  The all-sky Planck survey in 9 frequency bands was used to search for
emission from all 274 known Galactic supernova remnants. Of these, 16 were
detected in at least two Planck frequencies. The radio-through-microwave
spectral energy distributions were compiled to determine the emission mechanism
for microwave emission. In only one case, IC 443, is there high-frequency
emission clearly from dust associated with the supernova remnant.In all cases,
the low-frequency emission is from synchrotron radiation. A single power law,
as predicted for a population of relativistic particles with energy
distribution that extends continuously to high energies, is evident for many
sources, including the Crab and PKS 1209-51/52. A decrease in flux density
relative to the extrapolation of radio emission is evident in several sources.
Their spectral energy distributions can be approximated as broken power laws,
$S_\nu\propto\nu^{-\alpha}$, with the spectral index, alpha, increasing by
0.5-1 above a break frequency in the range 10-60 GHz. The break could be due to
synchrotron losses.
