Planetary nebula distances in Gaia DR2

Context. Planetary nebula distance scales often suffer from model-dependent solutions. Model-independent trigonometric parallaxes have been rare. Space-based trigonometric parallaxes are now available for a larger sample using the second Data Release of Gaia.Aims. We aim to derive a high-quality approach for selection criteria of trigonometric parallaxes for planetary nebulae and discuss possible caveats and restrictions in the use of this Data Release.Methods. A few hundred sources from previous distance scale surveys were manually cross-identified with data from the second Gaia Data Release (DR2) because coordinate-based matching does not work reliably. The data were compared with the results of previous distance scales and to the results of a recent similar study that used the first Data Release Gaia DR1.Results. While the few available previous ground-based trigonometric parallaxes as well as those obtained with the Hubble Space Telescope perfectly match the new data sets, older statistical distance scales, reaching larger distances, do show small systematic differences. When we restrict the comparison to the central stars for which the photometric colors of Gaia show a negligible contamination by the surrounding nebula, the difference is negligible for statistical distances based on radio flux, while those derived from Hα surface brightness still show minor differences. The DR2 study significantly improves the previous recalibration of the statistical distance scales using DR1/TGAS