Scientists believe our solar system may have once had a fifth gas planet that was ejected from the solar system and "orphaned".
A new study by David Nesvorny, from the Southwest Research Institute in Colorado, U.S., used different computer simulations to explore what the solar system looked like four billion years ago.
He discovered that back then planets had not yet settled into their existing orbits as they migrated and moved around.
However, after a series of tests he worked out that the solar system we recognise today could never have emerged without the existence of a fifth planet
He came to this conclusion by using several different starting positions and ran computer simulations using the four gas planets - Jupiter, Saturn, Uranus and Neptune - but discovered they were to large and one would destroy another eventually.
Even using configurations where the gas planets survived, the solar system's rocky planets like Mars and Venus did not.
However, once Mr Nesvorny had introduced a fifth planet into the equation he found the odds of the current solar system emerging increased dramatically.
This theory is backed up by the recent discovery of many rogue planets.
Mr Nesvorny wrote: "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."
The orphaned planet is believed to be an ice planet similar in size and make-up to Neptune and Uranus.
Mr Nesvorny believes it was ejected from the solar system into the Milky Way at some point as it could not survive Jupiter's gravitational force, Scientific American said.