  Knowledge of both the mass and radius of an exoplanet allows us to estimate
its mean density, and therefore its composition. Exoplanets seem to fill a very
large parameter space in terms of mass and composition, and unlike the
solar-system's planets, exoplanets also have intermediate masses (~ 5 - 50
M_Earth) with various densities. In this letter, we investigate the behavior of
the Mass-Radius relation for methane (CH_4) planets and show that when methane
planets are massive enough (Mp >~ 15 M_Earth), the methane can dissociate and
lead to a differentiated planet with a carbon core, a methane envelope, and a
hydrogen atmosphere. The contribution of a rocky core to the behavior of CH_4
planet is considered as well. We also develop interior models for several
detected intermediate-mass planets that could, in principle, be
methane/methane-rich planets. The example of methane planets emphasizes the
complexity of the Mass-Radius relation and the challenge involved in uniquely
inferring the planetary composition.
