  Exoplanet Proxima b will be an important laboratory for the search for
extraterrestrial life for the decades ahead. Here we discuss the prospects of
detecting carbon dioxide at 15 um using a spectral filtering technique with the
Medium Resolution Spectrograph (MRS) mode of the Mid-Infrared Instrument (MIRI)
on the James Webb Space Telescope (JWST). At superior conjunction, the planet
is expected to show a contrast of up to 100 ppm with respect to the star. At a
spectral resolving power of R=1790-2640, about 100 spectral CO2 features are
visible within the 13.2-15.8 um (3B) band, which can be combined to boost the
planet atmospheric signal by a factor 3-4, depending on the atmospheric
temperature structure and CO2 abundance. If atmospheric conditions are
favorable (assuming an Earth-like atmosphere), with this new application to the
cross-correlation technique carbon dioxide can be detected within a few days of
JWST observations. However, this can only be achieved if both the instrumental
spectral response and the stellar spectrum can be determined to a relative
precision of <= 1e-4 between adjacent spectral channels. Absolute flux
calibration is not required, and the method is insensitive to the strong
broadband variability of the host star. Precise calibration of the spectral
features of the host star may only be attainable by obtaining deep observations
of the system during inferior conjunction that serve as a reference. The
high-pass filter spectroscopic technique with the MIRI MRS can be tested on
warm Jupiters, Neptunes, and super-Earths with significantly higher planet/star
contrast ratios than the Proxima system.
