Complex sentence: He dreams of regaining his leaders' trust by taking part in "Operation Impending Doom II", so Zim is assigned to Earth, a planet which the Almighty Tallest believe has little to no chance of existing.
Split into simpler sentences: He dreams of regaining his leaders' trust by taking part in "Operation Impending Doom II". So Zim was assigned to a planet which the Almighty Tallest believe has little to no chance of existing.