Synopsis: This book could be classified as speculative science non-fiction as it has chapters on what aliens in various environments could be like. The environments are explained by first showing the type of star needed, what size and how far away from the star the host planet needs to be, and ideas on what the resulting life forms would look like. The author's goal was to construct environments that were close to what we have on Earth but to have a much lighter or heavier gravity, a glaciated world, an ocean world, a jungle world, or a gas giant such as Jupiter.