Volatile enrichment in low-mass planets: Signatures of past planetary disruption?

Abstract

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 hydrogen- and helium-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, 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, 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 high-energy stellar irradiation. Our results show that volatile-rich gas can form a "volatile-enriched planet" (VEP). The resulting envelopes can contain up to 10^-6 Earth masses -- comparable to Earth's atmosphere -- for Earth-like planets, yielding transit depths of tens to hundreds of parts per million. Such signatures may persist for 1 to 100 million 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 and may help explain the properties of systems such as TOI-421b and WASP-107b.