Observing transiting exoplanets with JWST/NIRSpec

Recent publications resulting from observations conducted with the Hubble Space Telescope (HST) have highlighted the diagnostic power of near-infrared spectroscopy for the study of the atmospheric properties of transiting exoplanets. In this context, the James Webb Space Telescope (JWST) and it suite of instruments will have an unprecedented combination of sensitivity and spectral coverage. In this article, we focus on one of these instruments, the near-infrared spectrograph NIRSpec. NIRSpec will offer an aperture spectroscopy mode dedicated to the characterization of transiting exoplanets. It will cover the 0.6-5.3 μm spectral domain with 3 ranges of spectral resolution (R 100, 1000 and 2700). The predicted noise floor (photon noise and detector noise only, no systematics included) is lower than 100 ppm for a single 1-hour in-transit observation of an 7th magnitude star, indicating that transit spectroscopy programs with NIRSpec will routinely have photon-noise limited noise floors of a few tens of ppm. In terms of brightness limits, at high spectral resolution, NIRSpec will be able to observe planets transiting stars with J-band magnitudes up to 6.5 in the worst case and 4.5 in the best case.