Introduction

This section states the purpose of the book, describes how distances to the stars are quantified, discusses how stars and planets are classified, and how their potential habitability and climates are determined. The tour of the known galaxy begins with Planet Nine, possibly located in a distant part of our own solar system but as yet undiscovered. The concept of rogue planets, worlds not orbiting any star but travelling between them, is introduced, along with a description of their possible types.