Computer simulations conducted by David Nesvorny of the Southwest Research Institute show that the outer Solar System may have possessed five giant gas planets, one of which was booted out as Jupiter jumped around in its orbit.

Dynamical instabilities are common in a newborn solar system as planets settle in their new-found orbits. Evidence for a gravitational disturbance of rocky planetary building blocks in our own Solar System is recorded in the impact crater history of our battered Moon's ancient surface just 600 million years after its formation. These cosmic collisions resulted from the outer giant planets' gravity unsettling smaller objects and sending them careering headlong into the inner Solar System.

At the same time, other small objects were flung into the outer icy reaches of the Solar System's Kuiper Belt, and Jupiter is thought to have migrated inwards on its orbit. But, says Nesvorny, a slow-moving Jupiter would likely have wrecked havoc in the inner Solar System, the transfer of momentum possibly even causing Earth to collide with Mars or Venus.

The "jumping-Jupiter" theory is indeed less harmful to the inner Solar System, but at the cost of either Neptune or Uranus being ejected instead, Nesvorny's computer simulations revealed.

Placing a fifth planet with a similar mass to Uranus or Neptune into the system seemed to solve the conundrum. In this scenario, one planet was booted out of the Solar System due to interactions with Jupiter, leaving the current four planets and the ability of Jupiter to still jump inwards, but without implicating the rocky inner planets.

“The possibility that the Solar System had more than four giant planets initially, and ejected some, appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, indicating the planet ejection process could be a common occurrence,” adds Nesvorny.