The atmosphere scatters light, even starlight. In addition, sunlight is SO brilliant, that some of it gets scattered completely around the Earth and so appears in the darkest night sky. Furthermore, the upper atmosphere glows faintly due to various chemical reactions and incoming charged particles like cosmic rays. When one points a camera ANYWHERE in the atmosphere aimed at the sky, one ALWAYS gets this noise in addition to any other light that may be present coming from outside. This noise is likely to be brighter than the light coming from any Earth-like planet.

There is a principle of Optics called the Liouville Theorem that states that a telescope (or other optical system) can not make any extended source (as opposed to a point source) brighter (more intense per unit spherical area) that it was originally. A telescope can only make such an object LARGER, but not brighter. If you have ever looked though a large telescope at Mars or Jupiter, you will know what I mean. They are NO BRIGHTER when seen through a telescope than when seen by the naked eye.

Therefore, not even the largest telescope can increase the signal-to-noise ratio of an extended object. The ONLY way to improve the signal-to-noise ratio is to go above the atmosphere and peer through the vacuum of space rather than the atmosphere.