  Taking advantage of the all-sky coverage and broad frequency range of the
Planck satellite, we study the Sunyaev-Zeldovich (SZ) and pressure profiles of
62 nearby massive clusters detected at high significance in the 14-month
nominal survey. Careful reconstruction of the SZ signal indicates that most
clusters are individually detected at least out to R500. By stacking the radial
profiles, we have statistically detected the radial SZ signal out to 3 x R500,
i.e., at a density contrast of about 50-100, though the dispersion about the
mean profile dominates the statistical errors across the whole radial range.
Our measurement is fully consistent with previous Planck results on integrated
SZ fluxes, further strengthening the agreement between SZ and X-ray
measurements inside R500. Correcting for the effects of the Planck beam, we
have calculated the corresponding pressure profiles. This new constraint from
SZ measurements is consistent with the X-ray constraints from XMM-Newton in the
region in which the profiles overlap (i.e., [0.1-1]R500), and is in fairly good
agreement with theoretical predictions within the expected dispersion. At
larger radii the average pressure profile is slightly flatter than most
predictions from numerical simulations. Combining the SZ and X-ray observed
profiles into a joint fit to a generalised pressure profile gives best-fit
parameters [P0, c500, gamma, alpha, beta] = [6.41, 1.81, 0.31, 1.33, 4.13].
Using a reasonable hypothesis for the gas temperature in the cluster outskirts
we reconstruct from our stacked pressure profile the gas mass fraction profile
out to 3 x R500. Within the temperature driven uncertainties, our Planck
constraints are compatible with the cosmic baryon fraction and expected gas
fraction in halos.
