Neutrino anisotropies after Planck

We present new constraints on the rest-frame sound speed, c₄₅₅^2, and the viscosity parameter, cₕ₈ₒ^2, of the cosmic neutrino background from the recent measurements of the cosmic microwave background anisotropies provided by the Planck satellite. While broadly consistent with the expectations of c₄₅₅^2=cₕ₈ₒ^2=1/3 in the standard scenario, the Planck data set hints at a higher value of the viscosity parameter, with cₕ₈ₒ^2=0. 600. 18 at 68% C. L. , and a lower value of the sound speed, with c₄₅₅^2=0. 3040. 013 at 68% C. L. We find a correlation between the neutrino parameters and the lensing amplitude of the temperature power spectrum A₋. When the latter parameter is allowed to vary, we find a better consistency with the standard model with cₕ₈ₒ^2=0. 510. 22, c₄₅₅^2=0. 3110. 019, and A₋=1. 080. 18 at 68% C. L. This result indicates that the anomalous large value of A₋ measured by Planck could be connected to nonstandard neutrino properties. Including additional data sets from baryon acoustic oscillation surveys and the Hubble Space Telescope constraint on the Hubble constant, we obtain cₕ₈ₒ^2=0. 400. 19, c₄₅₅^2=0. 3190. 019, and A₋=1. 150. 17 at 68% C. L. ; including the lensing power spectrum, we obtain cₕ₈ₒ^2=0. 500. 19, c₄₅₅^2=0. 3140. 015, and A₋=1. 0250. 076 at 68% C. L. Finally, we investigate further degeneracies between the clustering parameters and other cosmological parameters.