and apparently are shining by a
dull reflected light. It is unlikely that they will prove to be
self-luminous. If they should turn out dark bodies in fact, shining only
by the reflected light of the stars around which they revolve, we should
have the first case of planets--dark bodies--noticed among the fixed
stars."

Of course, Dr. See has no reference in this statement to the immense
dark bodies which, in recent years, have been discovered by
spectroscopic methods revolving around some of the visible stars,
although invisible themselves. The obscure objects that he describes
belong to a different class, and might be likened, except perhaps in
magnitude, to the companion of Sirius, which, though a light-giving
body, exhibits nevertheless a singular defect of luminosity in relation
to its mass. Sirius has only twice the mass, but ten thousand times the
luminosity, of its strange companion! Yet the latter is evidently rather
a faint, or partially extinguished, sun than an opaque body shining only
with light borrowed from its dazzling neighbor. The objects seen by Dr.
See, on the contrary, are "apparently shining by a dull reflected
light."

If, however (as he evidently thinks is probable), these objects should
prove to be really non-luminous, it would not follow that they are to be
regarded as more like the planets of the solar system than like the dark
companions of certain other stars. A planet, in the sense which we
attach to the word, can not be comparable in mass and size with the sun
around which it revolves. The sun is a thousand times larger than the
greatest of its attendant planets, Jupiter, and more than a million
times larger than the earth. It is extremely doubtful whether the
relation of sun and planet could exist between two bodies of anything
like equal size, or even if one exceeded the other many times in
magnitude. It is only when the difference is so great that the smaller
of the two bodies is insignificant in comparison with the larger, that
the former could become a cool, life-bearing globe, nourished by the
beneficent rays of its organic comrade and master.

Judged by our terrestrial experience, which is all we have to go by, the
magnitude of a planet, if it is to bear life resembling that of the
earth, is limited by other considerations. Even Jupiter, which, as far
as our knowledge extends, represents the extreme limit of great
planetary size, may be too large ever to become the abode of living