Characterizing Transiting Exoplanet Atmospheres With JWST

We explore how well spectra from the James Webb Space Telescope ( JWST ) will likely constrain bulk atmospheric properties of transiting exoplanets. We start by modeling the atmospheres of archetypal hot Jupiter, warm Neptune, warm sub-Neptune, and cool super-Earth planets with atmospheres that are clear, cloudy, or of high mean molecular weight (HMMW). Next we simulate the λ = 1–11 μ m transmission and emission spectra of these systems for several JWST instrument modes for single-transit or single-eclipse events. We then perform retrievals to determine how well temperatures and molecular mixing ratios (CH 4 , CO, CO 2 , H 2 O, NH 3 ) can be constrained. We find that λ = 1–2.5 μ m transmission spectra will often constrain the major molecular constituents of clear solar-composition atmospheres well. Cloudy or HMMW atmospheres will often require full 1–11 μ m spectra for good constraints, and emission data may be more useful in cases of sufficiently high F p and high F p / F * . Strong temperature inversions in the solar-composition hot-Jupiter atmosphere should be detectable with 1–2.5+ μ m emission spectra, and 1–5+ μ m emission spectra will constrain the temperature–pressure profiles of warm planets. Transmission spectra over 1–5+ μ m will constrain [Fe/H] values to better than 0.5 dex for the clear atmospheres of the hot and warm planets studied. Carbon-to-oxygen ratios can be constrained to better than a factor of 2 in some systems. We expect that these results will provide useful predictions of the scientific value of single-event JWST spectra until its on-orbit performance is known.