Hi. First post here. I'm writing a sci-fi novel, and the above is what I need. Thanks in advance for your help. By "similar surface gravity" I mean that someone from the smaller world could move to the heavier world with a minimum of health risks (they're going to have enough trouble with the local biosphere). By "vastly different sizes" I mean that I want the differential to be as vast as possible while keeping the above in mind and the fact that I want the larger world to still be a terrestrial world with continents similar in size to Earth's (and more of them). The characters are human and the smaller world was terraformed. It has a minimal amount of water, but it's spread out rather strategically, so it's not a desert world. There was an interstellar civilization once upon a time, but it fell and cast these worlds into a new bronze age, and the smaller has risen to be approximately modern while the highest civilization on the larger is about 19th century; basically, there will be no phlebotinum-based solutions to their problems, so if the smaller world can't colonize the larger one, I'm out of a story.