In our solar system, an extra giant planet, or possibly two, might once have accompanied Jupiter, Saturn, Neptune and Uranus. Computer models showing how our solar system formed suggested the planets once gravitationally slung one another across space, only settling into their current orbits over the course of billions of years. Scientist David Nesvorny at the Southwest Research Institute in Boulder, Colo., found that a solar system that began with four giant planets only had a 2.5 percent chance of leading to the orbits presently seen now. The model suggests our current solar system began with five giants, including a now lost world comparable in mass to Uranus and Neptune. This extra planet may have been an “ice giant” rich in icy matter just like Uranus and Neptun.
When the solar system was about 600 million years old, it underwent a major period of instability that scattered the giant planets and smaller worlds. Eventually, gravitational encounters with Jupiter would have flung the mystery world to interstellar space about 4 billion years ago. According to David Nesvorny this is just a beginning, It will need quite a lot of work to see if there actually was the fifth planet.