The Neptunian ridge through the lens of TESS

Abstract

The Neptunian desert and savanna are separated by a ridge, an overdensity at orbital periods of approximately 3-6 d identified with Kepler. Like the hot Jupiter pileup at larger radii, this feature has been linked to the accumulation of successful outcomes of high-eccentricity migration (HEM). We revisit the ridge independently using TESS occurrence posteriors for FGK main-sequence stars. We quantify its excess over adjacent regions and compare six occurrence profiles using leave-one-out cross-validation and Bayesian evidence. For planets with radii of approximately 5-8 $R_\oplus$, the ridge has twice the occurrence density of the denser neighbouring region ($J=2.05^{+0.52}_{-0.44}$), with 99.9% posterior support (one-sided Gaussian equivalent of $3.1σ$). This contrast agrees with the discovery study and earlier Kepler occurrence measurements. Both predictive performance and Bayesian evidence strongly favour profiles peaking within the ridge over a single power law. The lognormal is preferred, suggesting a narrower inner wing and a more extended outer wing, although evidence for asymmetry remains weak. In the same period window, the analysis recovers the hot Jupiter pileup (approximately 10-16 $R_\oplus$) with a contrast consistent with that of the ridge and excludes an analogous excess among sub-Neptunes (approximately 2-4 $R_\oplus$) with $3.4σ$ posterior support. TESS thus provides an independent detection of the Neptunian ridge. Its possible asymmetry would be compatible with HEM, in which tidal disruption could produce a sharper inner edge and variations in circularisation efficiency a more gradual outer decline. Establishing whether the ridge has this shape requires more precise occurrence measurements, with PLATO's expected planet yield offering a promising opportunity to help unveil its morphology.