  The chemical composition of an extrasolar planet is fundamental to its
formation, evolution and habitability. In this study, we explore a new way to
measure the chemical composition of the building blocks of extrasolar planets,
by measuring the gas composition of the disrupted planetesimals around white
dwarf stars. As a first attempt, we used the photo-ionization code Cloudy to
model the circumstellar gas emission around a white dwarf Gaia J0611$-$6931
under some simplified assumptions. We found most of the emission lines are
saturated and the line ratios approaching the ratios of thermal emission;
therefore only lower limits to the number density can be derived. Silicon is
the best constrained element in the circumstellar gas and we derived a lower
limit of 10$^{10.3}$ cm$^{-3}$. In addition, we placed a lower limit on the
total amount of gas to be 1.8 $\times$ 10$^{19}$ g. Further study is needed to
better constrain the parameters of the gas disk and connect it to other white
dwarfs with circumstellar gas absorption.
