The Living Worlds
From its lonely vantage point, nestled between the Orion and Sagittarius arms of the Milky Way galaxy, Earth's blue-green surface seems unique, a singular island of complexity in a harsh and frigid universe. Seen from a wider angle, nothing could be further from the truth. When races expand from their home solar systems, they discover that the cosmos is awash in living worlds. These planets fall into a limited number of classes, as defined by their morphologies and their suns. Not all suns are suitable hosts for biology. Stars much larger than Earth's sun are too short-lived to allow the formation of complex life: the worlds that circle them remain at a bacterial stage until the rising luminosity of the star boils the ocean and cooks the carbon dioxide out of the rocks, sterilizing the planet under a superheated shell of gas. Biologies are always exclusively bacterial for the first aeons of their planet's history, owing to the lack of oxygen in the atmosp...