Companion architectures of sub-Saturns: Distinct migration pathways across the Neptunian landscape
Year: 2026
Type: article
Abstract: Close-in sub-Saturns (4.0, _⊕ łeq R_ łeq 8.5, _⊕) are distributed unevenly across the period--radius plane: They are depleted in the Neptunian desert at periods P łesssim 3.2,d, accumulate in a narrow overdensity at P ≃ 3.2--5.7,d (the Neptunian ridge), and thin out in the savanna at P ≳ 5.7,d. R p R We tested whether sub-Saturns in different landscape regions have systematically different planetary companion architectures, as expected if desert and ridge planets arrived through high-eccentricity migration and if savanna planets migrated quiescently. We compiled 90 sub-Saturns in 86 systems with both transit and radial velocity (RV) data, constructed per-system RV and transit injection-recovery completeness maps, and combined them into detection probability surfaces for companions. From these maps we derived completeness-corrected occurrence rates for different companion types with a joint Poisson occurrence model. Companion architectures differ significantly across the landscape. Nearby companions (P_ < 200,d) accompany comp -4.3 ^ +3.2 % of savanna sub-Saturns but only 36_ -14 ^ +16 % of desert and ridge sub-Saturns. This contrast is driven almost entirely by small (M_ < 20, _⊕) companions, accompanying 88.2_ comp M -5.5 ^ +4.2 % of savanna but only 29_ -14 ^ +17 % of desert and ridge sub-Saturns, while medium-mass and giant companions within 200,d are rare (łesssim 13%) everywhere, so the sub-Saturn typically dominates its inner system. Moreover savanna sub-Saturns reside in compact multi-planet systems. These contrasts are robust to crosscuts in sub-Saturn radius, bulk density, eccentricity, and host-star properties. Desert and ridge sub-Saturns reside in dynamically emptied systems whose nearby companion rates resemble those of hot Jupiters, while savanna sub-Saturns inhabit compact multi-planet systems resembling those of warm Jupiters. This parallel supports two migration channels within a single population: high-eccentricity migration delivering planets to the desert and ridge, and quiescent disk migration or in situ formation populating the savanna.
Source: Astronomy and Astrophysics
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W7168187897