Day was like a bright, moonlight night. Night then meant that the dim Sun had set and, in effect, it merely made the landscape slightly darker.
They compared notes late into the night in the rocket plane. By dawn, when again the dim glow shone, they had come to some very definite conclusions about the planet.
A number of the drawings on the walls seemed to have some religious significance. They focused on the phases of a moon. There were symbolic representations of this moon, passing through its phases; presumably Plutonian religious and social practices were related to it.
"But where is this moon?" Burl had asked.
"I think," Russ answered, "that what some astronomers had suspected about Pluto was right. It did not originate in the solar system, but was captured from outer space. Originally it revolved around another sun, some star which was light-years away. How it tore loose from that star we'll probably never know--the star might have simply become too dim, their planet might have been on a shaky orbit, an experiment of theirs might have jarred it loose, many things could have happened.
"Once beyond the gravitational grip of its parent sun, the planet wandered through the darkness of interstellar space until it came within the influence of our own Sun. How long this took would again be a guess. Possibly not more than a few thousand years, I'd say, since somehow a remnant of the population managed to survive. This suggests that they had some warning. Enough time passed for them to build the big structure we noticed at the north pole, probably to store food, build underground greenhouses and make sealed homes for a few families. Inside this giant building the last of the Plutonian people kept going.
"Then came the moment when their planet fell into an orbit around our Sun. I'd guess they emerged to find that the new Sun was too far away ever to heat up Pluto again, or to permit the rebuilding of an atmosphere. So they worked out a new scheme. This was to blow up the Sun into a nova--make it a giant and thereby bring its heat all the way out to Pluto--warming this world again, lighting it again, unfreezing its gases and waters. So they set up the Sun-tap stations."
"That also accounts," added Haines, "for their limited number of spaceships and their need for secret operations."