  Hydrocarbon pyrolysis is a complex process involving large numbers of
chemical species and types of chemical reactions. Its quantitative description
is important for planetary sciences, in particular, for understanding the
processes occurring in the interior of icy planets, such as Uranus and Neptune,
where small hydrocarbons are subjected to high temperature and pressure. We
propose a computationally cheap methodology based on an originally developed
ten-reaction model, and the configurational model from random graph theory.
This methodology yields to accurate predictions for molecule size distributions
for a variety of initial chemical compositions and temperatures ranging from
3200K to 5000K. Specifically, we show that the size distribution of small
molecules is particularly well predicted, and the size of the largest molecule
can be accurately predicted provided that it is not too large.
