S1·E2 The Doomed Planet

The Fireball XL5 narrowly misses a planet that bas broken out of it's orbit and realize the planet is on a collision course for the planet Membrono. After concluding the planets are uninhabited, Steve realizes he saw a flying saucer and after the ship lands at Venus' house, the XL5 is lured back to save the planet Membrono from destruction.