How leaky? A large parameter study of leaky dust traps to quantify the transport of pebbles and ice in protoplanetary discs

Adrien Houge; Anders Johansen; Andrea Banzatti; Sierra Grant

arxiv: 2604.11925 · v1 · submitted 2026-04-13 · 🌌 astro-ph.EP

How leaky? A large parameter study of leaky dust traps to quantify the transport of pebbles and ice in protoplanetary discs

Adrien Houge , Anders Johansen , Andrea Banzatti , Sierra Grant This is my paper

Pith reviewed 2026-05-10 15:38 UTC · model grok-4.3

classification 🌌 astro-ph.EP

keywords protoplanetary discsdust trapspebble transportice transportC/O ratiodust evolutionplanet-disc interactionplanetesimal formation

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The pith

Dust traps in protoplanetary discs are leakier than earlier models assumed, so most outer traps sustain oxygen-rich inner discs with gas-phase C/O below 1.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper runs over 300 detailed simulations of how dust particles grow, fragment, and move past planet-carved traps in protoplanetary discs. It replaces simplified single-size dust approximations with full coagulation and fragmentation tracking to measure leakage rates across wide ranges of viscosity, turbulence, planet mass, and location. The central result is that outer traps beyond 5 au are permeable enough in most cases to let oxygen-bearing pebbles and ice reach the inner disc, preserving low C/O ratios for long times. Inner traps prove leakier still under similar planet conditions, though their blocking power changes with time. Only special combinations of low viscosity, weak turbulence, and efficient planetesimal formation allow traps to block material strongly enough to raise inner C/O above 1, and even then planetesimal formation itself drives the chemical shift more than the trap.

Core claim

We find that dust traps are leakier than previously thought, on a broader parameter space, such that most outer traps (r > 5 au) will result in a long-lived O-rich inner disc with gas-phase C/O < 1. In similar conditions (e.g., carved by the same planet mass), we find inner traps are much leakier than outer traps, though their relative efficiency in reducing the pebble flux is time-dependent. Highly blocking traps altering the inner disc composition dramatically (leading, e.g., to C/O > 1) are possible to set up but necessitate low viscosity and weak turbulence, along with efficient planetesimal formation by the streaming instability. In that case, we find that it is the formation of planets

What carries the argument

The DustPy one-dimensional code that evolves the full dust size distribution via coagulation and fragmentation while computing radial transport to quantify the fraction of pebbles and ice that leak past planetary dust traps.

If this is right

Most outer dust traps beyond 5 au permit enough pebble and ice transport to maintain oxygen-rich inner discs with gas-phase C/O < 1 over long timescales.
Inner dust traps carved by planets are leakier than outer traps under comparable conditions, with their blocking efficiency varying over time.
Dramatically raising inner-disc C/O above 1 requires low viscosity, weak turbulence, and high planetesimal formation efficiency.
In cases where inner-disc composition changes strongly, planetesimal formation rather than the dust trap itself drives the alteration.
The reduction in pebble flux by traps is time-dependent for inner locations.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Models linking outer planets to inner-disc chemistry may need to incorporate higher leakage rates across wider turbulence and viscosity ranges.
The time dependence of inner-trap leakage implies that chemical signatures could differ between early and late disc stages.
JWST measurements of inner C/O ratios could serve as indirect probes of planetesimal formation efficiency in outer disc regions.

Load-bearing premise

The one-dimensional model with its chosen rules for particle sticking and breaking, together with the selected ranges of disc conditions, accurately captures real three-dimensional disc physics and that planetesimal formation efficiency can be varied independently.

What would settle it

Direct observation or higher-dimensional simulation showing that outer traps at r > 5 au block more than half the incoming pebble and ice flux under typical turbulence strengths, or JWST spectra indicating many inner discs with gas-phase C/O > 1 despite the presence of outer planets.

Figures

Figures reproduced from arXiv: 2604.11925 by Adrien Houge, Anders Johansen, Andrea Banzatti, Sierra Grant.

**Figure 1.** Figure 1: Schematic representing a protoplanetary disc with a gap created by the presence of a giant planet. The gas pressure profile is modified, resulting in the filtering of particles: pebbles are blocked while small grains leak past the gap. The key processes influencing the leakiness of dust traps included in our work are highlighted. & Tielens 1997; Tazaki & Dominik 2022). We set the fragmentation velocity to… view at source ↗

**Figure 2.** Figure 2: Radial profile of the gas surface density Σg (upper panels) and disc viscosity αvisc (lower panels) including a dust trap caused by different planet mass and location, whose shape and depth is computed using empirical fits from Kanagawa et al. (2017). We pick here our lowest value of the viscosity (αvisc,0 = 10−3 ) which produces the deepest gaps in our parameter study. For comparison, Mp = 300 M⊕ corresp… view at source ↗

**Figure 3.** Figure 3: Vertically-integrated dust density distribution as a function of particle size for a disc with a gap carved by a 100 M⊕ planet at 10 au (upper panels) compared to a smooth disc (lower panels). In the model showed here, the disc viscosity is set to αvisc = 10−3 and the local turbulence to δturb = 3.2 × 10−4 . The colour bar indicates the surface density of dust in a given size range at a given radius. The t… view at source ↗

**Figure 4.** Figure 4: Dust velocity as a function of distance from the star and dust Stokes number in a disc hosting a 100 M⊕ planet that has carved a gap at 10 au (upper panel) compared to a smooth disc (lower panel) for a global viscosity αvisc = 10−3 and local turbulence δturb = 3.2×10−4 . Negative velocity represents the inward motion of dust grains (red) while a positive velocity shows the outward motion (blue). The dark … view at source ↗

**Figure 5.** Figure 5: Key diagnostic quantities to quantify the leakiness of dust traps, here for the fiducial model of a disc with a 100 M⊕ planet that has carved a gap at 10 au for a global viscosity αvisc = 10−3 and local turbulence δturb = 3.2 × 10−4 . Upper panel: Pebble flux Fpeb outside the planetary gap (30 au, blue) and reaching the inner disc at (1 au, red), for a planethosting disc (solid line) or smooth disc (dashe… view at source ↗

**Figure 6.** Figure 6: Trap blocking efficiency B (Eq. 4) of a dust trap by t = 5 Myr for a range of planet mass Mp (30, 100, 300 M⊕) and location ap (5, 10, 40 au). The fragmentation velocity is set to vfrag = 1 m s−1 (Sect. 2.1). For each case, we run simulations for 25 pairs of viscosity αvisc and local turbulence δturb. In total, 300 simulations were used to build this figure. As discussed in Sect. 3.1, a blocking efficiency… view at source ↗

**Figure 7.** Figure 7: Trap blocking efficiency of a dust trap carved by a 100 M⊕ planet at 10 au by t = 5 Myr for resistant dust grains (vfrag = 3 m s−1 ), for a range of viscosity αvisc and local turbulence δturb. We indicate an estimate of the Stokes number of fragmentation-limited pebbles (Stfrag = v 2 frag/3δturbc 2 s ) in the dust trap under the x-axis, similarly to [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗

**Figure 8.** Figure 8: Mass of planetesimals formed in the dust trap generated by a planet at 10 au by the end of our simulations for a range of viscosity αvisc, local turbulence δturb and planet mass Mp. We see that under specific conditions, dust traps can be strong enough to form planetesimals. In that case, a large fraction of the available outer disc dust mass (see [PITH_FULL_IMAGE:figures/full_fig_p010_8.png] view at source ↗

**Figure 9.** Figure 9: Pebble flux at 1 au and dust trap blocking efficiency B as a function of time for simulations with or without planetesimal formation. We focus on the case of a gap carved by a 100 M⊕ planet at 10 au, in which case only three simulations reach the requirements for planetesimal formation (δturb = 10−4 and αvisc ∈ [10−3 , 3.2 × 10−3 ], see [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗

**Figure 10.** Figure 10: Comparison between the dust trap blocking efficiency found by our full grain size distribution model based on DustPy, BDustPy, and the simpler two-population algorithm, B2pop (Birnstiel et al. 2012) here incorporated within chemcomp (Schneider & Bitsch 2021), for identical disc models with fragile grains (upper panel) and resistant grains (lower panel). We see that simpler dust evolution models can overe… view at source ↗

**Figure 11.** Figure 11: Pebble flux through the iceline (left axis) and observable cold water vapour mass (right axis) vs. time for different dust trap leakiness and parameters. The different colours indicate simulations with δturb = 10−4 and varying viscosity from αvisc = 10−3 (most blocking, dark blue) to αvisc = 10−2 (fully permeable, light blue). We present these four cases because as seen in [PITH_FULL_IMAGE:figures/full_f… view at source ↗

**Figure 12.** Figure 12: Mass flux of oxygen (stored in water ice, solid lines) and carbon (in gas-phase CH4, dashed lines) at 1 au as a function of time, for a disc with a gap carved by a 100 M⊕ planet at 10 au. As in [PITH_FULL_IMAGE:figures/full_fig_p015_12.png] view at source ↗

**Figure 13.** Figure 13: Ratio of the cumulative mass flux (Eq. 3) reaching 1 au of simulations with outer traps at 40 au relative to simulations with inner traps at 5 au both carved by a 100 M⊕ planet, for different blocking efficiency regime as in [PITH_FULL_IMAGE:figures/full_fig_p016_13.png] view at source ↗

read the original abstract

In protoplanetary discs, the presence of dust traps can significantly alter the transport of solids from the outer to the inner regions, and hence they are often invoked as an explanation for the chemical diversity of inner discs observed with JWST (e.g., varying oxygen abundances and C/O ratios). As a detailed treatment of dust transport around dust traps is computationally expensive, earlier works investigating the impact of outer traps on the inner disc composition have often used simplified dust models representing the size distribution with a single effective size and drift speed. In this paper, we revisit the impact of outer traps on dust transport using the state-of-the-art one-dimensional dust evolution code \texttt{DustPy}, which simulates the transport and evolution of dust particles including detailed coagulation and fragmentation. We quantify and map the leakiness of dust traps across a broad parameter space, performing over 300 simulations while varying the disc viscosity, turbulence strength, planet mass and location, and dust fragmentation velocity. We find that dust traps are leakier than previously thought, on a broader parameter space, such that most outer traps (r > 5 au) will result in a long-lived O-rich inner disc with gas-phase C/O < 1. In similar conditions (e.g., carved by the same planet mass), we find inner traps are much leakier than outer traps, though their relative efficiency in reducing the pebble flux is time-dependent. Highly blocking traps altering the inner disc composition dramatically (leading, e.g., to C/O > 1) are possible to set up but necessitate low viscosity and weak turbulence, along with efficient planetesimal formation by the streaming instability. In that case, we find that is the formation of planetesimals, rather than the dust traps themselves, that is capable of significantly altering the inner disc composition.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

This large DustPy grid shows outer dust traps leak more than single-size models predicted, so most keep inner discs O-rich, though 1D radial limits need checking.

read the letter

The main thing your colleague should know is that this paper concludes dust traps are leakier than previously modeled, so most outer ones will not create long-lived high C/O inner discs. They achieve this by running more than 300 simulations in DustPy that follow the complete dust size evolution with coagulation and fragmentation. The parameter space covers disc viscosity, turbulence strength, planet mass and location, and dust fragmentation velocity. This broad mapping replaces the single effective size used in earlier studies. The findings indicate that traps rarely block enough to flip the inner composition unless viscosity and turbulence are low and planetesimal formation is efficient. In those cases, the planetesimals themselves reduce the flux more than the trap. Inner traps prove leakier than outer ones for the same planet, but the pebble flux reduction changes over time. A clear soft spot is the reliance on one-dimensional radial transport. Real discs involve three-dimensional effects like meridional circulation that might change how much material leaks past the trap. The independent treatment of planetesimal formation is a fair choice for a parameter study, yet it may not capture how formation depends on local dust overdensities. Readers working on protoplanetary disc chemistry and volatile transport will find the leakiness maps useful. It helps interpret JWST observations of inner disc compositions. I recommend putting it through peer review. The scale of the study makes it worth a detailed look at the methods and assumptions.

Referee Report

2 major / 1 minor

Summary. The paper claims that by running over 300 DustPy simulations of dust evolution in discs with planet-carved traps, varying viscosity, turbulence, planet parameters, and fragmentation velocity, dust traps are found to be leakier than in prior simplified models. Consequently, most outer traps at r > 5 au permit enough pebble/ice transport to sustain O-rich inner discs with C/O < 1 over long times. Dramatic blocking leading to C/O > 1 requires low viscosity, weak turbulence, and efficient planetesimal formation, which the authors treat as an independent parameter. Inner traps are leakier than outer ones in comparable setups.

Significance. If the 1D results are representative, this work significantly revises our understanding of how outer dust traps influence inner disc chemistry, suggesting that traps alone rarely produce the high C/O ratios sometimes invoked to explain observations. The extensive parameter study using detailed coagulation physics is a clear strength, providing a quantitative map of leakiness that can guide future work. However, the translation from 1D to 3D disc physics remains an open question for the robustness of the 'most traps are leaky' conclusion.

major comments (2)

[§5 (Discussion)] The assertion that 'most outer traps (r > 5 au) will result in a long-lived O-rich inner disc with gas-phase C/O < 1' is based on the leakiness thresholds mapped in the simulation grid. This claim's applicability to real discs is load-bearing on the fidelity of the 1D radial advection-diffusion treatment. The skeptic's note correctly identifies that 3D meridional flows and azimuthal structures could alter the net flux; the manuscript should include a dedicated paragraph estimating the magnitude of this uncertainty or referencing 3D simulations that bound it.
[§3.2 (Model parameters)] Treating planetesimal formation efficiency as an independent input (varied separately from trap properties) is reasonable for exploration but may not reflect the coupled physics, as higher dust concentrations in traps promote streaming instability. This could narrow the parameter space for highly blocking traps. A brief sensitivity analysis coupling these would be appropriate.

minor comments (1)

[Abstract] The number of simulations is given as 'over 300'; a precise count and breakdown by parameter combinations in the methods section would enhance transparency.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and positive report, which highlights both the strengths of our broad parameter study and important caveats regarding the 1D framework. We address each major comment below and have prepared revisions to strengthen the manuscript.

read point-by-point responses

Referee: [§5 (Discussion)] The assertion that 'most outer traps (r > 5 au) will result in a long-lived O-rich inner disc with gas-phase C/O < 1' is based on the leakiness thresholds mapped in the simulation grid. This claim's applicability to real discs is load-bearing on the fidelity of the 1D radial advection-diffusion treatment. The skeptic's note correctly identifies that 3D meridional flows and azimuthal structures could alter the net flux; the manuscript should include a dedicated paragraph estimating the magnitude of this uncertainty or referencing 3D simulations that bound it.

Authors: We agree that the translation from 1D to 3D remains an important open question for the robustness of our conclusions. Our existing skeptic's note already flags this issue, but we will expand it into a dedicated paragraph in §5. This paragraph will discuss the potential effects of meridional flows and azimuthal structures on net pebble flux, reference relevant 3D hydrodynamical studies that quantify gap leakage (e.g., those examining meridional circulation in planet-carved gaps), and provide a qualitative estimate of the uncertainty. We will qualify our claims accordingly while preserving the quantitative 1D results. revision: yes
Referee: [§3.2 (Model parameters)] Treating planetesimal formation efficiency as an independent input (varied separately from trap properties) is reasonable for exploration but may not reflect the coupled physics, as higher dust concentrations in traps promote streaming instability. This could narrow the parameter space for highly blocking traps. A brief sensitivity analysis coupling these would be appropriate.

Authors: We acknowledge that planetesimal formation efficiency is physically coupled to local dust density via the streaming instability. Our choice to vary it independently was made to systematically map its influence across the large grid. For the revision, we will add a brief sensitivity discussion in §3.2 and §5, exploring how a density-dependent efficiency (e.g., via a simple threshold model) would affect the parameter space for strong blocking. A full dynamical coupling is beyond the scope of this 1D study, but the added text will note that such coupling would likely make highly efficient blocking even rarer. revision: partial

Circularity Check

0 steps flagged

Numerical parameter study derives leakiness from direct DustPy integrations with no reduction to self-defined inputs

full rationale

The paper executes over 300 independent DustPy simulations varying viscosity, turbulence, planet mass/location, and fragmentation velocity to map trap leakiness and resulting inner-disc C/O ratios. All reported findings (most outer traps leaky, inner traps leakier, dramatic blocking requires planetesimal formation) are direct outputs of these integrations of the advection-diffusion-coagulation equations. No step renames a fitted parameter as a prediction, imports a uniqueness theorem from the authors' prior work, or reduces the central claim to a self-citation chain. The derivation chain is therefore self-contained and non-circular.

Axiom & Free-Parameter Ledger

4 free parameters · 2 axioms · 0 invented entities

The study rests on standard dust coagulation and fragmentation physics implemented in DustPy plus the assumption that 1D radial transport captures the dominant effects.

free parameters (4)

disc viscosity
Varied across simulations as a key control on trap depth and dust diffusion.
turbulence strength
Varied independently to control diffusion and fragmentation.
planet mass and location
Varied to set trap properties.
dust fragmentation velocity
Varied to control maximum particle size and sticking efficiency.

axioms (2)

domain assumption DustPy's coagulation and fragmentation kernels accurately represent collisional outcomes in protoplanetary discs.
Invoked by using the code for all runs.
domain assumption 1D radial averaging suffices to capture net pebble flux past traps.
Implicit in the choice of 1D code.

pith-pipeline@v0.9.0 · 5646 in / 1409 out tokens · 34227 ms · 2026-05-10T15:38:46.713400+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

4 extracted references · 4 canonical work pages

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