A planet is not a world. Planet? A ball of rock. World? A Four-dimensional wonder. On a world there must be mysterious mountains. Let there be bottomless lakes peopled with antique monsters. Let there be strange footprints in high snowfields, green ruins in endless jungles, bells beneath the sea; echo valleys and cities of gold. This is the yeast in the planetary crust, without which the imagination of men will not rise.
We build worlds, we don't just terraform planets. Robots could do that.
Terry Pratchett - Strata