tokens: only investigate H 2 –H 2 O atmospheres, the sole opacity sources in the cloud-free models are the vibration-rotation bands of water vapor in the near- and mid-IR, collision-induced absorption (CIA), and Rayleigh scattering off of H 2 and H 2 O gas (see Freedman et al. 2008 , 2014 and Lupu et al. 2014 for a more detailed description of the opacity data). Exo-Transmit includes H 2 –H 2 CIA opacities but not H 2 –H 2 O or H 2 O–H 2 O. The non-inclusion of H 2 O CIA should not affect our results because these opacities tend to be only weakly wavelength dependent. For our cloudy models, the cloud opacity is treated as an infinite opacity source at the location of the cloud deck. 2.2. Modeling JWST Observations and Instrumental Noise We simulate three different instrument modes: the NIRISS Single Object Slitless Spectrometer (SOSS) ( R = 700, 0.7–2.7 μ m), NIRSpec G395H+f290lp ( R = 2000, 2.8–5 μ m), and the MIRI Low Resolution Spectrometer (LRS) ( R = 100, 5–14 μ m). To calculate the flux and background rates, and , we use the beta version of Space Telescope Science Institute’s online Exposure Time Calculator 6 6 https://devjwstetc.stsci.edu (ETC). The ETC does not contain a systematic noise floor, which has been suggested to be anywhere from 20 to 30 ppm for the near-IR instruments and 50 ppm for MIRI (Greene et al. 2016 ). These noise estimates may shift somewhat but will not impact the conclusions in this paper. The duty cycle is calculated by determining the number of allowable groups in an integration before detector saturation. A group, in JWST terminology, is one or more consecutively read frames—all exoplanet spectroscopy modes have a single frame per group. To determine the number of groups per integration, n grp , we sequentially increase n grp in the ETC, until a single pixel on the detector becomes saturated. The duty cycle is calculated by . We compute noise simulations for a GJ-1214-like host star with a magnitude J = 8, as a realistic system that might be observed with JWST and also discovered by TESS (Sullivan et al. 2015 ). Additionally, a GJ-1214-like star at J = 8 will have a distance of 6.6 pc, which acts as an optimistic comparison with de Wit Seager’s ( 2013 ) assumed system at 15 pc. We calculate , 9, and 29 for NIRISS SOSS, NIRSpec G395M, and MIRI LRS, respectively, corresponding to duty cycles d = 0.33, 0.80, and 0.93. Using the duty cycle, the total shot noise is F out is the flux (e − /s) for the host star computed from the JWST ETC. F in is the in-transit flux, , where is obtained from the transmission spectrum model described in Section 2.1 . The in-transit and out-of-transit time components, t in and t out , are the transit duration and the out-of-transit observing time, respectively, multiplied by the duty cycle. We compute the total noise viawhere n int is the total number of integrations during the entire transit, B λ is the extracted background rate (observatory background plus dark current in e − /s), computed in
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