A compromise between a normal planet and a Dyson sphere. A ringworld would be an artificial, band-shaped structure placed around a star within the star's habitable zone. Although, unlike a Dyson sphere, it would intercept only a small fraction of the total radiation output of the star, the incident energy would still be many millions of times greater than that falling on a planet. The ringworld would be easier to build than a Dyson sphere, allow freer access to open space, and be completely habitable (in contrast with most of the interior surface of a Dyson sphere which would be subject to reduced gravity conditions.) As a bonus, those living on a ringworld would be able to see the stars.
Larry Niven speculated about such a structure in his novels Ringworld and The Ringworld Engineers.
