Well, the bigger the planet the more Halley's it's going to need. You can check this by viewing the system and looking at the list of planets in the top right. If a planet can be moved it will show the amount of engines needed to move it. Once you meet the requirement click the planet in the system viewer (it should say "ready"), click engage and pick where you want to crash the planet. Make sure you zoom into the planet you want to destroy and left click. Click annihilate and it should take off.
i have a ton of planets in this system many of which are minimum mass and very small radius, all i see is "metal spots" on the planet in the system menu, i.imgur.com/7vjrdTy.png like so
The radius has to be somewhere under 200 for it to be movable. The moon radius I use is either 150 or 100 and 3 Halley's are required for those. I keep the mass at 1000 which is the smallest if you're not manually putting it in.
I think its the mass that makes it so the haleys can move planets? i don't really know, i didn't test it yet.
I just tested a system to try and figure out if mass has anything to do with planet movement and with the mass cranked up to 10k and a radius of 100, it still only needs 3 Halley's to move. It seems mass has no effect on planet movement.
In that case, that's something that needs to be fixed since it's mass, not radius, that gives way to inertia.
I see you're new to the forums, and welcome! You should do a search before posting, in accordance with the forum rules. You should also check out the FAQ, which answers your question. Here's a tutorial: http://pamatches.com/2013/how-to-destroy-planets-with-the-halley/ Here's more info about the Halley: http://pamatches.com/wiki/buildings/defensive/halley/