Considering that this is a show about outer space, and that they have already met alien races at this point, why did Zero find it so hard to believe that XL5's crew had seen a flying saucer? Was it simply because it was saucer shaped?
I find it difficult (not to mention frightening) to believe that a single ship could have the firepower to blow up an entire planet. Divert its course, perhaps, but destroy it completely?
Also, when the planet was on fire (which looked rather ridiculous) XL5 apparently could not divert its own course, and was in danger of crashing. Then the planet exploded. I should think that being right next to an exploding planet would be a wee bitty bit dangerous in itself.
Steve Zodiac has an extremely narrow and egocentric view of what constitutes "life". Whizzing through a thick, lush jungle of growing plants, and he says, "No life forms found." And after searching a very miniscule portion of the planet, he and the Professor agree that there is no point in searching any further.
As I understand it, XL patrols are three months long. In this episode, having returned to Earth, Venus states that she has had Zooney for some three months. Who looks after Zooney when she's gone, and who taught him to say "Welcome Home" (and only when someone arrives)? Commander Zero said that he had had Zooney home "last night". (He seems to like Zooney at this point. Wait til he gets to know him better!)
XL5 may have only been up in space to investigate the rogue planet, but given their reaction to it, that seems doubtful.
Why was Steve the only one to investigate the doomed planet for signs of life? Robert is capable of looking after the main body of the ship, so why not have all three of them down searching?
XL5 goes to check if anyone is living on Membrono, but nobody seemed worried that someone could be living on the rogue planet itself.
Why did the saucer pilot waste time letting XL5 return to Earth, following it there and luring it back to Membrono just to ask for help? Couldn't he have asked during Steve's first visit?
When going to meet whoever is in the flying saucer, Steve, Matt and Robert all go in Fireball Junior, leaving Venus alone on the main ship. That's rather unwise, is she really trained to fly XL5 all by herself in case of trouble? She is a doctor after all, not a pilot.