  We test for foreground residuals in the foreground cleaned Planck Cosmic
Microwave Background (CMB) maps outside and inside U73 mask commonly used for
cosmological analysis. The aim of this paper is to introduce a new method to
validate masks by looking at the differences in cleaned maps obtained by
different component separation methods. By analyzing the power spectrum as well
as the mean, variance and skewness of needlet coefficients on bands outside and
inside the U73 mask we first confirm that the pixels already masked by U73 are
highly contaminated and cannot be used for cosmological analysis. We further
find that the U73 mask needs extension in order to reduce large scale
foreground residuals to a level of less than $20\%$ of the standard deviation
of CMB fluctuations within the bands closest to the galactic equator. We also
find 276 point sources in the cleaned foreground maps which are currently not
masked by the U73 mask. Our final publicly available extended mask leaves
$65.9\%$ of the sky for cosmological analysis. Note that this extended mask may
be important for analyses on local sky patches; in full sky analyses the
additional residuals near the galactic equator may average out.
