The Secret of the Ninth Planetwas written at the dawn of the Space Age, only a few months after the launching of Sputnik, the first artificial satellite. In addition to telling an action packed story, one of the goals of the book was to include as much of the current available information about the various planets of the solar system as could be fit within the confines of the story. To do this the spacecraft Magellan embarks upon a voyage of discovery that would reach from one end of the solar system to the other.
At the time that it was written, the only information available about any of the planets was that which was obtainable by using earth based telescopes. This was a decade before the first Mariner spacecraft showed us a close up of the surface of Mars, and long before the two Voyagers gave us detailed glimpses of one outer planet after another on their own incredible voyages.
While today the portrayals of the planets in the book may seem wildly inaccurate and fanciful, they actually reflected the best guesses and speculations then coming out of the astronomical community. Their divergence from reality just shows how little was known about the rest of the solar system before the many space probes that have been launched in the intervening years.
As an example, for many years the perceived wisdom was that the planet Mercury was tidally locked to the sun so that the same side always faced towards that body. It has only been comparatively recently that this has been discovered to not be the case. At the other end of the solar system, I need only mention the current controversy over the planetary status of Pluto.
Wollheim can scarcely be faulted for using the data available at the time as the basis for his speculations. As to his imaginings of the life forms on the various planets, the book is a work of fiction, and they are not far different than the musings then current among astronomers.
The Secret of the Ninth Planet can then be read several ways. It can be taken as a straight forward science fiction adventure, but it can also be used to provide a glimpse into the science of a time that is now over half a century distant. Either way, I hope that you enjoy reading it.