Doc Sportello said:

That's funny I originally titled it The Hidden Planet, based on my love for the 1956 film The Forbidden Planet and obviously The Hidden Fortress. Later changing it to The Missing Planet to match Obi-Wan's dialogue exchange with r4, but I agree The Hidden Planet is the superior title. Very Flash Gordon.

"The Hidden Planet" it is!

Ha! that are two very nice references ...

I would have called one Episode "The Lost Planet" to keep it ambiguous, as to if the location is not known or it is destroyed ;-)