Earth is a human habitable planet in the milky way galaxy. It's where the human race were born before they had space traveling technology.
Characters like: Robert Montgomery, Sharon Montgomery, and John Ezekiel are from Earth, more specifically, Australia. John Ezekiel, was aboriginal decent, and grew up in the Coober Pedy, mining opals. Cutter also mentioned that he castrated bulls in outback, most likely on Earth, unless bulls were transported to other planets.