The unlucky "odd planet out" would have wound up falling into the sun, being ejected from the solar system, or crashing into another terrestrial planet.
This isn't too far-fetched in that the solar system is fundamentally chaotic, says Minton. "Solar systems don't know if they are going to be stable for billions of years." Minton says that the best dynamical computer simulation for relocating Earth has a rogue plant that is 75 percent Earth's mass smashing into Venus - in the ultimate planetary pinball game of "three's a crowd." This would have happened as little as 2 or 3 billion years ago, with Earth getting kicked out into its present orbit as a consequence.
This idea leaves me a little chagrined because it sounds like some of the wacky imaginary planet Nibiru predictions for the 2012 doomsday warnings. What's more, in the 1970s Sagan ridiculed Immanuel Velikovsky's ad hoc theories of a runaway Venus and other colliding planets. Velikovsky's ideas were built around comparative mythology and not dynamical modeling.
The Venus collision model is a "plausible idea" insists Minton, but "heavy toward science fiction." It would mean that Venus didn't finish forming until 2.5 billion years ago, and that would explain Venus' appearance and a geologically young looking volcanic planet, he says.
Regardless of this scenario, we're not necessarily safe into the future. In 1 of 2,500 dynamical simulations of the evolving solar system, Mercury, which is in a quasi-stable orbit, gets ejected from the solar system within 5 billion years from now. This triggers the ultimate Armageddon: a collision between Earth and Venus. Mars is tossed farther from the sun.