  We model the evolution of planets with various masses and compositions. We
investigate the effects of the composition and its depth dependence on the
long-term evolution of the planets. The effects of opacity and stellar
irradiation are also considered. It is shown that the change in radius due to
various compositions can be significantly smaller than the change in radius
caused by the opacity. Irradiation also affects the planetary contraction but
is found to be less important than the opacity effects. We suggest that the
mass-radius relationship used for characterization of observed extrasolar
planets should be taken with great caution since different physical conditions
can result in very different mass-radius relationships.
