Planck Early Results: Statistical properties of extragalactic radio
  sources in the Planck Early Release Compact Source Catalogue
  The data reported in Planck's Early Release Compact Source Catalogue (ERCSC)
are exploited to measure the number counts (dN/dS) of extragalactic radio
sources at 30, 44, 70, 100, 143 and 217 GHz. Due to the full-sky nature of the
catalogue, this measurement extends to the rarest and brightest sources in the
sky. At lower frequencies (30, 44, and 70 GHz) our counts are in very good
agreement with estimates based on WMAP data, being somewhat deeper at 30 and 70
GHz, and somewhat shallower at 44 GHz. Planck's source counts at 143 and 217
GHz join smoothly with the fainter ones provided by the SPT and ACT surveys
over small fractions of the sky. An analysis of source spectra, exploiting
Planck's uniquely broad spectral coverage, finds clear evidence of a steepening
of the mean spectral index above about 70 GHz. This implies that, at these
frequencies, the contamination of the CMB power spectrum by radio sources below
the detection limit is significantly lower than previously estimated.
