What does this research mean for the field?

Surface gas accretion in protoplanetary disks, combined with fragile and slowly drifting icy dust, leads to significant water vapor enrichment inside the snow line and naturally reproduces the observed mass-metallicity anti-correlation in gas giant exoplanets. Novelty: ClaimNovelty.NOVEL_FINDING. Consensus alignment: ConsensusAlignment.ESTABLISHES_NEW_DIRECTION.

What question did this study set out to answer?

This research aims to explore how heavy-element enrichment occurs in gas giant exoplanets' atmospheres, particularly focusing on the mass-metallicity relationship.

June 10, 2026

Heavy element enrichment of gas in surface-accretion disks. A possible origin of the mass--metallicity anti-correlation in exoplanets

Key Points

This research aims to explore how heavy-element enrichment occurs in gas giant exoplanets' atmospheres, particularly focusing on the mass-metallicity relationship.
Simulated gas and dust evolution in surface-accretion disk models, accounting for radial transport and dust dynamics.
Compared effects of vertically uniform versus surface-concentrated gas accretion on water vapor abundance.
Examined the impact of fragile icy dust on vapor enrichment and its implications for planetary atmospheres.
In the uniform-accretion disk model, icy grains only increase water vapor by ∼3 times due to their slow drift.
In the surface-accretion model, icy dust leads to an order of magnitude higher water vapor enrichment by removing ice-free gas.
A clear anti-correlation between water vapor concentration and residual gas mass is established, paralleling observed trends in exoplanets.

Abstract

Recent observations, including those by JWST, suggest that the atmospheres of many gas giant exoplanets have super-stellar metallicity that is anti-correlated with planetary mass. Several studies suggest that the super-stellar metallicity can be explained by accretion of vapor-enriched disk gas produced by the sublimation of rapidly drifting icy pebbles. However, recent disk observations and experiments suggest that icy dust is fragile at low temperatures, calling into question the conventional picture that icy grains grow efficiently and drift rapidly. We present a new scenario for heavy-element enrichment in the inner disk by fragile slowly drifting icy dust, assuming that magnetohydrodynamical disk winds drive gas accretion near the disk surface rather than at the midplane. We simulate the evolution of gas and dust in a surface-accretion disk, taking into account the radial transport of gas and dust, collision growth and fragmentation of fragile dust, and the condensation and sublimation of H₂O. Two accretion disk models are presented, in which gas accretion flows are assumed to be either vertically uniform or narrowly concentrated near the disk surface. In the uniform-accretion disk model, fragile icy grains increase the water vapor abundance inside the snow line only by a factor of ∼3 due to their slow drift. In contrast, in the surface-accretion disk model, the slow drift of icy dust leads to water vapor enrichment that is higher by an order of magnitude, owing to the selective removal of ice-free gas from the disk. Furthermore, surface accretion yields an anti-correlation between the water vapor concentration in the inner disk and the residual disk gas mass, analogous to the anti-correlation between atmospheric metallicity and planet mass observed in extrasolar giant planets. Surface gas accretion naturally establishes vapor-rich environments inside the snow line when icy dust is fragile. This study provides a novel perspective on the formation environments that dictate the composition of gas giant atmospheres.

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