1. What does the "zoo" tell us about the aliens from Pluto? About their religious practices? About how they view other living beings?

2. Why are the explorers from Earth excited about finding so many alien beings in the zoo?

3. Some of the astronomy in The Secret of the Ninth Planet is out of date. Did this affect your enjoyment of the novel? Is the success of the story dependent on the accuracy of the science, or is it enjoyable anyway? Are there other science-fiction novels that are old but enjoyable even if some their science is dated?

4. What are the qualities that make Burl such a good companion on the journey to save the solar system?

5. Why are there no women involved in the action?

6. Wollheim says that his antigravity propulsion system is different from those of other writers because his is based on known...