Five carbon- and nitrogen-bearing species in a hot giant planet's atmosphere

The atmospheres of gaseous giant exoplanets orbiting close to their parent stars (hot Jupiters) have been probed for nearly two decades^1,2. They allow us to investigate the chemical and physical properties of planetary atmospheres under extreme irradiation conditions³. Previous observations of hot Jupiters as they transit in front of their host stars have revealed the frequent presence of water vapour⁴ and carbon monoxide⁵ in their atmospheres; this has been studied in terms of scaled solar composition⁶ under the usual assumption of chemical equilibrium. Both molecules as well as hydrogen cyanide were found in the atmosphere of HD 209458b^5,7,8, a well studied hot Jupiter (with equilibrium temperature around 1,500 kelvin), whereas ammonia was tentatively detected there⁹ and subsequently refuted¹⁰. 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^3,11,12, this would suggest that HD 209458b formed far from its present location and subsequently migrated inwards^11,13. 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.