  The determination of the abundance of volatiles in extrasolar planets is very
important as it can provide constraints on transport in protoplanetary disks
and on the formation location of planets. However, constraining the internal
structure of low-mass planets from transit measurements is known to be a
degenerate problem. Using planetary structure and evolution models, we show how
observations of transiting planets can be used to constrain their internal
composition, in particular the amount of volatiles in the planetary interior,
and consequently the amount of gas (defined in this paper to be only H and He)
that the planet harbors. We show for low-mass gas-poor planets that are located
close to their central star that assuming evaporation has efficiently removed
the entire gas envelope, it is possible to constrain the volatile fraction of
close-in transiting planets. We illustrate this method on the example of 55 Cnc
e and show that under the assumption of the absence of gas, the measured mass
and radius imply at least 20 % of volatiles in the interior. For planets at
larger distances, we show that the observation of transiting planets at
different evolutionary ages can be used to set statistical constraints on the
volatile content of planets. These results can be used in the context of future
missions like PLATO to better understand the internal composition of planets.
