The period when our solar system began its individual life was remote
beyond the possibility of conception. Naturalists are pretty well
agreed that living beings began to exist upon the earth at least a
hundred million years ago; but the beginnings of our solar system must
be placed at a date very many times as remote from the present day.[1]

[Footnote 1: Some astronomers, particularly the distinguished Professor
Newcomb, hold that the sun can not have been supplying heat as at
present for more than about ten million years, and that all geological
time must be thus limited. The geologist believes that this reckoning is
far too short.]

According to the nebular theory, the original vapour of the solar
system began to fall in toward its centre and to whirl about that
point at a time long before the mass had shrunk to the present limits
of the solar system as defined by the path of the outermost planets.
At successive stages of the concentration, rings after the manner of
those of Saturn separated from the disklike mass, each breaking up and
consolidating into a body of nebulous matter which followed in the
same path, generally forming rings which became by the same process
the moons or satellites of the sphere. In this way the sun produced
eight planets which are known, and possibly others of small size on
the outer verge of the system which have eluded discovery. According
to this view, the planetary masses were born in succession, the
farthest away being the oldest. It is, however, held by an able
authority that the mass of the solar system would first form a rather
flat disk, the several rings forming and breaking into planets at
about the same time. The conditions in Saturn, where the inner ring
remains parted, favours the view just stated.