  In the past decade, the analysis of exoplanet atmospheric spectra has
revealed the presence of water vapour in almost all the planets observed, with
the exception of a fraction of overcast planets. Indeed, water vapour presents
a large absorption signature in the wavelength coverage of the Hubble Space
Telescope's (HST) Wide Field Camera 3 (WFC3), which is the main space-based
observatory for atmospheric studies of exoplanets, making its detection very
robust. However, while carbon-bearing species such as methane, carbon monoxide
and carbon dioxide are also predicted from current chemical models, their
direct detection and abundance characterisation has remained a challenge. Here
we analyse the transmission spectrum of the puffy, clear hot-Jupiter KELT-11 b
from the HST WFC3 camera. We find that the spectrum is consistent with the
presence of water vapor and an additional absorption at longer wavelengths than
1.5um, which could well be explained by a mix of carbon bearing molecules. CO2,
when included is systematically detected. One of the main difficulties to
constrain the abundance of those molecules is their weak signatures across the
HST WFC3 wavelength coverage, particularly when compared to those of water.
Through a comprehensive retrieval analysis, we attempt to explain the main
degeneracies present in this dataset and explore some of the recurrent
challenges that are occurring in retrieval studies (e.g: the impact of model
selection, the use of free vs self-consistent chemistry and the combination of
instrument observations). Our results make this planet an exceptional example
of chemical laboratory where to test current physical and chemical models of
hot-Jupiters' atmospheres.
