The situation today is that , if we consider the solar system on its own , the planets are in some dynamical equilibium , which means they interchange energy ( call it momentum ) . As a result their eccentricities , inclinations and semi-major axis vary . One can say that if one object is slowing down another object must accelerate . Simulations learn that the outer planets are higly involved in this exchange . Also Saturn and Jupiter exchange energy . The inner planets are more stable . For the Earth it is known that Earths orbit has a changing eccentricity . Wikipedia can provide some interesting links.
A link which provides simulations about this system and data can also be found in the www.orbitsimulator.com message bord .