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

Abstract

The atmospheres of gaseous giant exoplanets orbiting close to their parent stars (hot Jupiters) have been probed for nearly two decades<SUP>1,2</SUP>. They allow us to investigate the chemical and physical properties of planetary atmospheres under extreme irradiation conditions<SUP>3</SUP>. Previous observations of hot Jupiters as they transit in front of their host stars have revealed the frequent presence of water vapour<SUP>4</SUP> and carbon monoxide<SUP>5</SUP> in their atmospheres; this has been studied in terms of scaled solar composition<SUP>6</SUP> under the usual assumption of chemical equilibrium. Both molecules as well as hydrogen cyanide were found in the atmosphere of HD 209458b<SUP>5,7,8</SUP>, a well studied hot Jupiter (with equilibrium temperature around 1,500 kelvin), whereas ammonia was tentatively detected there<SUP>9</SUP> and subsequently refuted<SUP>10</SUP>. Here we report observations of HD 209458b that indicate the presence of water (H<SUB>2</SUB>O), carbon monoxide (CO), hydrogen cyanide (HCN), methane (CH<SUB>4</SUB>), ammonia (NH<SUB>3</SUB>) and acetylene (C<SUB>2</SUB>H<SUB>2</SUB>), 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<SUP>3,11,12</SUP>, this would suggest that HD 209458b formed far from its present location and subsequently migrated inwards<SUP>11,13</SUP>. 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.