  We study the formation conditions of icy planetesimals in protoplanetary
disks in order to determine the composition of ices in small and cold
extrasolar planets. Assuming that ices are formed from hydrates, clathrates,
and pure condensates, we calculate their mass fractions with respect to the
total quantity of ices included in planetesimals, for a grid of disk models. We
find that the composition of ices weakly depends on the adopted disk
thermodynamic conditions, and is rather influenced by the initial composition
of the gas phase. The use of a plausible range of molecular abundance ratios
and the variation of the relative elemental carbon over oxygen ratio in the gas
phase of protoplanetary disks, allow us to apply our model to a wide range of
planetary systems. Our results can thus be used to constrain the icy/volatile
phase composition of cold planets evidenced by microlensing surveys,
hypothetical ocean-planets and carbon planets, which could be detected by Corot
or Kepler.
