The 12th Planet has a similar process ongoing within its core, but being composed of heavier substances than your Sun, this process is slowed. You can equate this to mixing medicine into food, rather than taking it directly. This is tolerated because the incident or rate of medicine reacting with a taste bud is reduced, or slowed. On the 12th Planet, the heat released diffuses in all directions, so the surface of the planet is warm but not hot. Your feet would not blister, but they may feel cool in places where the rock strata do not provide a good passage for heat, should you visit that planet.
Light only escapes the core where what is essentially volcanic activity under the water occurs.
Of course, this would occur if there was volcanic activity on the land surface of the planet, but there is little land surface, and this long ago hardened. Volcanic activity would not be on the surface of the Earth, either, should her proportion of water to land be increased. Water is pooling in the low spots, and the low spots have less hardened surface mantle as a barrier. Thin places break, and give release to the pressure from the churning molten core. The oceans of the 12th Planet, therefore, have places where diffused light is rising, and other places where the oceans would appear to you much as your own do here on Earth.