 The series contains a number of assumptions about the solar system which are outlandish by modern standards but which still seemed plausible, at least to the general public, in the time the stories were written.All of the planets of the solar system, and many of the moons and asteroids, are suitable for life, and most are already occupied by humanoid extraterrestrial races.The initial adventures take place in the planets of the solar system but later stories (after the character invents the "vibration drive") take the hero to other stars, other dimensions and even the distant past and almost to the end of the Universe.