Top: images of the G21.5-0.9 environment at the nine Planck frequencies (Table 1), increasing from 30 GHz at top left to 857 GHz at bottom right. Each image is 100′ on a side, with galactic coordinate orientation. Bottom: microwave SED of G21.5-0.9. Filled circles are Planck measurements from this paper, with 3σ error bars. The open symbol at 1 GHz is from the Green catalogue. The dashed line emanating from the 1 GHz flux density is a flat spectrum, for illustration. The dotted line is a broken power law, with a break frequency at 40 GHz, a spectral index of 0.05 at lower frequencies and 0.55 at higher frequencies. Open diamonds show prior radio flux densities from Morsi & Reich (1987) and Salter et al. (1989).