Last June a team of scientists published the results of a simulation that suggested the possibility that in the course of the solar system history Jupiter moved towards the Sun and then back outwards, to its present orbit. Now, again at the Southwest Research Institute in San Antonio, Texas, Dr. David Nesvorny has published the result of 6,000 simulations that suggest a more complex scenario in which there was a fifth giant planet with a mass similar to that of Uranus and Neptune.

Clues about the early solar system history come from the analysis of the Kuiper Belt and Moon craters. What we can deduce is that there was an instability in the orbits of the planets when the solar system was about six hundred million years old. In essence, giant planets and smaller bodies moved substantially, some towards the Sun, often crashing into the inner planets and the Moon, others towards the outskirts of the solar system.

A slow motion of Jupiter, however, would have destabilized the orbits of the inner planets. Dr. David Nesvorny was suggested that the Jupiter’s orbit may have changed relatively quickly, which explains why the inner planets’ orbits are stable. According to the simulations however in this case the gravitational dynamics of the various planets would most likely have lead Uranus or Neptune or even both planets to be moved enough to be torn out of their solar orbit and get lost into space.

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Given that the percentage of chances that the four solar system giant planets would all end into stable orbits was only 2.5% Dr. David Nesvorny thought that there was something wrong. At that point he tried to add a fifth giant planet to the simulations and the probability that the solar system took its present configuration of planetary orbits increased tenfold. The added giant planet would be the one that got lost in interstellar space.

In recent years astronomers have discovered many “orphans” planets that don’t orbit any star. It’s therefore possible that in several star systems during the first phase of their life there was an unstable situation that tore one or more planets out of their star’s orbit. Who knows, maybe someday the lost Earth’s “sibling” will be identified.