Factual error: Cook draws his sun and planets in the sand, saying that it's "my galaxy." When Norda draws hers, he calls it another galaxy. Wrong. He's drawn a solar system, not a galaxy. (A galaxy is a collection of millions of solar systems.) The terms are in no way synonymous - there's a vast difference, and a trained astronaut would definitely know that, so it's the writer's error, not a character mistake. Their little ships can't possibly cross galactic voids (that would require tens of thousands of years, even at many times the speed of light), so Cook and Norda's planets are in different star systems in the same galaxy - this one.