Here's why:

* Gravity: If a planet were behind the sun, its gravitational pull would be noticeable in the orbits of other planets. We'd see perturbations in their orbits that we don't observe.

* Sunlight: A planet behind the sun would block some of the sun's light, creating a slight dimming that we would be able to detect.

* Observatory Technology: Modern telescopes are capable of observing objects far beyond the sun. We've mapped the outer solar system and haven't found any large planets lurking behind the sun.

The idea of a "counter-Earth" behind the sun has been popular in science fiction, but it's not scientifically supported.