The solar system may have had five giant planets instead of four but ejected one, thus sparing the Earth.

This was the finding of a recent study, "Young Solar System's Fifth Giant Planet?" funded by the National Lunar Science Institute and the National Science Foundation.

According to its author, Dr. David Nesvorny, clues about the early evolution of the solar system coming from the analysis of the trans-Neptunian population of small bodies known as the Kuiper Belt, and from the lunar cratering record, suggest that the orbits of giant planets were affected by dynamical instability when the solar system was only about 600 million years old.

The instability resulted in the scattering of the giant planets and the smaller bodies, with the smaller bodies moving into the Kuiper Belt, producing impacts on the terrestrial planets and the moon. Under this scenario, slow changes in Jupiter's orbit could have resulted in the possible collision of Earth and Mars or Venus.

Wondering whether the early solar system could have had five giant planets instead of four, Nesvorny ran simulations with an additional giant planet with mass similar to that of Uranus or Neptune. His study suggested that one planet was ejected from the solar system by Jupiter, leaving four giant planets behind, and Jupiter jumped, leaving the terrestrial planets undisturbed.

"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," Nesvorny said.

Nesvorny's paper was published online by The Astrophysical Journal Letters.