Thomas Fisher, of Kings High School, asks :-
How could scientists tell if there was a planet orbiting the sun on the same path as the earth but on the other side of the sun?
Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.
A planet hidden on the far side of the sun would make itself known by its gravity pull on the other planets, comets and asteroids. In calculating the movements of the planets, etc, astronomers consider not just the gravity pull of the sun. For really accurate predictions they must also allow for the gravity of all the other planets.
A planet in the same orbit as ours but on the other side of the sun would affect the orbit of anything passing near it. Astronomers would find that the nearby planets, comets and asteroids were not keeping to their predicted tracks. Even a planet smaller than our moon would show up this way.
This is how Neptune was found. Astronomers noticed that Uranus, discovered in 1781, was being slowed down by the gravity of a more distant planet. Mathematicians were able to calculate almost exactly where the unknown planet was. Thus Neptune was discovered in 1846.
In fact, two planets on opposite sides of the same orbit would be an unstable situation. They would eventually get into slightly different orbits and meet.