Sub-Snowline Formation of Gas-Giant Planets in Binary Systems

Abstract

Gas-giant planets are thought to require conditions beyond the water snow line to build solid cores efficiently. In close binary star systems, the companion's gravity additionally limits the region of stable orbits, potentially excluding the zone where giants should form.} We aim to identify binary systems in which gas giants exist despite the snow line lying in the dynamically unstable zone, and to develop a physically motivated formation channel that explains and predicts their observed locations. We analyse a catalogue of 811 circumstellar binary systems from \citet{Thebault2025}, identifying those hosting gas giants. ($M_p \geq 0.15\,M_\mathrm{Jup}$) with snow lines larger than $0.8\,a_c$ as defined by \citet{Quarles_2020}. We compare their metallicity and eccentricity distributions with the background population, model snow-line evolution with MESA, and fit a linear relation between observed planet semi-major axes and the tidal truncation radius from \citet{Pichardo2005}.} Among 393 gas-giant hosts, we identify 17 systems whose snow line lies in the dynamically unstable zone. Their metallicity and eccentricity distributions are consistent with the background population. We propose that a dust trap formed near the tidal truncation radius of the protoplanetary disc can explain sub-snowline giant formation. The observed planet positions follow $a_\mathrm{planet} = (0.569 \pm 0.05)\,r_t$ ($R^2 = 0.94$), enabling system-by-system predictive power. Evolved systems deviate from this relation, independently supporting a second-generation planet origin for those cases. The tidal truncation of a protoplanetary disc by the stellar companion provides a natural mechanism for sub-snowline gas-giant formation in binaries. The resulting empirical relation yields testable predictions for binary eccentricities in systems lacking direct orbital measurements.