Selected individual Gaia spectra plotted over a density map of 20 million flux-normalised Gaia BP/RP spectra. Top row: individual Gaia spectra of our compact PNe literature sample (red lines, see Sect. 4.1). The BP spectra are distinguished by strong [O III]λλ4960, 5008 and Hα emission. The RP spectra show Hα emission, with secondary peaks around 7100–7300 Å from [Ar I/III] and [O II]; the [S III]λλ9071, 9533 lines are also discernible. One of the PNe (G334.8−07.4) has a strong stellar continuum, following the bulk of the Gaia SEDs. Bottom row: same as above, but showing the candidates we selected blindly from all 200 million Gaia sources matching our search criteria for strong line emitters (Sect. 4.3).