Why can't I attack certain planets?

I had a problem a couple of days ago: I built Halleys on a planet, but could it smash only in some of the other planets. I first thought I had a bug, but I saw the same thing in a live stream just now.

Are there restrictions which planets I can annihilate? Does it have to do with the mass / radius of them?

Yes u can only annihilate planets that are bigger than the planet u wish to smash with.

I think it's a bug with how the planets interact. It's likely to be changed in the future so we can destroy any planet.