Going a little beyond Shaula's great answer, signal strength for such operations go as the inverse 4th power of distance. Imaging Venus this way took a huge amount of power from Arecibo. A planet around Alpha Centauri might be a million times as far away as Venus when we do this, and so would require 1024 times the power we used to image Venus. This number would get smaller with a larger dish, but it would still be a huge number, and require the patience Shaula described above.
Originally Posted by Shaula
Forming opinions as we speak