But this was not
the end of it, for within a few years three or four more tiny planets
were observed not far from the first one, and, as years rolled on, one
after another was discovered until now the number amounts to over six
hundred and others are perpetually being added to the list! Here was a
new feature in the solar system, a band of tiny planets not one of which
was to be compared in size with the least of those already known. The
largest may be about as large as Europe, and others perhaps about the
size of Wales, while there may be many that have only a few square miles
of surface altogether, and are too small for us to see. To account for
this strange discovery many theories were advanced.

One was that there had been a planet--it might be about the size of
Mars--which had burst up in a great explosion, and that these were the
pieces--a very interesting and exciting idea, but one which proved to be
impossible. The explanation now generally accepted is a little
complicated, and to understand it we must go back for a bit.

When we were talking of the earth and the moon we realized that once
long ago the moon must have been a part of the earth, at a time when the
earth was much larger and softer than she now is; to put it in the
correct way, we should say when she was less dense. There is no need to
explain the word 'dense,' for in its ordinary sense we use it every day,
but in an astronomical sense it does not mean exactly the same thing.
Everything is made up of minute particles or atoms, and when these
atoms are not very close together the body they compose is loose in
texture, while if they are closer together the body is firmer. For
instance, air is less dense than water, and water than earth, and earth
than steel. You see at once by this that the more density a thing has
the heavier it is; for as a body is attracted to another body by every
atom or particle in it, so if it has more particles it will be more
strongly attracted. Thus on the earth the denser things are really
heavier. But 'weight' is only a word we use in connection with the
earth; it means the earth's pulling power toward any particular thing at
the surface, and if we were right out in space away from the earth, the
pulling power of the earth would be less, and so the weight would be
less; and as it would be impossible always to state just how far away a
thing was from the earth, astronomers talk about density, which means
the number of parti