"Known Space" only serves as a background for the game. The game is intended to be set on the Ringworld itself, an enormous single world discovered at the far reaches of Known Space, a ring around a sun at approximately the orbit of the Earth. It is 997,000 miles wide, about 125 Earth-diameters. The total inner surface of the ring is equal to that of 3 million Earths. The ring is spun at a speed to provide 0.992G of gravity on the innerside, while 20 giant shadow squares at about the orbit of Mercury occlude the Sun to provide night. It was constructed by the Pak Protectors, now mostly extinct, who had a common origin with humans. The Ringworld is home to some 30 trillion sentient inhabitants from up to 2000 hominid species. The world is described in a series of novels by Niven, "Ringworld", "The Ringworld Engineers", and, after the game's publication, "The Ringworld Throne" and "Ringworld's Children".