reply to post by switching yard
The reason that the drawings show the fictional planet so far out is that it makes up for the problem that no one can see it. Two objects will remain
bound to each other as long as neither is moving fast enough, i.e. reaches what is known as escape velocity. Imagine throwing a rock really high into
the sky or throwing it only a little way up. In both cases the rock eventually slows down and for a moment is at rest with respect to the ground. Then
it speeds up and falls to the ground. Without air resistance the rock lands as fast as it went up. A planet in orbit also changes speed as it moves
away from the sun. It speeds up as it returns towards the Sun so there is no loss of energy.
But there is a place where energy is transferred, not lost. That happens mainly when the 2 are close to each other. The Earth's rotation slows down
and the Moon moves away from the Earth. The Sun slows down and the Earth moves away from the Sun. These are tiny changes on a yearly basis.
A planet with a highly eccentric orbit changes quite a bit. The Moon's orbit is circular enough that there is little change. A highly eccentric orbit
has a greater energy transfer. That makes the orbit of the planet change quite a bit. So a planet with a hghly eccentric orbit cannot have a regular
orbit such as a 3600 year orbit. The orbital period would change as the orbit changes.