Signatures of Nitrogen Chemistry in Hot Jupiter Atmospheres

Inferences of molecular compositions of exoplanetary atmospheres have generally focused on carbon, hydrogen, and oxygen-bearing molecules. Recently, additional absorption in Hubble Space Telescope Wide Field Camera 3 (WFC3) transmission spectra around 1.55 μm has been attributed to nitrogen-bearing chemical species: NH3 or HCN. These species, if present in significant abundance, would be strong indicators of disequilibrium chemical processes—e.g., vertical mixing and photochemistry. The derived N abundance, in turn, could also place important constraints on planetary formation mechanisms. Here, we examine the detectability of nitrogen chemistry in exoplanetary atmospheres. In addition to the WFC3 bandpass (1.1-1.7 μm), we find that observations in the K-band at ∼2.2 μm, achievable with present ground-based telescopes, sample a strong NH3 feature, while observations at ∼3.1 μm and ∼4.0 μm sample strong HCN features. In anticipation of such observations, we predict absorption feature amplitudes due to nitrogen chemistry in the 1-5 μm spectral range possible for a typical hot Jupiter. Finally, we conduct atmospheric retrievals of nine hot Jupiter transmission spectra in search of near-infrared absorption features suggestive of nitrogen chemistry. We report weak detections of NH3 in WASP-31b (2.2σ), HCN in WASP-63b (2.3σ), and excess absorption that could be attributed to NH3 in HD 209458b. High-precision observations from 1 to 5 μm (e.g., with the James Webb Space Telescope) will enable definitive detections of nitrogen chemistry, in turn serving as powerful diagnostics of disequilibrium atmospheric chemistry and planetary formation processes.