  Sub-Neptune planets are a very common type of planets. They are inferred to
harbour a primordial (H/He) envelope, on top of a (rocky) core, which dominates
the mass. Here, we investigate the long-term consequences of the core
properties on the planet mass-radius relation. We consider the role of various
core energy sources resulting from core formation, its differentiation, its
solidification (latent heat), core contraction and radioactive decay. We divide
the evolution of the rocky core into three phases: the formation phase, which
sets the initial conditions, the magma ocean phase, characterized by rapid heat
transport, and the solid state phase, where cooling is inefficient. We find
that for typical sub-Neptune planets of ~2-10 Earth masses and envelope mass
fractions of 0.5-10% the magma ocean phase lasts several Gyrs, much longer than
for terrestrial planets. The magma ocean phase effectively erases any signs of
the initial core thermodynamic state. After solidification, the reduced heat
flux from the rocky core causes a significant drop in the rocky core surface
temperature, but its effect on the planet radius is limited. In the long run,
radioactive heating is the most significant core energy source in our model.
Overall, the long term radius uncertainty by core thermal effects is up to 15%.
