Kepler DR25 cat. transit and detec. prob.

We delimited the Neptunian desert as the close-in region of the period-radius space with no planets at a 3{sigma} level, and provide the community with simple, ready-to-use approximate boundaries. We identified an overdensity of planets separating the Neptunian desert and savanna (3.2d<~ P_orb_<~5.7d) that stands out at a 4.7{sigma} level above the desert and at a 3.5{sigma} level above the savanna, which we propose to call the Neptunian ridge.

The period range of the ridge matches that of the well-known hot Jupiter pileup (~=3-5 days), which suggests that similar evolutionary processes might act on both populations. We find that the occurrence fraction between the pileup and warm Jupiters (f_pileup/warm_=5.3+/-1.1) is about twice that between the Neptunian ridge and savanna (f_ridge/savanna_=2.7+/-0.5). This indicates either that the processes that drive or maintain planets in the overdensity are more efficient for Jupiters or that the processes that drive or maintain planets in the warm region are more efficient for Neptunes.

Our revised landscape supports a previous hypothesis that a fraction of Neptunes were brought to the edge of the desert (i.e. the newly identified ridge) through high-eccentricity tidal migration (HEM) late in their life, surviving the evaporation that eroded Neptunes that arrived earlier on in the desert. The ridge thus appears as a true physical feature illustrating the interplay between photoevaporation and HEM processes, providing further evidence of their role in shaping the distribution of close-in Neptunes.