In the late 23rd century, humanity has settled fifteen younger worlds around nine stars, including Earth's solar system. Although Old Earth remains populated by the traditional variety of "full-spectrum" people, the younger worlds have developed "splinter" cultures, taking very divergent paths and developing specialized cultures. Most notable of these are: the Exotics, philosophers, mystics and psychologists; the Friendlies, puritan faith-holders who supplement the meager production of their rocky worlds by hiring out as mercenaries; and the Dorsai, true professional soldiers, highly skilled and expertly trained. Other worlds have specialized in different ways, in the sciences, mining, agriculture or commerce.