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.
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