Where the Planets Are In the beginning, the Earth was believed to be both flat and at the center of the universe. Then a sailor set sail to prove that the Earth is in fact round. Decades later science proved that the churches were wrong and in fact the Earth orbited around the Sun not the other way around. Now it is commonly known that the solar system that the human race lives in is just a small part of a much larger spinning body called a galaxy. Now science argues about whether or not Pluto is really a planet and how the solar system itself was originally formed. Current studies of the solar system show that not only due planets clean extra material out of a solar system but they are also capable of changing their orbits.
When a star is born the surrounding material that is left is called planetary protomaterial. This material is made up of layers of gases, ice crystals and other solid matter. The larger pieces start pulling in the smaller ones by using gravitational pulls and time. Over time, these larger pieces start to clear out a path around the star that they orbit. Following current thinking the reason that the largest of the planets are on the outer portion of the solar system is that is where the most material will be at the time that the planet formed. As the loose material gets added together to form planets, the material that is just outside of the gravity well of the growing planet gets pulled at just like everything else except that the pull is only enough to send it flying out of the orbital disk.
In this manner planets clean out the solar system of all unused material that would otherwise clutter up the space between other planets. Sometimes the protomaterial can act just like a line of bread crumbs, leading a growing planet further and further into the solar system. This happens when there is a large concentration of material that spirals out from that systems's un. "Wit the disk-proto planet interactions theorized by Gold reich and Tremaine, the planet would be virtually locked to the inward flow of gas accreting onto the proto star and might either plunge into the star of decouple from the gas when it drew close to the star. [Another way] is the scattering of large planets that either formed in or moved into orbits too close to one another for long-term stability. In this process, the outcomes would be quite unpredictable but generally would yield very eccentric orbits for both planets." (Migrating) While all this movement is happening, the smaller objects that do not become part of a planet get thrown out of the immediate planetary orbital plane.
Stable orbits are final achieved when the planets gain one of three different distance ratios from one another. The first one, a 2: 1 ratio, is most commonly found between the planets that are the farthest from the system's star. As the planets distance from the system star decreases so does the stable orbit ratio. For the planets that are in the middle of the orbital disk, a ratio of 3: 2 is needed to maintain a stable orbit.
The planets closest to the system star require a 5: 3 ratio to maintain an orbit that will survive. Once a stable orbital system is achieved, the solar system becomes a balanced system where each planet helps to keep its neighbors in their own orbits. Any outside influence (i. e.
stray comets or asteroids) can change a once stable set of orbits into a cascading change that will change the orbits of all the planets in a solar system. In 1994, this solar system got to see an example of outside influence in action when the Shoemaker-Levy 9 comet came plowing in. If the Shoemaker-Levy 9 comet had hit a planet that did not have the mass to absorb the repeated impacts it would have changed the target planet's orbit thereby changing the orbit of every other planet in the solar system. With this in mind, "one thing is certain: the idea that planets can change their orbits dramatically is here to stay." (Migrating) References: Malhotra, Rent.
"Migration Planets." Scientific American. Sept 1999. 25 Aug 1999.