence on vrel. The system enters a regime called oligarchic g
rowth (e.g., Kokubo and Ida, 1998) in which neighboring embryos are forced to grow at similar rates. When a 
body starts to grow larger than its neighbors, it begins to increase the vrel of planetesimals in its vicinity, decreasin
g its accretion efficiency, and thus letting its neighbors catch up. The end result of this stage is a system 
of roughly comparably sized and spaced planetary embryos embedded in a swarm of planetesimals with a total mass roughly comparable to the total mass of embryos. 
In the third and final stage of terrestrial planet accretion, the gravitational effect of the planetesimals begins
 to fade as their numbers decrease, and the planetary embryos begin to perturb one another onto crossing
 orbits. Planets then begin grow from collisions between embryos and the accretion of remaining 
planetesimals. This stage is characterized by relatively violent, stochastic large collisions as compared
 to the previous stages, where the continual accretion of small bodies dominates