Investigating the detectability of hydrocarbons in exoplanet atmospheres with JWST

Abstract

Aims. We investigate at what abundances various hydrocarbon molecules (e.g. acetylene (C2H2), ethylene (C2H4), and methane (CH4)) become detectable when observing the atmospheres of various planets using the James Webb Space Telescope (JWST). Methods. We focused on atmospheric models based on the parameters of a small sample of planets: HD 189733b, HD 209458b (hot Jupiters orbiting bright stars); HD 97658b (a sub-Neptune/super-Earth orbiting a bright star); and Kepler-30c (a warm Jupiter orbiting a faint star). We computed model transmission spectra, assuming equilibrium chemistry and clear atmospheres for all planets apart from HD 189733b, where we also computed spectra with a moderate cloud layer included. We used the Bayesian retrieval package ARCiS for the model atmospheres, and simulated observed spectra from different instruments that will be on board JWST using the PandExo package. We subsequently ran retrievals on these spectra to determine whether the parameters input into the forward models, with a focus on molecular abundances, can be accurately retrieved from these simulated spectra. Results. We find that generally we can detect and retrieve abundances of the hydrocarbon species as long as they have a volume mixing ratio above approximately 1 × 10−7–1 × 10−6, at least for the brighter targets. There are variations based on planet type and instrument(s) used, and these limits will likely change depending on the abundance of other strong absorbers. We also find scenarios where the presence of one hydrocarbon is confused with another, particularly when a small wavelength region is covered; this is often improved when two instruments are combined. Conclusions. The molecules C2H2, CH4, and C2H4 will all be detectable with JWST, provided they are present in high enough abundances, and that the optimal instruments are chosen for the exoplanet system being observed. Our results indicate that generally a combination of two instruments, either NIRSpec G395M and MIRI LRS, or NIRCam F322W2 and MIRI LRS, are best for observing these hydrocarbons in bright exoplanet systems with planets of various sizes, with NIRSpec G395M and MIRI LRS the best option for the HD 189733b-like atmosphere with clouds included. The use of NIRSpec Prism is tentatively found to be best for fainter targets, potentially in combination with the MIRI LRS slit mode, although the target we test is too faint to draw any strong conclusions. Instrument sensitivity, noise, and wavelength range are all thought to play a role in being able to distinguish spectral features.

Clouds form on the hot Saturn JWST ERO target WASP-96b

“…The primary spectral features of both these molecules are not in the region of the spectra covered by the HST and VLT observations; around 2.3 and 5 µm for CO, and around 3.3 µm for CH 4 . As illustrated by Gasman et al (2022), other absorption features of CH 4 which are present in the region of the HST data are often masked by stronger H 2 O features. This highlights the benefits of the JWST observations of WASP-96b (Pontoppidan et al 2022), which do cover this region.…”

Clouds form on the hot Saturn JWST ERO target WASP-96b

“…The primary spectral features of both these molecules are not in the region of the spectra covered by the HST and VLT observations; around 2.3 and 5 µm for CO, and around 3.3 µm for CH 4 . As illustrated by Gasman et al (2022), other absorption features of CH 4 that are present in the region of the HST data are often masked by stronger H 2 O features. Figure C.2 shows the volume-mixing ratios of various molecules predicted to be present at the equator of the morning (left) and evening (right) terminators, based on the GCM and kinetic cloud models of this work.…”

On the 12C2H2 near-infrared spectrum: absolute transition frequencies and an improved spectroscopic network at the kHz accuracy level

“…Second, the near-infrared spectrum of acetylene is of considerable interest in the field of optical frequency metrology: (a) the P( 16) n 1 + n 3 transition of 13 C 2 H 2 has been included by the International Committee for Weights and Measures (CIPM) in the list of recommended reference lines for the practical realization of the definition of the meter, and (b) acetylene was used as a molecular target in Doppler-broadening gas thermometry for the optical determination of the Boltzmann constant, 36 and subsequently enabled for the implementation of the new SI definition of the unit Kelvin. 37 Third, the acetylene spectrum can be extremely useful for studying the atmosphere of young Earth-like exoplanets, 38,39 requiring the availability of high-quality spectroscopic parameters.…”