"Surely all this can be overcome. They can bring their own food, build domed stations of lowered air pressure, devise specially designed ships."
"They can. And how glibly you can describe feats that are easy to a race in its youth. It is simply that they don't have to do any of that. There are millions of worlds suitable for them in the Galaxy. They don't need this one which isn't."
"How do you know? All this is their information again."
"This I was able to check independently. I am an astronomer, after all."
"That is true. Let me hear what you have to say then, while we walk."
"Then, sir, consider that for a long time our astronomers have believed that two general classes of planetary bodies existed. First, the planets which formed 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."