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The best estimate is that the earth formed about 4.6 billion years
ago, not
12 billion. According to a recent article in the Sep-Oct 2001 issue
of
American Scientist (pages 432-442), the protoearth accretion was 99%
complete 40 million years after "the settling of dust in the
accretionary
disk of the protosolar system."
See Cameron, A. G.W. 1978. Solar accretion disk and planetary
formation.
In Origin of the Solar System, ed. S. F. Dermott, pp. 49-74. New
York:
Wiley.