Fractional uncertainty in radii of extrasolar giant planets
(at 0.05 AU from solar-type stars) due to uncertainties in physical
parameters (top) and input physics (bottom), as a function of
mass. The corresponding absolute uncertainty on the fraction of the
planetary mass which is due to heavy elements is directly proportional
to the radii uncertainty (a 10% uncertainty in model radii corresponds to
a ~9% uncertainty on the mass of heavy elements, i.e. in that case and
for a 1 Mjup (i.e. 318 Mearth) planet, the mass of
heavy elements would be known with an accuracy of 
~30 Mearth). The albedo was assumed to lie between 0.1 and 0.5,
the age between 3 and 7 Ga.
(From Guillot 1999; Science 286, 72).