To continue to survive, an Earth-like world must also continue to be lucky

Carl Sagan, Shadows of Forgotten Ancestors, page 14.

None of these worlds, you remind yourself, has volition; none intends to be in a particular orbit. But those that are on well-behaved, circular orbits tend to grow and prosper, while those on giddy, wild, eccentirc, or recklessly tilted orbits tend to be removed. As time goes on, the confusion and chaos of the early Soar System slowly settle down into a steadily more orderly, simple, regularly spaced, and, to your eyses, increasingly beautiful set of trajectories. Some bodies are selected to survive, others to be annihliated or exiled. This selection of worlds occurs through the operation of a few extremely simple laws of motion and gravity. Despite the good neighbor policy of the well-mannered worlds, you can occassionally make out a flagrant rogue worldlet on collision trajectory. Even a body with the most circumspect circular orbit has no warrantee against utter annihliation. To continue to survive, an Earth-like world must also continue to be lucky.