  The Planck mission, thanks to its large frequency range and all-sky coverage,
has a unique potential for systematically detecting the brightest, and rarest,
submillimetre sources on the sky, including distant objects in the
high-redshift Universe traced by their dust emission. A novel method, based on
a component-separation procedure using a combination of Planck and IRAS data,
has been applied to select the most luminous cold submm sources with spectral
energy distributions peaking between 353 and 857GHz at 5' resolution. A total
of 2151 Planck high-z source candidates (the PHZ) have been detected in the
cleanest 26% of the sky, with flux density at 545GHz above 500mJy. Embedded in
the cosmic infrared background close to the confusion limit, these high-z
candidates exhibit colder colours than their surroundings, consistent with
redshifts z>2, assuming a dust temperature of 35K and a spectral index of 1.5.
First follow-up observations obtained from optical to submm have confirmed that
this list consists of two distinct populations. A small fraction (around 3%) of
the sources have been identified as strongly gravitationally lensed
star-forming galaxies, which are amongst the brightest submm lensed objects
(with flux density at 545GHz ranging from 350mJy up to 1Jy) at redshift 2 to 4.
However, the vast majority of the PHZ sources appear as overdensities of dusty
star-forming galaxies, having colours consistent with z>2, and may be
considered as proto-cluster candidates. The PHZ provides an original sample,
complementary to the Planck Sunyaev-Zeldovich Catalogue; by extending the
population of the virialized massive galaxy clusters to a population of sources
at z>1.5, the PHZ may contain the progenitors of today's clusters. Hence the
PHZ opens a new window on the study of the early ages of structure formation,
and the understanding of the intensively star-forming phase at high-z.
