Analytical models of exoplanetary atmospheres. III. Gaseous C–H–O–N chemistry with nine molecules

Heng, Kevin; Tsai, Shang-Min (2016). Analytical models of exoplanetary atmospheres. III. Gaseous C–H–O–N chemistry with nine molecules. Astrophysical journal, 829(2), p. 104. Institute of Physics Publishing IOP 10.3847/0004-637X/829/2/104

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We present novel, analytical, equilibrium-chemistry formulae for the abundances of molecules in hot exoplanetary atmospheres that include the carbon, oxygen and nitrogen networks. Our hydrogen-dominated solutions involve acetylene (C₂H₂), ammonia (NH₃), carbon dioxide (CO₂), carbon monoxide (CO), ethylene (C2H₄), hydrogen cyanide (HCN), methane (CH₄), molecular nitrogen (N₂) and water (H₂O). By considering only the gas phase, we prove that the mixing ratio of carbon monoxide is governed by a decic equation (polynomial equation of degree 10). We validate our solutions against numerical calculations of equilibrium chemistry that perform Gibbs free energy minimization and demonstrate that they are accurate at the ∼1% level for temperatures from 500 to 3000 K. In hydrogen-dominated atmospheres, the ratio of abundances of HCN to CH₄ is nearly constant across a wide range of carbon-to-oxygen ratios, which makes it a robust diagnostic of the metallicity in the gas phase. Our validated formulae allow for the convenient benchmarking of chemical kinetics codes and provide an efficient way of enforcing chemical equilibrium in atmospheric retrieval calculations.

Item Type:

Journal Article (Original Article)


08 Faculty of Science > Physics Institute > Space Research and Planetary Sciences
08 Faculty of Science > Physics Institute > NCCR PlanetS
10 Strategic Research Centers > Center for Space and Habitability (CSH)

UniBE Contributor:

Heng, Kevin, Tsai, Shang-Min


500 Science > 520 Astronomy
600 Technology > 620 Engineering
500 Science > 530 Physics




Institute of Physics Publishing IOP




Danielle Zemp

Date Deposited:

30 Jun 2017 14:53

Last Modified:

05 Dec 2022 15:03

Publisher DOI:





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