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Despite the many studies in the last decades, the low-ionisation structures(LISs) of planetary nebulae (PNe) still hold several mysteries. Recent imagingsurveys have demonstrated that LISs are composed of molecular gas. Here wereport H$_2$ emission in the LISs of NGC 7009 and NGC 6543 by means of verydeep narrow-band H$_2$ images taken with NIRI@Gemini. The surface brightness ofthe H2 1-0 S(1) line is estimated to be (0.46-2.9)$\times$10$^{-4}$ ergs$^{-1}$ cm$^{-2}$ sr$^{-1}$ in NGC 7009 and (0.29-0.48)$\times$10$^{-4}$ ergs$^{-1}$ cm$^{-2}$ sr$^{-1}$ in NGC 6543, with signal-to-noise ratios of 10-42and 3-4, respectively. These findings provide further confirmation of hiddenH$_2$ gas in LISs. The emission is discussed in terms of the recent proposeddiagnostic diagram R(H$_2$)=H$_2$ 1-0 S(1)/H$_2$ 2-1 S(1) versusR(Br$\gamma$)=H$_2$ 1-0 S(1)/Br$\gamma$, which was suggested to trace themechanism responsible for the H$_2$ excitation. Comparing our observations toshock and ultraviolet (UV) molecular excitation models, as well as a number ofobservations compiled from the literature showed that we cannot conclude foreither UV or shocks as the mechanism behind the molecular emission.

H$_2$ emission in the low-ionisation structures of the Planetary Nebulae NGC 7009 and NGC 6543