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.

近二十年来，人们一直在探索靠近其母星（热木星）运行的气态巨型系外行星的大气层^1,2。它们使我们能够研究行星大气在极端辐射条件下的化学和物理特性³。先前对热木星在其宿主恒星前凌日的观察表明，它们的大气层中经常存在水蒸气⁴和一氧化碳⁵；在通常的化学平衡假设下，这已根据按比例缩放的太阳成分^{6进行了研究。在 HD 209458b 5,7,8}的大气中发现了这两种分子以及氰化氢，一个经过充分研究的热木星（平衡温度约为 1,500 开尔文），而氨在那里被初步检测到⁹，随后被驳斥¹⁰。在这里，我们报告了 HD 209458b 的观测结果，表明存在水 (H ₂ O)、一氧化碳 (CO)、氰化氢 (HCN)、甲烷 (CH ₄ )、氨 (NH ₃ ) 和乙炔 (C ₂ H ₂), 每个分子具有 5.3 到 9.9 个标准偏差的统计显着性。解释检测到的物种的辐射和化学平衡大气模型表明富含碳的化学物质，其碳氧比接近或大于 1，高于太阳能值 (0.55)。根据大气化学与行星形成和迁移情景相关的现有模型^3,11,12，这表明 HD 209458b 在远离其当前位置的地方形成，随后向内迁移^11,13。其他热木星也可能表现出比以前发现的更丰富的化学成分，这将质疑人们经常做出的假设，即它们具有类太阳和富含氧气的成分。