In the fourth century BC, Greek philosopher Philolaus proposed the existence of a planet that he called Counter-Earth. He believed that Counter-Earth was always on the opposite side of the solar system from Earth. This meant that the Sun, Earth, and Counter-Earth would always be on the same line.
Philolaus believed that Counter-Earth was invisible from Earth because Counter-Earth was always obscured by the Sun. Today, we know that it could have never existed. If it had, we would have seen it from Earth because every planet in the solar system is affected by the gravitational pull of other planets.
The gravitational pull of Mercury and Venus would have altered the orbit of Counter-Earth and shifted it from its position on the opposite side of the solar system. This would have made it visible from Earth. Counter-Earth would have strayed closer to Earth over time, and both planets would have eventually met.
One of two things would have happened when they met. Earth and Counter-Earth could have collided to form a new Earth. Or they could have missed each other. If they had, their gravitational pulls would have been so great that they would have been thrown into new orbits.