Planets outside of the solar system are not detected using light from the body itself. They are detected by their gravitational pull on their host star (i.e. tides; causing a slight twinkle in the light emitted by the star), or a transit (eclipse) of the star (causing the star to become slightly dim for a period). In both cases the telescope is looking at stars, and isn't fixated on one star but rather a field of stars, waiting for one to show activity.

Never mind the fact that even if we have a telescope that big, the issue is that these telescopes show a microscopically small field of view. There's a whole sphere that would need to be observed, and repeatedly (since the bodies move).