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We characterized the population of galaxy clusters detected with the SZeffect with Planck, by measuring the cool core state of the objects in awell-defined subsample of the Planck catalogue. We used as indicator theconcentration parameter Santos et al. (2008). The fraction of cool coreclusters is $29 \pm 4 \%$ and does not show significant indications ofevolution in the redshift range covered by our sample. We compare thedistribution of the concentration parameter in the Planck sample with the oneof the X-ray selected sample MACS (Mann & Ebeling, 2011): the distributions aresignificantly different and the cool core fraction in MACS is much higher ($59\pm 5 \%$). Since X-ray selected samples are known to be biased towards coolcores due to the presence of their prominent surface brightness peak, wesimulated the impact of the "cool core bias" following Eckert et al. (2011). Wefound that it plays a large role in the difference between the fractions ofcool cores in the two samples. We examined other selection effects that couldin principle bias SZ-surveys against cool cores but we found that their impactis not sufficient to explain the difference between Planck and MACS. Thepopulation of X-ray under-luminous objects, which are found in SZ-surveys butmissing in X-ray samples (Planck Collaboration 2016), could possibly contributeto the difference, as we found most of them to be non cool cores, but thishypothesis deserves further investigation.

The cool core state of Planck SZ-selected clusters versus X-ray selected samples: evidence for cool core bias