If planets get bigger (a good thing!), then they would get a massive HP/armor (AP) count from stone and dirt, even if individual stone and dirt blocks have nominal decoration HP/AP values. The radius of the core could heavily influence HP/AP values also. Since planets don't overheat, the "ship HP" that would normally lead to overheating is instead simply added to the HP count of the core to increase its health, which would need to be totally depleted (while slowly regenerating!) before the planet can be blown apart.
I wonder what such a count would look like, to give a planet enough points to feel substantial.
E.g. Core Radius = "R"
Planet Armor = 50% + radius / 1000 + average block armor rating (total no more than 99%)
HP = R * R * R + 9*R + block HP (E.g. 100 radius planet: ~3M HP added to core health, 400 radius planet: ~200M HP added to core health)
(HP total no more than 2.1B HP, but might stay at 2.1 B even when blocks are destroyed, as long as the total would be more than 2.1 B)
Since the radius doesn't change until the planet is actually blown apart, the radius part of the equation can be calculated separately when the planet is generated, so that part of the HP boost never needs to be recalculated while the planet is being "reduced" either by weapons fire or strip-mining, or the planet can be increased by on-planet building. (That underground base can suddenly improve the planet's "health" and survivability, being built out of sturdier materials.)