"I had this partly in mind from the first." The Industrialist went on. "It is for that reason I agreed to see them after I received your letter. Not to agree to an unsettling and impossible trade, but to judge their real purposes. I did not count on their evading the meeting." He sighed. "I suppose it isn't our fault. You are right in one thing, at any rate. The world has been at peace too long. We are losing a healthy sense of suspicion." The Astronomer's mild voice rose to an unusual pitch and he said, "I _will_ speak. I tell you that there is no reason to suppose they can possibly be hostile. They are small, yes, but that is only important because it is a reflection of the fact that their native worlds are small. Our world has what is for them a normal gravity, but because of our much higher gravitational potential, our atmosphere is too dense to support them comfortably over sustained periods. For a similar reason the use of the world as a base for interstellar travel, except for trade in certain items, is uneconomical. And there are important differences in chemistry of life due to the basic differences in soils. They couldn't eat our food or we theirs." "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."