Normally, a planet's core was scorching hot.
Not only was most of the energy from a planet's birth sealed within its core, but during the early stages of planetary solidification, the scorching molten matter produced inside the planet also migrated toward the core under the effects of gravity and seepage. If the planet was primarily rocky, the radioactive materials accumulated near its core could usually reach concentrations millions of times greater than those on the surface. In such an environment, a highly radioactive core was almost like a thermal power plant carrying out slow nuclear reactions continuously, and with no way for that heat to escape, it would eventually heat the core into a blazing ball of liquid metal. Under such astonishing pressure and temperature, even diamonds became as soft as butter, and no matter how high the temperature rose, matter here could only liquefy, never vaporize.
That was the ordinary version of a planetary core.
Some planetary cores were cooled—cold compared with scorching ones. Even long-lived planets moved steadily toward their end through constant evolution. The activity of their cores gradually diminished, radioactive materials slowly decayed until they could no longer release heat, and the transition from a planet's youth to middle age was a process of layer-by-layer cooling from the surface down to the core. Far from a star, that cooling happened even faster. When a planet entered old age, its inner core approached the solidification point of iron-based metals. Strip away its crust and mantle, and you would see a giant dark-red alloy sphere, declaring the passage of time amid desolate space. Lonely, cold, and silent, the dimming fire at its center was the twilight candlelight of an aged planet.
That was the artistic version of a planetary core.
Then there were some planetary cores that took