-active--then we
have another source of heat for the sun that will last indefinitely.

THE PLANETS

LIFE IN OTHER WORLDS?

Sec. 1

It is quite clear that there cannot be life on the stars. Nothing solid
or even liquid can exist in such furnaces as they are. Life exists only
on planets, and even on these its possibilities are limited. Whether all
the stars, or how many of them, have planetary families like our sun, we
cannot positively say. If they have, such planets would be too faint and
small to be visible tens of trillions of miles away. Some astronomers
think that our sun may be exceptional in having planets, but their
reasons are speculative and unconvincing. Probably a large proportion at
least of the stars have planets, and we may therefore survey the globes
of our own solar system and in a general way extend the results to the
rest of the universe.

In considering the possibility of life as we know it we may at once rule
out the most distant planets from the sun, Uranus and Neptune. They are
probably intrinsically too hot. We may also pass over the nearest planet
to the sun, Mercury. We have reason to believe that it turns on its axis
in the same period as it revolves round the sun, and it must therefore
always present the same side to the sun. This means that the heat on the
sunlit side of Mercury is above boiling-point, while the cold on the
other side must be between two and three hundred degrees below
freezing-point.

The Planet Venus

The planet Venus, the bright globe which is known to all as the morning
and evening "star," seems at first sight more promising as regards the
possibility of life. It is of nearly the same size as the earth, and it
has a good atmosphere, but there are many astronomers who believe that,
like Mercury, it always presents the same face to the sun, and it would
therefore have the same disadvantage--a broiling heat on the sunny side
and the cold of space on the opposite side. We are not sure. The
surface of Venus is so bright--the light of the sun is reflected to us
by such dense masses of cloud and dust--that it is difficult to trace
any permanent markings on it, and thus ascertain how long it takes to
rotate on its axis. Many astronomers believe that they have succeeded,
and that the planet always turns the same face to the sun. If it does,
we can hardly conceive of life on its surface, in spite of the
cloud-screen.

[Illustration: FIG. 14.--THE MOON

Showing