Thank your lucky stars that our solar system was only big enough for eight planets. Apparently a ninth one—and a gas giant, no less—was nudged out billions of years ago and made room for Earth to survive. In computer simulations of our solar system's evolution, Colorado scientist David Nesvorny kept witnessing the Earth's demise or gas giant planets destroying one other. Then he added an extra planet, and presto, the solar system evolved into its current form, Mother Nature Network reports.
So what happened to gas giant number five? In the simulations, the gas giants started off bound together until Jupiter and Saturn pushed out lighter planets like Neptune and Uranus—and the mystery planet was simply ejected into space. Where it ended up, Nesvorny's computer didn't say. (Read more solar system stories.)