This is a question that is oft asked of CE, and I thought I understood the FFG rules until I read the Reference section at the end.
In the main rules, it seems clear that you target the hyperspace gate at a planet in the defensive system. The defensive player then defends with their colony (if any) on that planet. This means that you can attack empty planets or planets with no colony belonging to the defensive player, and the defense has zero ships in the challenge. This sounds like the original Eon rules.
However, in the Reference section it states under Launch: "The offense aims the hyperspace gate at one of the defense's colonies in the target system." (emphasis mine). A colony is earlier defined to be 1 or more ships on a planet. This sounds more like the wording in my West End/Games Workshop edition.
So, what is the correct interpretation? Can the offense target any planet in the defense system?