Close-in Neptune properties as a function of orbital period. Vertical green lines mark the Ridge boundaries. Middle panels: Measured properties, colored in blue (resp. magenta) if less dense (resp. denser) than 1 g cm^-3. 3D spin-orbit angles from Tepcat are shown when known to better than 30° precision; eccentricity, mass, and radius from the NASA Exoplanet Archive when known to better than 0.1 and 20%. Density is recomputed from these mass and radius. Star and square symbols have the same meaning as in Fig. 1. Left panels: Schematic representation of the proposed atmospheric (broad colored arrows, see text) and dynamical (gray arrows) pathways, as described in the text. The black dashed line in the density diagram marks the approximate location of the proposed density brink. Right panels: Except for spin-orbit angles, which have too few measurements, these histograms show the fractions of fluffy and dense planets relative to the Savanna (dashed) or Desert+Ridge (solid) populations.