Transit spectroscopy with James Webb Space Telescope: systematics, starspots and stitching

The James Webb Space Telescope (JWST) is predicted to make great advances in the field of exoplanet atmospheres. Its 25 m² mirror means that it can reach unprecedented levels of precision in observations of transit spectra, and can thus characterize the atmospheres of planets orbiting stars several hundred pc away. Its coverage of the infrared spectral region between 0.6 and 28 μm allows the abundances of key molecules to be probed during the transit of a planet in front of the host star, and when the same planet is eclipsed constraints can be placed on its temperature structure. In this work, we explore the possibility of using low-spectral-resolution observations by JWST/Near-Infrared Spectrograph and JWST/Mid-Infrared Instrumen-Low Resolution Spectrometer together to optimize wavelength coverage and break degeneracies in the atmospheric retrieval problem for a range of exoplanets from hot Jupiters to super-Earths. This approach involves stitching together non-simultaneous observations in different wavelength regions, rendering it necessary to consider the effect of time-varying instrumental and astrophysical systematics. We present the results of a series of retrieval feasibility tests examining the effects of instrument systematics and starspots on the recoverability of the true atmospheric state, and demonstrate that correcting for these systematics is key for successful exoplanet science with JWST.