ormed at distances far enough from their stellar nucleus to
become cool enough to capture hydrogen. These would be large planets
rich in hydrogen, ammonia and methane. We have examples of these in the
giant outer planets. The second class would include those planets formed
so near the stellar center that the high temperature would make it
impossible to capture much hydrogen. These would be smaller planets,
comparatively poorer in hydrogen and richer in oxygen. We know that type
very well since we live on one. Ours is the only solar system we know in
detail, however, and it has been reasonable for us to assume that these
were the _only_ two planetary classes."

"I take it then that there is another."

"Yes. There is a super-dense class, still smaller, poorer in hydrogen,
than the inner planets of the solar system. The ratio of occurrence of
hydrogen-ammonia planets and these super-dense water-oxygen worlds of
theirs over the entire Galaxy--and remember that they have actually
conducted a survey of significant sample volumes of the Galaxy which we,
without interstellar travel, cannot do--is about 3 to 1. This leaves
them seven million super-dense worlds for exploration and colonization."

The Industrialist looked at the blue sky and the green-covered trees
among which they were making their way. He said, "And worlds like ours?"

The Astronomer said, softly, "Ours is the first solar system they have
found which contains them. Apparently the development of our solar
system was unique and did not follow the ordinary rules."

The Industrialist considered that. "What it amounts to is that these
creatures from space are asteroid-dwellers."

"No, no. The asteroids are something else again. They occur, I was told,
in one out of eight stellar systems, but they're completely different
from what we've been discussing."

"And how does your being an astronomer change the fact that you are
still only quoting their unsupported statements?"

"But they did not restrict themselves to bald items of information. They
presented me with a theory of stellar evolution which I had to accept
and which is more nearly valid than anything our own astronomy has ever
been able to devise, if we except possible lost theories dating from
Beforethewars. Mind you, their theory had a rigidly mathematical
development and it predicted just such a Galaxy as they describe. So you
see, they have all the worlds they wish. They are not land-hungry.
Certainl