Hmmm... I was thinking of Defender when reading, but didn't suggested it because of "loosing a planet" part. I knew about the sequel, but I thought it is more or less same as Defender.
There is one arcade game which is probably the best defender clone there is. You naturally need MAME to play it, and I hate myself for forgetting a name. As soon as I rediscover the game, I'll let you know.
Anyway:
Stargate
1983, Atari
moby