Computer simulation shows Solar System once had an extra planet

In an effort to determine just how the solar system was formed, Nesvorny performed a series of some 6,000 computer simulations. When using just the four giant planets, every simulation found that they were too large and ended up destroying each other. In the simulations where they did manage to make it in one piece, the rocky planets such as Mars and Venus, were instead destroyed. According to his results, the current solar system structure would have a very low probability of occurring if it started with only four rocky planets and four gas planets.

After running these simulations, Nesvorny decided to add a fifth large planet into the mix. With the addition of this large planet, results found that the odds of our current solar system increased significantly.

The most successful simulations show that Jupiter, Saturn, Uranus, Neptune and a fifth planet, similar to that of Neptune or Uranus, started out all tightly packed and orbiting some 15 times further from the sun then our planet Earth. The lighter planets are sent out further by Jupiter and Saturn. A close encounter with Jupiter then ejects this mysterious fifth planet out of the solar system.

Recent discoveries of free-floating planets in interstellar space show that the ejection of planets could have been common, according to the study.

More information: Young Solar System's Fifth Giant Planet? arXiv:1109.2949v1 [astro-ph.EP] http://arxiv.org/abs/1109.2949

Abstract
Recent studies of solar system formation suggest that the solar system's giant planets formed and migrated in the protoplanetary disk to reach resonant orbits with all planets inside 15 AU from the Sun. After the gas disk's dispersal, Uranus and Neptune were likely scattered by gas giants, and approached their current orbits while dispersing the transplanetary disk of planetesimals, whose remains survived to this time in the region known as the Kuiper belt. Here we performed N-body integrations of the scattering phase between giant planets in an attempt to determine which initial states are plausible. We found that the dynamical simulations starting with a resonant system of four giant planets have a low success rate in matching the present orbits of giant planets, and various other constraints (e.g., survival of the terrestrial planets). The dynamical evolution is typically too violent, if Jupiter and Saturn start in the 3:2 resonance, and leads to final systems with fewer than four planets. Several initial states stand out in that they show a relatively large likelihood of success in matching the constraints. Some of the statistically best results were obtained when assuming that the solar system initially had five giant planets and one ice giant, with the mass comparable to that of Uranus and Neptune, was ejected to interstellar space by Jupiter. This possibility appears to be conceivable in view of the recent discovery of a large number free-floating planets in interstellar space, which indicates that planet ejection should be common.

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