One of the grandest pictures that the scientific mind has ever thrown
upon the screen is that of the Nebular Hypothesis. According to
Laplace's famous form of this theory (1796), the solar system was once a
gigantic glowing mass, spinning slowly and uniformly around its centre.
As the incandescent world-cloud of gas cooled and its speed of rotation
increased the shrinking mass gave off a separate whirling ring, which
broke up and gathered together again as the first and most distant
planet. The main mass gave off another ring and another till all the
planets, including the earth, were formed. The central mass persisted as
the sun.
Laplace spoke of his theory, which Kant had anticipated forty-one years
before, with scientific caution: "conjectures which I present with all
the distrust which everything not the result of observation or of
calculation ought to inspire." Subsequent research justified his
distrust, for it has been shown that the original nebula need not have
been hot and need not have been gaseous. Moreover, there are great
difficulties in Laplace's theory of the separation of successive rings
from the main mass, and of the condensation of a whirling gaseous ring
into a planet.
So it has come about that the picture of a hot gaseous nebula revolving
as a unit body has given place to other pictures. Thus Sir Norman
Lockyer pointed out (1890) that the earth is gathering to itself
millions of meteorites every day; this has been going on for millions of
years; in distant ages the accretion may have been vastly more rapid and
voluminous; and so the earth has grown! Now the meteoritic contributions
are undoubted, but they require a centre to attract them, and the
difficulty is to account for the beginning of a collecting centre or
planetary nucleus. Moreover, meteorites are sporadic and erratic,
scattered hither and thither rather than collecting into unit-bodies. As
Professor Chamberlin says, "meteorites have rather the characteristics
of the wreckage of some earlier organisation than of the parentage of
our planetary system." Several other theories have been propounded to
account for the origin of the earth, but the one that has found most
favour in the eyes of authorities is that of Chamberlin and Moulton.
According to this theory a great nebular mass condensed to form the sun,
from which under the attraction of passing stars planet after planet,
the earth included, was heaved off in the form of knotted spiral nebulæ,
like many of those now observed in the heavens.
Of great importance were the "knots," for they served as collecting
centres drawing flying matter into their clutches. Whatever part of the
primitive bolt escaped and scattered was drawn out into independent
orbits round the sun, forming the "planetesimals" which behave like
minute planets. These planetesimals formed the food on which the knots
subsequently fed.