  The atmospheres of gaseous giant exoplanets orbiting close to their parent
stars (hot Jupiters) have been probed for nearly two decades. They allow us to
investigate the chemical and physical properties of planetary atmospheres under
extreme irradiation conditions. Previous observations of hot Jupiters as they
transit in front of their host stars have revealed the frequent presence of
water vapour and carbon monoxide in their atmospheres; this has been studied in
terms of scaled solar composition under the usual assumption of chemical
equilibrium. Both molecules as well as hydrogen cyanide were found in the
atmosphere of HD 209458b, a well studied hot Jupiter (with equilibrium
temperature around 1,500 kelvin), whereas ammonia was tentatively detected
there and subsequently refuted. Here we report observations of HD 209458b that
indicate the presence of water (H2O), carbon monoxide (CO), hydrogen cyanide
(HCN), methane (CH4), ammonia (NH3) and acetylene (C2H2), with statistical
significance of 5.3 to 9.9 standard deviations per molecule. Atmospheric models
in radiative and chemical equilibrium that account for the detected species
indicate a carbon-rich chemistry with a carbon-to-oxygen ratio close to or
greater than 1, higher than the solar value (0.55). According to existing
models relating the atmospheric chemistry to planet formation and migration
scenarios, this would suggest that HD 209458b formed far from its present
location and subsequently migrated inwards. Other hot Jupiters may also show a
richer chemistry than has been previously found, which would bring into
question the frequently made assumption that they have solar-like and
oxygen-rich compositions.
