  Current observation techniques are able to probe the atmosphere of some giant
exoplanets and get some clues about their atmospheric composition. However, the
chemical compositions derived from observations are not fully understood, as
for instance in the case of the CH4/CO abundance ratio, which is often inferred
different from what has been predicted by chemical models. Recently, the warm
Neptune GJ3470b has been discovered and because of its close distance from us
and high transit depth, it is a very promising candidate for follow up
characterisation of its atmosphere. We study the atmospheric composition of
GJ3470b in order to compare with the current observations of this planet, to
prepare the future ones, but also as a typical case study to understand the
chemical composition of warm (sub-)Neptunes. The metallicity of such
atmospheres is totally uncertain, and vary probably to values up to 100x solar.
We explore the space of unknown parameters to predict the range of possible
atmospheric compositions. Within the parameter space explored we find that in
most cases methane is the major carbon-bearing species. We however find that in
some cases, typically for high metallicities with a sufficiently high
temperature the CH4/CO abundance ratio can become lower than unity, as
suggested by some multiwavelength photometric observations of other warm
(sub-)Neptunes, such as GJ1214b and GJ436b. As for the emission spectrum of
GJ3470b, brightness temperatures at infrared wavelengths may vary between 400
and 800K depending on the thermal profile and metallicity. Combined with a hot
temperature profile, a substantial enrichment in heavy elements by a factor of
100 with respect to the solar composition can shift the carbon balance in
favour of carbon monoxide at the expense of CH4. Nevertheless, current
observations of this planet do not allow yet to determine which model is more
accurate.
