Like @DanTheMan said, it probably has to be an enemy planet with enemy halleys.

I do have Titans. This achievement is also in the original game and quite a few people have unlocked it.

Ah, I think you may have solved it. The problem may be that it's my own planet, not an enemy planet.

According to the guides you used to be able to use your own planet but I have a feeling they've changed that! Thanks for the help.