  Solar and extrasolar comets and extrasolar planets are the subject of
numerous studies in order to determine their chemical composition and internal
structure. In the case of planetesimals, their compositions are important as
they govern in part the composition of future planets. The present works aims
at determining the chemical composition of icy planetesimals, believed to be
similar to present day comets, formed in stellar systems of solar chemical
composition. The main objective of this work is to provide valuable theoretical
data on chemical composition for models of planetesimals and comets, and models
of planet formation and evolution. We have developed a model that calculates
the composition of ices formed during the cooling of the stellar nebula.
Coupled with a model of refractory element formation, it allows us to determine
the chemical composition and mass ratio of ices to rocks in icy planetesimals
throughout in the protoplanetary disc. We provide relationships for ice line
positions (for different volatile species) in the disc, and chemical
compositions and mass ratios of ice relative to rock for icy planetesimals in
stellar systems of solar chemical composition. From an initial homogeneous
composition of the nebula, a wide variety of chemical compositions of
planetesimals were produced as a function of the mass of the disc and distance
to the star. Ices incorporated in planetesimals are mainly composed of H2O, CO,
CO2, CH3OH, and NH3. The ice/rock mass ratio is equal to 1+-0.5 in icy
planetesimals following assumptions. This last value is in good agreement with
observations of solar system comets, but remains lower than usual assumptions
made in planet formation models, taking this ratio to be of 2-3.
