2022A&A...659A.114G

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 x 10^–7-1 x 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.