The mass of the sun is much larger than all the planets.
For example: the mass of the sun is 10^6 times mass of the earth.
So the interaction forces between planets are much smaller than interaction force between planet and the sun.
It means that the motion for each planet are all similar.
You can check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=9.0]Kepler Motion [/url] or [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=629.0]How to determined the trajectory of a planetary motion?[/url]