When the sun becomes a red giant, this planet is done for. <br>Before that (circa a billion years from now), the suns heat output will rise so much that life on the earths surface can't survive any longer.<br>EDIT: This also means an increase in the sun's brightness of about 10%, so sunglasses are a must for the "near" apocalypse in the far future.<br>Until then, the earth is not expected to cool down to a solid rock and it's not going to exist long enough to become one. <br>Meaning: Him saying that the earth will not run out of geothermal energy is correct. It can't run out when it's gone.