Scientists believe the solar system as we know it was once just a cloud of dust surrounding a newborn star. Gravity caused those dust particle to form into pebbles, then boulders, then eventually coalescing into the planets we know today. Over the course of 100 million years, the eight planets we know today went from nothing more than space dust to the massive (sort of) celestial object we know today.
That is the tale that astronomers believe today. All except for one, and those that have now joined in believing his new theory. According to Discover, Astronomer David Nesvorny of the Southwest Research Institute in Colorado contends that there may have been a ninth planet at some point during our solar system's history. (And he doesn't mean Pluto, which is now classified as a dwarf planet) Nesvorny thinks that there was once a gas giant that flew off the gravitational handle, so to speak, billions of years ago.
Nesvorny came up with this theory while experimenting with models of our solar system's history. He discovered that in these early models, the four gas giants that we know today, were constantly jostling for position. In our early solar system, these gaseous planets would have been much closer together, and the gravitational pull of Juptier would have thrown one of the others into space.
So he proposed that a fifth gaseous giant existed around this time. When he ran the test models again, low and behold, the system worked. Jupiter knocked out the introduced planet and spared Saturn, Uranus and Neptune, giving us the model we have today.
The simulations also revealed that when this scenario is played out, it also shifted the orbits of the remaining planets. Jupiter moved closer to the sun, while Uranus and Neptune moved even farther away from there original positions -- reflecting the current position of these planets today.
It could also explain what scientists call the Late Heavy Bombardment -- a period in our solar system's history when small objects on the edge of the solar system were flung toward Earth and the inner planets.
According to Discover, Nesvorny's study was published in The Astrophysical Journal Letters right around the time that other scientists discovered that the Milky Way may contain hundreds of billions of "lost" planets floating through interstellar space. This fifth gas giant could be one of them.[Discover]