The term, 12th Planet, is not scientifically exact but relates to the historical and widely read book that Sitchen wrote, titled The 12th Planet. In this book he explains that the ancient visitors from this traveling comet considered the Moon to be a planet, and counted the Sun as the first. The periodic Earth cataclysms caused by the 12th Planet have been in place for eons, since the Earth was cold and without life. As this statement will raise questions in some minds, let us explain. The Earth was cold as the Sun had not yet lit. All this is a matter of astrophysics, and not relevant to the discussion at hand. The 12th Planet, or giant comet, assumed its orbit around the Sun due to gravitational and motion issues, which were at play coming out of what some Earthlings refer to as the big bang. This was in fact only a little bang, a local affair, however.
The orbit of the 12th Planet is long and narrow. This is not dependent on gravitational and orbital matters within your Solar System, but on a larger scheme, which causes the trip back into your Solar System to be but a minor part of the itinerary. Why does the 12th Planet swing so far away from your Solar System, and why bother to return, having done so?
There is a balance between the attraction of your Sun and another, unseen by you but nevertheless present and in force. The 12th Planet travels interminably between these two forces, not able to settle on an orbit around just one because of the momentum and path it originally took. It is caught. The path of the 12th Planet is such that it spends most of its life out in dark space, slowly moving from one giant tug to another. As it approaches one of these giants, your Sun being one, it picks up speed, and reaches a maximum speed as it passes the attraction. Having passed, it now has double the gravitational attraction on one side, and quickly switches back in the other direction, zooming just as rapidly much along the path it just took. Out in space again, caught between the two giants that dominate its life, it settles down to a sedate few thousand years, only to zip around the Sun's counterpart in a like manner and head back toward your Solar System.