One of the pleasures of reading and writing science fiction is to find ourselves on a very different world from our own. The real purpose of such worlds is to give us a new perspective on our own, so I suppose it doesn't matter how unrealistic—how unscientific—they may be.
But it does matter. We want such worlds to be as concrete as possible, the unexpected outcomes of the same physics and chemistry that created this planet. And then we want to explore the implications of such worlds.
This was easier when we didn't know so much. Edgar Rice Burroughs could give us jungles on Venus and princesses on Mars, not to mention Pellucidar right inside our own planet. And the incomparable Chesley Bonestell could show us faraway worlds that were both exotic and recognizable.
Scientists, rather than SF authors, usually have more imagination about other planets. And one in particular is Neil F. Comins, who last year published a very entertaining book: What If the Earth Had Two Moons? He covers more than just that—including an Earth circling a double star (not like Tatooine), or an Earth circling a gas giant (nothing like Pandora). Along the way, we learn a lot about astronomy, planet formation, and the behaviour of galaxies. It's imagination-stretching.
As long as I'm on the subject, here are a couple of other books that aspiring world-builders should study:
Rare Earth: Why Complex Life is Uncommon in the Universe, by Peter Ward and Donald Brownlee, makes a persuasive argument that we are the outrageously lucky beneficiaries of a rare combination of circumstances. Remove or alter any of those circumstances, they say, and you'll end up with a world of bacteria. (It also comes with a great Chesley Bonestell cover, originally in Life Magazine. But when you read Comins, you'll know that when the moon was that close to us, enormous tides of molten lava were sweeping over the surface of the earth.)
If you're a pushover for far-future worlds, I strongly recommend The Five Ages of the Universe: Inside the Physics of Eternity, by Fred C. Adams and Greg Laughlin. They put time in a very different perspective; when the last stars blink out, trillions and trillions of years will remain. What happens then, and to whom?
I am increasingly pessimistic about the prospects for human travel between stars (or even as far as Venus and Mars). A civilization capable of interstellar travel would likely find it easier to build its own new worlds instead of schlepping light-years in cramped, unhealthy tin cans just so they can land on a planet that might not be so earthlike after all. But it sure is fun to enjoy some highly informed speculation on distant worlds.