The solar system appears to have a new ninth planet. Today, two scientists announced evidence that a body nearly the size of Neptune—but as yet unseen—orbits the sun every 15,000 years.

[Teeqoz]

The Fury said:
Riiiight. I believe that there is a planet in exact perfect orbit on the other side of the sun. I call it Counter Earth (in honour of Herbert Wyndham). It's there, we just can't see it.

 

Actually there are such things called lagrangian points, which mean that two bodies share the same orbit. However the lagrangian point that is 180 degrees off the orbit of the earth is unstable, so a planet couldn't be there. There are however examples of natural satelites (or "moons" if I must) that share orbits. If I recall correctly, Saturn had three moons in the same orbit.

https://en.wikipedia.org/wiki/Lagrangian_point