  Until now, our knowledge of the extragalactic Universe at mid-IR wavelengths
(> 5 microns) was limited to rare active galactic nuclei (AGN) and the
brightest normal galaxies up to z~3. The advent of the JWST with its
Mid-Infrared Instrument (MIRI) will revolutionise the ability of the mid-IR
regime as a key wavelength domain to probe the high-z Universe. In this work we
present a first study of JWST MIRI 7.7 micron sources selected with > 3 sigma
significance from the lensing cluster field SMACS J0723.3-7327. We model their
spectral energy distribution fitting with 13 JWST and HST broad bands, in order
to obtain photometric redshifts and derived physical parameters for all these
sources. We find that this 7.7 micron galaxy sample is mainly composed of
normal galaxies up to z=4 and has a tail of about 2% of sources at higher
redshifts to z~ 9-10. The vast majority of our galaxies have [3.6]-[7.7]<0
colours and very few of them need high dust extinction values (A_V=3 -6 mag)
for their SED fitting. The resulting lensing-corrected stellar masses span the
range 10^7-10^{11} Msun. Overall, our results clearly show that the first MIRI
7.7 micron observations of deep fields are already useful to probe the
high-redshift Universe and suggest that the deeper 7.7 micron observations to
be available very soon will open up, for the first time, the epoch of
reionisation at mid-IR wavelengths.
