Most of the planets in Nilespace are small, most of them have the size of planetoids, the largest have the size of Earth's moon. Almost all of them are located near a long axis through the sphere, called the Nile. The planets doesn't have an orbit around a central object, such as a sun, but orbit around the Nile.
The small planets are mostly earth bodies, there are some larger waterbodies and gasbodies. One small fire body is circulating around a planet called Naucratis.
The Nile has a special magical feature: one can sail on its magic. With a special Nilesail everybody, not only priests and mages, can sail a boat through space.
The god Ra brings light in Nilespace. He sails with the sun in his boat, Manjet, through space. In the morning he appears out of the only big planet in the system (Ra's Planet), and in the evening he disappears into the planet again to travel through the land of the dead, the Nightnile. In the morning he will appear again in the sky. There is only one complication: there seems to be two appearances of Ra, when one of them sails in Nilespace the other is sailing on the nightnile.
Translation into AD&D rules.