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

TL;DR

This paper explores how volatile-rich gas expelled during planetary disruption can be captured by outer planets, potentially forming detectable secondary atmospheres that persist for millions of years, influencing low-mass planet composition.

Contribution

It introduces a model for volatile gas capture during planetary disruption, demonstrating the formation of transient, volatile-enriched atmospheres on low-mass planets, a novel pathway in planetary evolution.

Findings

Volatile gas can be captured by outer planets forming transient atmospheres.

Envelopes can contain up to 10^-6 Earth masses, similar to Earth's atmosphere.

Signatures of these atmospheres may last from 1 to 100 million years.

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.