the cosmic dust would gather about
a number of smaller centres. Thus might be explained the hundreds of
planetoids, or minor planets, which we find between Mars and Jupiter. If
these smaller bodies came within the sphere of influence of one of
the larger planets, yet were travelling quickly enough to resist its
attraction, they would be compelled to revolve round it, and we could
thus explain the ten satellites of Saturn and the eight of Jupiter. Our
moon, we shall see, had a different origin.

We shall find this new hypothesis crossing the familiar lines at
many points in the next few chapters. We will consider those further
consequences as they arise, but may say at once that, while the new
theory has greatly helped us in tracing the formation of the planetary
system, astronomers are strongly opposed to its claim that the planets
did not pass through an incandescent stage. The actual features of our
spiral nebulae seem clearly to exhibit that stage. The shape of the
planets--globular bodies, flattened at the poles--strongly suggests that
they were once liquid. The condition in which we find Saturn and Jupiter
very forcibly confirms this suggestion; the latest study of those
planets supports the current opinion that they are still red-hot, and
even seems to detect the glow of their surfaces in their mantles of
cloud. These points will be considered more fully presently. For
the moment it is enough to note that, as far as the early stages of
planetary development are concerned, the generally accepted theory
rests on a mass of positive evidence, while the new hypothesis is
purely theoretical. We therefore follow the prevailing view with some
confidence.

Those of the spiral nebulae which face the earth squarely afford an
excellent suggestion of the way in which planets are probably formed. In
some of these nebulae the arms consist of almost continuous streams
of faintly luminous matter; in others the matter is gathering about
distinct centres; in others again the nebulous matter is, for the most
part, collected in large glowing spheres. They seem to be successive
stages, and to reveal to us the origin of our planets. The position
of each planet in our solar system would be determined by the chance
position of the denser stuff shot out by the erupting sun. I have seen
Vesuvius hurl up into the sky, amongst its blasts of gas and steam,
white-hot masses of rock weighing fifty tons. In the far fiercer
outburst of the erup