§ 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.