  Characterization of directly imaged exoplanets is one of the most eagerly
anticipated science functions of the James Webb Space Telescope. MIRI, the
mid-IR instrument has the capability to provide unique spatially resolved
photometric data points in a spectral range never achieved so far for such
objects. We aim to present the very first on-sky contrast measurements of the
MIRI's coronagraphs. In addition to a classical Lyot coronagraph at the longest
wavelength, this observing mode implements the concept of the four quadrant
phase mask for the very first time in a space telescope. We observed single
stars together with a series of reference stars to measure raw contrasts as
they are delivered on the detector, as well as reference subtracted contrasts.
MIRI's coronagraphs achieve raw contrasts greater than $10^3$ at the smallest
angular separations (within $1''$) and about $10^5$ further out (beyond
$5\sim6''$). Subtracting the residual diffracted light left unattenuated by the
coronagraph has the potential to bring the final contrast down to the
background and detector limited noise floor at most angular separations (a few
times $10^4$ at less than $1''$). MIRI coronagraphs behave as expected from
simulations. In particular the raw contrasts for all four coronagraphs are
fully consistent with the diffractive model. Contrasts obtained with
subtracting reference stars also meet expectations and are fully demonstrated
for two four quadrant phase masks (F1065C and F1140C). The worst contrast,
measured at F1550C, is very likely due to a variation of the phase aberrations
at the primary mirror during the observations, and not an issue of the
coronagraph itself. We did not perform reference star subtraction with the Lyot
mask at F2300C, but we anticipate that it would bring the contrast down to the
noise floor.
