>>3372242>>3371606The age of the Earth isn't actually very old cosmically speaking (roughly 4.5 Gyrs). Our Sun is middle-aged and is considered relatively old in our solar neighborhood. But the Earth, and terrestrial planets (let alone gas planets) can remain geologically active exponentially longer than standard G class V (dwarf) stars can remain on main sequence. And so if you put the Earth into interstellar space, the core could remain molten geologically active for billions of years longer than our sun would be able to survive.
