beings of a high organization. The force of gravitation on the surface
of Jupiter exceeds that on the earth's surface as 2.64 to 1.
Considering the effects of this on the weight and motion of bodies, the
density of the atmosphere, etc., it is evident that Jupiter would, to
say the very least, be an exceedingly uncomfortable place of abode for
beings resembling ourselves. But Jupiter, if it is ever to become a
solid, rocky globe like ours, must shrink enormously in volume, since
its density is only 0.24 as compared with the earth. Now, the surface
gravity of a planet depends on its mass and its radius, being directly
as the former and inversely as the square of the latter. But in
shrinking Jupiter will lose none of its mass, although its radius will
become much smaller. The force of gravity will consequently increase on
its surface as the planet gets smaller and more dense.

The present mean diameter of Jupiter is 86,500 miles, while its mass
exceeds that of the earth in the ratio of 316 to 1. Suppose Jupiter
shrunk to three quarters of its present diameter, or 64,800 miles, then
its surface gravity would exceed the earth's nearly five times. With one
half its present diameter the surface gravity would become more than ten
times that of the earth. On such a planet a man's bones would snap
beneath his weight, even granting that he could remain upright at all!
It would seem, then, that, unless we are to abandon terrestrial
analogies altogether and "go it blind," we must set an upper limit to
the magnitude of a habitable planet, and that Jupiter represents such
upper limit, if, indeed, he does not transcend it.

The question then becomes, Can the faint objects seen by Dr. See and his
fellow-observers, in the near neighborhood of certain stars, be planets
in the sense just described, or are they necessarily far greater in
magnitude than the largest planet, in the accepted sense of that word,
which can be admitted into the category--viz., the planet Jupiter? This
resolves itself into another question: At what distance would Jupiter be
visible with a powerful telescope, supposing it to receive from a
neighboring star an amount of illumination not less than that which it
gets from the sun? To be sure, we do not know how far away the faint
objects described by Dr. See are; but, at any rate, we can safely assume
that they are at the distance of the nearest stars, say somewhere about
three hundred thousand times the earth's dista