Volatile enrichment in low-mass planets as signatures of past planetary disruption

Tidal disruption and engulfment events around main-sequence stars -- such as the luminous red nova ZTF SLRN-2020 (a candidate planetary-engulfment event) -- reveal the destruction of close-in giant planets. While current observations focus on stellar accretion and inner dust emission, the fate of the volatile-rich material expelled during disruption remains poorly understood. We investigate whether the H/He-rich gas expelled from the disrupted planet’s envelope and atmosphere can escape the inner system and be gravitationally captured by a low-mass outer planet (volatile-enriched planet (VEP)), potentially forming a transient atmosphere and producing detectable volatile contamination. We model the outward diffusion of gas from a tidally stripped giant using 2D hydrodynamical simulations with FARGO3D complemented by analytical estimates of volatile observability and atmospheric escape. We assess the efficiency of gas capture by outer planets and the survival timescales of the resulting secondary atmospheres under XUV-driven erosion. Our simulations show that volatile-rich gas can form a VEP. The resulting envelopes can contain 10^-10–10^ M_⊕ -- up to the mass of Earth's atmosphere -- for Earth-like planets, yielding transit depths of tens to hundreds of parts per million. Such signatures may persist for 10⁶–10⁸ years, depending on planetary mass, orbit, and stellar activity. This scenario offers a viable pathway for the formation of volatile-rich atmospheres in evolved low-mass planets. When accompanied by dynamical signatures such as eccentric orbits, these chemical anomalies may trace past planetary disruption. This framework may help us interpret the atmospheric and orbital properties of systems such as TOI-421b, a warm sub-Neptune with a H/He-rich envelope and moderate eccentricity, and WASP-107b, a low-density Neptune-mass planet showing ongoing He I escape, shedding new light on the late-stage evolution of planetary systems.