This movie makes so many mistakes that they spoil it.

It's not possible to be a planet on the same distance from Sun as Earth is and not be spotted. We didn't use only telescopes to find planets, we used calculations on known planets orbits to figure where other planets were. Even if there was a planet "hidden" behind the Sun, we'd find it from the seeable ones.

And if the was a planet moving across other planets as this one was, it'd wreck havoc on their orbits. They could even be thrown away from the Sun. Even if it didn't collide with Earth, we'd get farther or closer to the Sun and climate effects would kill most of life on Earth.