  The Mid-Infrared instrument (MIRI) on board the James Webb Space Telescope
will perform the first ever characterization of young giant exoplanets observed
by direct imaging in the 5-28 microns spectral range. This wavelength range is
key for both determining the bolometric luminosity of the cool known exoplanets
and for accessing the strongest ammonia bands. In conjunction with shorter
wavelength observations, MIRI will enable a more accurate characterization of
the exoplanetary atmospheric properties. Here we consider a subsample of the
currently known exoplanets detected by direct imaging and we discuss their
detectability with MIRI, either using the coronagraphic or the spectroscopic
modes. By using the Exo-REM atmosphere model we calculate the mid-infrared
emission spectra of fourteen exoplanets, and we simulate MIRI coronagraphic or
spectroscopic observations. Specifically we analyze four coronagraphic
observational setups, which depend on (i) the target-star and reference-star
offset (0, 3, 14 mas), (ii) the wave-front-error (130, 204 nm rms), (iii) the
telescope jitter amplitude (1.6, 7 mas). We then determine the signal-to-noise
and integration time values for the coronagraphic targets whose planet-to-star
contrasts range from 3.9 to 10.1 mag. We conclude that all the MIRI targets
should be observable with different degrees of difficulty, which depends on the
final in-flight instrument performances. Furthermore, we test for detection of
ammonia in the atmosphere of the coolest targets. Finally, we present the case
of HR 8799 b to discuss what MIRI observations can bring to the knowledge of a
planetary atmosphere, either alone or in combination with shorter wavelength
observations.
