Normalised 1st-order shape coefficients (assuming 0 indexing) of the BP and RP Gaia spectra. The logarithmic density of all 200 million Gaia sources in this space is indicated by the grey cells. Our sample of compact PNe (Table 1) is shown by the red crosses if they had Gaia DR3 BP/RP spectra, and known or candidate PNe from Chornay & Walton (2021) by light blue crosses. The latter follow the bulk of the Gaia sources, suggesting that their SEDs are continuum-dominated; a small subset shows low RP₁ values and approach our sample of compact PNe. Compact PNe candidates with RP₁ < −0.4 are shown in orange, but were rejected/disqualified upon further inspection (Sect. 4.3).