Ringworlds? First off let me say I enjoyed Larry Niven's Ringworld book. But his version of a ringworld just isn't workable. "Scrith" would need to be about as strong as the Strong Nuclear Force to survive the load that would be put on a ribbon of matter 1km thick and spinning around a star at over 1,000kps. Plus it would be unstable about the star; it's not orbiting it like a planet. So I've tried to think of something that might work. My ringworld is made of higgsinium, is several meters or more thick(don't know how thick it'd need to be) several hundred thousand km tall and this ring actually orbits the star at normal orbital speed. The inside of the higgsinium ring is between 1-2 something AU out from its parent G type star. Oceans, soil, air etc. are held by the ringworld by real gravity from the huge amount of mass packed into such a small area. Since the strength of gravity is due not only to mass but how close something is to that mass; not sure if it'd work just right though. The Moon is less than 1/200th the mass of Earth, but due to its much smaller diameter it still has 1/6th the gravity. Could you get an Earth-like environment on the ring? How high up would the air go? Would it be stable? What have I forgotten?