In Isaac Asimov's Foundation series, the main characters are looking for the long-lost Earth. There are two major features of the solar system that seem to make it unusual among the millions of inhabited systems: the rings of Saturn and the size of the Moon.
The conversations below are mostly between Golan Trevize, ex-military, well-versed in astronomy, and Janov Pelorat, a naïve and recluse history teacher.
The rings of Saturn
"That's what the poem was speaking of. Three wide rings, concentric, wider than the planet itself."
Trevize said, "I never heard of such a thing. I don't think rings can be that wide. Compared to the planet they circle, they are always very narrow."
"Is that sort of thing common?" asked Bliss, awed.
"No," said Trevize. "Almost every gas giant has rings of debris, but they tend to be faint and narrow. I once saw one in which the rings were narrow, but quite bright. But I never saw anything like this; or heard of it, either."
Pelorat said, "That's clearly the ringed giant the legends speak of. If this is really unique-"
"Really unique, as far as I know, or as far as the computer knows," said Trevize.
"A giant satellite is more difficult to accept. No other inhabited world in the Galaxy has such a satellite. Large satellites are invariably associated with the uninhabited and uninhabitable gas-giants. As a Skeptic, then, I prefer not to accept the existence of the moon."
"Yes. It's rather farther from the planet than one might expect but it's definitely revolving about it. It's only the size of a small planet; in fact, it's smaller than any of the four inner planets circling the sun. Still, it's large for a satellite. It's at least two thousand kilometers in diameter, which makes it in the size range of the large satellites that revolve about gas giants."
"No larger?" Pelorat seemed disappointed. "Then it's not a giant satellite?"
"Yes, it is. A satellite with a diameter of two to three thousand kilometers that is circling an enormous gas giant is one thing. That same satellite circling a small, rocky habitable planet is quite another. That satellite has a diameter over a quarter that of Earth. Where have you heard of such near-parity involving a habitable planet?"
Pelorat said timidly, "I know very little of such things."
Trevize said, "Then take my word for it, Janov. It's unique. We're looking at something that is practically a double planet, and there are few habitable planets that have anything more than pebbles orbiting them. Janov, if you consider that gas giant with its enormous ring system in sixth place, and this planet with its enormous satellite in third—both of which your legends told you about, against all credibility, before you ever saw them-then that world you're looking at must be Earth. It cannot conceivably be anything else. We've found it, Janov; we've found it."
My question is in two parts:
- Are the various descriptions and facts given above generally correct?
- How unusual are Saturn's rings and the Moon compared to other systems we've observed?