Victor
Safronov’s Model Of How Planets Evolved

THE
model proposed by Victor Safronov (1917-1999), Soviet astronomer, for evolution
of planets around a star from an initial interstellar cloud informs our modern
understanding of the dynamics of planet formation. His theory was in contrast to
the then prevailing theory that conceived planets to be end product of a process
of gravitational fragmentation of the proto-planetary disc directly into
planets. Safronov theorised that planets form from aggregation of countless
smaller bodies: dust grains and debris left over in the solar system after
formation of the Sun. Initially the dust or small grain move about due to
Brownian motion, or turbulent motions in the gas to cause the collisions that
can lead to coalescing. However, when the bodies reach sizes of approximately
one kilometer, then they can attract each other directly through their mutual
gravity, aiding further growth into moon-sized protoplanets enormously. Many
such planetesimals are indeed formed in the accretion disc, many eventually
break apart during violent collisions, but a few of the largest planetesimals
survive such encounters and continue to grow into protoplanets and later
planets. In this model planets are end product of secondary accretion in a disk
around a primary star. Planets in this sense occur only in highly evolved (old)
systems, which have reached the final cleanup phase of accretion, with the major
bodies in stable non-intersecting orbits. In this model planets are the solitary
bodies that prevail in the creative-destructive evolution of a disk, and are
dynamically distinct from the populations of leftover debris.