The Neptunian ridge as a natural outcome of high-eccentricity tidal migration

TL;DR

This paper demonstrates that high-eccentricity tidal migration naturally explains the Neptunian ridge and desert boundary features observed in the period-radius and period-density distributions of exoplanets.

Contribution

It provides a formalism linking tidal survival constraints with observed planet distributions, reproducing the ridge and boundary features with a unified model.

Findings

The HEM model reproduces the slope of the desert boundary across sub-Neptune to super-Neptune regimes.

The model predicts clustering of survivors just beyond the disruption limit, forming the ridge overdensity.

The population follows a density-dependent survival pattern, with ridge planets near 1.7 g/cm³.

Abstract

Recent occurrence-rate analyses have shown that the transition between the Neptunian desert and the savanna is not smooth but instead exhibits an overdensity of planets at $P_{\rm orb}\simeq3$-$6$ d, known as the Neptunian ridge. We confronted the high-eccentricity tidal migration (HEM) scenario with this updated desert-ridge-savanna landscape. We mapped the HEM tidal survival constraints onto the period-radius plane using empirically inferred mass-radius relations and provided an independent consistency check in the period-density plane. The HEM tidal survival formalism reproduces the slope of the desert boundary across the sub-Neptune to super-Neptune/sub-Saturn regime ($1.8\,\rm R_\oplus \lesssim R_{\rm p} \lesssim 6\,\rm R_\oplus$), with a single representative tidal encounter parameter setting the overall period offset. In the Jovian regime, the boundary remains broadly consistent with the survival limit, with residual deviations likely due to radius inflation or orbital decay. Incorporating the observed density dispersion transforms the disruption limit into a finite tidal survival band that traces the ridge. Because tidal dissipation rises steeply towards the disruption threshold, HEM survivors are expected to circularise just beyond this limit, clustering within the band and naturally producing the ridge overdensity. In the period-density plane, the population follows the predicted density-dependent survival and clustering pattern, with a persistent concentration of ridge planets near $\rho_{\rm p}\simeq1.7\,\mathrm{g\,cm^{-3}}$. High-eccentricity tidal migration thus provides a self-consistent explanation for the ridge and desert boundary geometry.