  In principle, the combined measurements of the mass and radius a giant
exoplanet allow one to determine the relative fraction of hydrogen and helium
and of heavy elements in the planet. However, uncertainties on the underlying
physics imply that some known transiting planets appear anomalously large, and
this generally prevent any firm conclusion when a planet is considered on an
individual basis. On the basis of a sample of 9 transiting planets known at the
time, Guillot et al. A&A 453, L21 (1996), concluded that all planets could be
explained with the same set of hypotheses, either by large but plausible
modifications of the equations of state, opacities, or by the addition of an
energy source, probably related to the dissipation of kinetic energy by tides.
On this basis, they concluded that the amount of heavy elements in close-in
giant planets is correlated with the metallicity of the parent star.
Furthermore they showed that planets around metal-rich stars can possess large
amounts of heavy elements, up to 100 Earth masses. These results are confirmed
by studying the present sample of 18 transiting planets with masses between
that of Saturn and twice the mass of Jupiter.
