  Sub-Neptune planets are very common in our galaxy and show a large diversity
in their mass-radius relation. In sub-Neptunes most of the planet mass is in
the rocky part (hereafter core) which is surrounded by a modest hydrogen-helium
envelope. As a result, the total initial heat content of such a planet is
dominated by that of the core. Nonetheless, most studies contend that the core
cooling will only have a minor effect on the radius evolution of the gaseous
envelope, because the core's cooling is in sync with the envelope, i.e., most
of the initial heat is released early on timescales of about 10-100 Myr. In
this Letter we examine the importance of the core cooling rate for the thermal
evolution of the envelope. Thus, we relax the early core cooling assumption and
present a model where the core is characterized by two parameters: the initial
temperature and the cooling time. We find that core cooling can significantly
enhance the radius of the planet when it operates on a timescale similar to the
observed age, i.e. several Gyr. Consequently, the interpretation of
sub-Neptunes' mass-radius observations depends on the assumed core thermal
properties and the uncertainty therein. The degeneracy of composition and core
thermal properties can be reduced by obtaining better estimates of the planet
ages (in addition to their radii and masses) as envisioned by future
observations.
