  The formation of a solar system is believed to have followed a multi-stage
process around a protostar. Whipple first noted that planetesimal growth by
particle agglomeration is strongly influenced by gas drag; there is a
"bottleneck" at the meter scale with such bodies rapidly spiraling into the
central star, whereas much smaller or larger particles do not. Thus, successful
planetary accretion requires rapid planetesimal growth to km scale. A commonly
accepted picture is that for collisional velocities $V_c$ above a certain
threshold collisional velocity, ${V_{th}} \sim$ 0.1-10 cm s$^{-1}$, particle
agglomeration is not possible; elastic rebound overcomes attractive surface and
intermolecular forces. However, if perfect sticking is assumed for all
collisions the bottleneck can be overcome by rapid planetesimal growth. While
previous work has dealt explicitly with the influences of collisional pressures
and the possibility of particle fracture or penetration, the basic role of the
phase behavior of matter--phase diagrams, amorphs and polymorphs--has been
neglected. Here it is demonstrated that novel aspects of surface phase
transitions provide a physical basis for efficient sticking through collisional
melting or amphorph-/polymorphization and fusion to extend the collisional
velocity range of primary accretion to $\Delta V_c \sim$ 1-100 m s$^{-1}$,
which bound both turbulent RMS speeds and the velocity differences between
boulder sized and small grains $\sim$ 1-50 m s$^{-1}$. Thus, as inspiraling
meter sized bodies collide with smaller particles in this high velocity
collisional fusion regime they grow rapidly to km scales and hence settle into
stable Keplerian orbits in $\sim$ 10$^5$ years before photoevaporative wind
clears the disk of source material.
