The violence of The Secret of the Ninth Planet is the likeliest aspect of the novel to cause concern for readers. When Burl and his companions use an atomic bomb to blow up a sun-tapping station located in a Martian city, they kill many of the Martian inhabitants. The Martians are like insects and purely instinctual, which means that their complicity in the plot to make the sun into a nova would be entirely without their consent—they cannot reason and therefore have not been conscious coconspirators. For some readers, the killing of Martians would probably be no more meaningful than stepping on an anthill, and the situation is so desperate that their deaths would probably seem inevitable and justifiable. Still, for some readers who are sensitive to the rights of other living things, the killing of the fascinating Martians may be unsettling.

A social...