A fifth gas giant ejected from our solar system?

Our solar system has eight official planets – four gas giant worlds like Jupiter and four small rocky worlds like Earth. But until the solar system was 600 million years old, it might have had a fifth gas giant planet. What’s more, Earth might have been spared from a collision with Mars or Venus by the process in which this fifth giant world was ejected from our solar system.

Dr. David Nesvorny of the Southwest Research Institute in Boulder, Colorado is author of an article on this subject published in The Astrophysical Journal Letters this month (November, 2011). He used what is called the jumping Jupiter theory to draw the conclusion that the solar system might have originally had five gas giant worlds.

According to Nesvorny, 600 million years into our solar system’s history, the orbits of the giant planets were affected by a dynamical instability. That is a gravitational instability caused by the fact of many bodies in orbit around the sun. As a result of this instability, the giant planets and smaller bodies scattered away from each other.

According to this scenario, some small bodies moved into the Kuiper Belt to become objects like Pluto and Eris. Other small bodies traveled inward, producing impacts on the terrestrial planets and leaving behind the craters on the moon we still see today. The giant planets moved as well, Nesvorny said. Jupiter, for example, which is the largest body in our solar system today besides the sun, scattered most small bodies outward and moved inward.

This scenario presents a problem, however. Slow changes in Jupiter’s orbit, such as the ones expected from interaction with small bodies, would have conveyed too much momentum to the orbits of the terrestrial planets, stirring up or disrupting the inner solar system and possibly causing Earth to collide with Mars or Venus. Nesvorny said:

Colleagues suggested a clever way around this problem. They proposed that Jupiter’s orbit quickly changed when Jupiter scattered off of Uranus or Neptune during the dynamical instability in the outer solar system.

The jumping-Jupiter theory, as it is known, is less harmful to the inner solar system, because the orbital coupling between the terrestrial planets and Jupiter is weak if Jupiter jumps.

Nesvorny conducted thousands of computer simulations of the early solar system to test the jumping-Jupiter theory. He found that, as hoped for, in the simulations Jupiter did in fact jump by scattering from Uranus or Neptune. When it jumped, however, Uranus or Neptune was knocked out of the solar system. Since both planets still orbit the sun, Nesvorny said:

Something was clearly wrong.

Motivated by these results, Nesvorny wondered whether the early solar system could have had five giant planets instead of four.

By running the simulations with an additional giant planet with mass similar to that of Uranus or Neptune, things suddenly fell in place. One planet was ejected from the solar system by Jupiter, leaving four giant planets behind, and Jupiter jumped, leaving the terrestrial planets undisturbed. He said:

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.

Bottom line: Computer simulations by an astronomer at the Southwest Research Institute in Boulder suggests that our solar system might once have had five gas giant planets, instead of four. Dr. David Nesvorny used what is called the jumping Jupiter theory to reach this conclusion.