Numerous as are the heavenly bodies visible to the naked eye, and
glorious as are their manifestations, it is probable that in our own
system there are great numbers as yet undiscovered. Just two hundred
years ago this year, Huyghens announced the discovery of one satellite
of Saturn, and expressed the opinion that the six planets and six
satellites then known, and making up the perfect number of _twelve_,
composed the whole of our planetary system. In 1729 an astronomical
writer expressed the opinion that there might be other bodies in our
system, but that the limit of telescopic power had been reached, and no
further discoveries were likely to be made.[A] The orbit of one comet
only had been definitively calculated. Since that time the power of the
telescope has been indefinitely increased; two primary planets of the
first class, ten satellites, and forty-three small planets revolving
between Mars and Jupiter, have been discovered, the orbits of six or
seven hundred comets, some of brief period, have been ascertained;--and
it has been computed, that hundreds of thousands of these mysterious
bodies wander through our system. There is no reason to think that all
the primary planets, which revolve about the sun, have been discovered.
An indefinite increase in the number of asteroids may be anticipated;
while outside of Neptune, between our sun and the nearest fixed star,
supposing the attraction of the sun to prevail through half the
distance, there is room for ten more primary planets succeeding each
other at distances increasing in a geometrical ratio. The first of
these will, unquestionably, be discovered as soon as the perturbations
of Neptune shall have been accurately observed; and with maps of the
heavens, on which the smallest telescopic stars are laid down, it may be
discovered much sooner.
[Footnote A: _Memoirs of A.A.S._, vol. iii, 275.]