There are marked advantages in starting the evolution of the

solar system from a spiral nebula, aside from the fact that
spirals are abundant, and therefore represent a standard
product of development
There are marked advantages in starting the evolution of the
solar system from a spiral nebula, aside from the fact that
spirals are abundant, and therefore represent a standard
product of development. The material is thinly and very
irregularly distributed in a plane passing through the Sun, and
the motions around the Sun are all in the same direction. The
great difficulty in the Laplace hypothesis, as to the constancy
of the moment of momentum, is here eliminated. There are
well-defined condensations of nuclei at quite different
distances from the Sun. According to this hypothesis the
principal nuclei are the beginnings of the future planets. They
draw into themselves the materials with which they come in
contact by virtue of the crossings of the orbits of various
sizes and various eccentricities. The growth of the planets is
gradual, for the sweeping up and combining process must be
excessively slow. The satellites are started from those smaller
nuclei which happen to be moving with just the right speeds not
to escape entirely the attractions of the principal nuclei, nor
to fall into them. The planes of the planetary orbits and, in
general, the planes of the satellite orbits should agree quite
closely with each other, but they could differ and should
differ from that of the Sun"s equator.


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