People's Democracy

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.