What is behind the sun? Are there any other planets behind the sun?

Most likely, we have found every object in our solar system that we would refer to as a planet.

Since the Earth is not stationary, what you mean when you say it is "behind the sun" would require an orbit that is synchronized with ours.

A "counter Earth" at a place known as L3, which is the Langrangian point behind the sun (from our perspective) where the gravitational and "centrifugal" forces would be balanced, would be the closest thing to such an occurrence.

Such a theory has two shortcomings:

L3 is erratic.

Since we can now observe things from space, we can conclude that L3 does not contain any planets of that type.

It's interesting to note that while L1 and L2 are also unstable Langrangian points, L4 and L5 are stable. These points are related to the Earth-Sun orbit and contain interplanetary dust as well as at least one asteroid that orbits the sun alongside us.

From our perspective on Earth, there are no planets behind the sun. However, all the planets in our solar system orbit the sun, so they may appear to be behind it at certain times from our viewpoint.