Letter to the Editor

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

¹ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, CH-1290 Versoix, Switzerland
² Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
³ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴ CFisUC, Departamento de Física, Universidade de Coimbra, 3004-516 Coimbra, Portugal
⁵ LTE, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 75014 Paris, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 February 2026
Accepted: 17 April 2026

Abstract

Context. 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 orbital periods of ≃3 − 6 d, known as the Neptunian ridge.

Aims. We confronted the high-eccentricity tidal migration (HEM) scenario with the Neptunian desert–ridge–savanna landscape.

Methods. We mapped the HEM tidal survival constraints onto the period–radius plane using empirically inferred mass–radius relations, and provide an additional consistency check by projecting the tidal survival boundary onto the period–density plane.

Results. The HEM tidal survival formalism reproduces the slope of the Neptunian desert boundary across the sub-Neptune to super-Neptune/sub-Saturn regime (1.8 R_⊕ ≲ R_p ≲ 6 R_⊕), with a single representative value of the tidal encounter parameter setting the overall period offset. In the Jovian regime, the empirical boundary geometry remains broadly consistent with the tidal survival limit, although additional effects such as radius inflation and orbital decay may account for residual deviations. Incorporating the empirically inferred density dispersion transforms the disruption limit into a finite tidal survival band that traces the Neptunian ridge. Because tidal dissipation rises steeply towards the disruption threshold, HEM survivors are expected to circularise just beyond this limit, thereby clustering within the survival band and naturally generating the ridge overdensity. In the period–density plane, the observed population follows the predicted density-dependent tidal survival and clustering pattern, and the high-density ridge planets exhibit a persistent concentration near ρ_p ≃ 1.7 g cm⁻³, whose origin warrants further investigation.

Conclusions. High-eccentricity tidal migration provides a self-consistent physical explanation for the origin of the Neptunian ridge and the geometry of the desert boundary.