Pebbles versus Planetesimals: The case of Trappist-1

Adrien Leleu; Gavin A. L. Coleman; Willy Benz; Yann Alibert

REVIEW 4 major objections 5 minor 1 cited by

Pebbles versus Planetesimals: The case of Trappist-1

T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read This paper claims that both pebble and planetesimal accretion can form Trappist-1-like planetary systems, and that the only robust observable difference is the planets' water content.

desk verdict Both pebble and planetesimal accretion can make Trappist-1-like systems in this model, but the only claimed observable difference — water content — rests on an uncertain envelope-recycling assumption that the authors themselves flag. read the letter →

arxiv 1908.04166 v1 pith:QS4PWDQT submitted 2019-08-12 astro-ph.EP

Gavin A. L. Coleman , Adrien Leleu , Yann Alibert , Willy Benz This is my paper · ORCID

classification astro-ph.EP

keywords Trappist-1pebbleaccretionplanetesimalresonantchainswaterfractionlow-massstarsplanetmigrationN-bodysimulations

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper asks whether the Trappist-1 system can reveal how planets form around very low mass stars by testing the two leading solid-accretion mechanisms: accretion of planetesimals and accretion of pebbles. Using N-body simulations with a viscous, photoevaporating disc, the authors find that a wide range of initial conditions in either scenario yields planetary systems matching Trappist-1's masses, orbital periods, and resonant chains. The two scenarios are therefore nearly indistinguishable observationally. The one clear difference is water content: with efficient recycling of a planet's envelope, pebble-built planets emerge extremely dry while planetesimal-built planets remain extremely wet. If accurate water fractions for Trappist-1 can be measured, they could point to the formation route or to missing physics.

What carries the argument

The mechanism that carries the argument is the ablation–recycling prescription for pebble accretion: as water-rich pebbles plunge through a planet's atmosphere, they are heated and ablate, and the released water is assumed to be mixed into the planet's tiny envelope and recycled back into the protoplanetary disc, using a prescribed envelope-mass threshold for ablation that is lowered for water-rich pebbles. This step is what converts pebble accretion from a wet formation route into a dry one. The rest of the model—an N-body integrator, a 1D viscously heated and photoevaporating disc, type I migration, and resonant-convoy dynamics—sets the masses and architectures, but it produces nearly identical systems in both scenarios; the ablation and recycling step is the only piece that separates them.

What would settle it

Measure the bulk water mass fraction of the Trappist-1 planets, for example through transmission spectroscopy or mass-radius interior modelling: if they are water-rich at the tens-of-percent level rather than the few-percent level produced by the dry pebble scenario, the paper's predicted discriminator is falsified.

Watch

Extended reading notes

Core claim

The central claim is that planet formation around M dwarfs like Trappist-1 is degenerate with respect to accretion mode: planetesimal accretion and pebble accretion both produce compact resonant chains of Earth-to-sub-Earth-mass planets inside roughly 20-day orbits, with similar eccentricities, inclinations, mass gradients, and period ratios. The paper's principal discriminator is the final water fraction. Planets that accrete pebbles lose the water content of those pebbles because the pebbles ablate in the planet's envelope and the ablated water is assumed to be efficiently recycled into the local disc; the remaining rocky core grows dry. Planetesimals are too large to be ablated, so planetesimal-built planets keep roughly 50 percent water. If ablation is neglected or the ablated water falls to the core instead of being recycled, pebble-built planets are also wet, restoring the degeneracy.

Load-bearing premise

The load-bearing premise is that a growing planet's thin envelope is fully recycled with the surrounding protoplanetary disc, so that water ablated from pebbles is carried away rather than retained; if recycling is inefficient, pebble-built planets stay wet and the two formation routes become observationally indistinguishable.

Editorial extensions

If this is right

If the paper is right, matching Trappist-1's masses, periods, and resonant chain does not by itself favour either pebble or planetesimal accretion; both routes are viable across a wide range of disc masses and solid distributions.
A measured water-poor composition for the Trappist-1 planets would support pebble accretion with efficient envelope recycling, while a water-rich composition would support planetesimal accretion or a breakdown of the recycling assumption.
Pebble-built systems are slightly more compact and coplanar than planetesimal-built ones, so future transit surveys that find many seven-plus-planet systems around M dwarfs may weakly prefer pebble accretion.
The planetesimal scenario leaves leftover planetesimals that can form debris discs, whereas the pebble scenario leaves little such material; detection of debris discs around low-mass stars would therefore favour planetesimal accretion.
Long-term irradiation and photolysis can remove water over gigayears, so the water-fraction signature may be blurred in evolved systems like Trappist-1 itself.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

A quantitative map of how envelope recycling efficiency varies with envelope mass could turn the binary dry-versus-wet outcome into a graded water-fraction diagnostic; adding such a parameter to the pebble model and comparing with measured bulk compositions would test this.
The ablation case predicts that water fraction decreases as planet mass increases, so a water-mass anticorrelation within the Trappist-1 planets would be a direct test that the paper does not present as its headline prediction.
The simulations reproduce apparent 8:5 period ratios through an unseen planet in a 2:1 resonance, which suggests that precise period-ratio statistics across M-dwarf systems could reveal hidden planets and help distinguish dynamical histories.
Observing younger, still-forming M-dwarf planetary systems before gigayear-scale water loss would offer a cleaner test of the dry-versus-wet dichotomy than the evolved Trappist-1 planets.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Gavin, this one is worth a look. The headline result is that pebble and planetesimal accretion both produce Trappist-1-like resonant chains in their N-body simulations, and the only discernible difference they find is water content: pebble-formed planets come out dry, planetesimal-formed planets wet, provided the envelope is fully recycled. That is a genuinely new comparative result, and it goes beyond Schoonenberg et al. by including envelope recycling and showing how the water fractions differ from that earlier work.

The modeling is thoughtful and fairly comprehensive for the time: multiple disc masses, pebble fractions, planetesimal sizes, migration traps, resonant structures, and observability biases. The similarity criterion is a sensible way to compare multi-planet systems without pretending to fit Trappist-1. The authors also deserve credit for stating their caveats openly, including the ones that weaken their own headline.

The soft spots are real, but they are acknowledged. The water-content dichotomy is the only observable discriminator between the two scenarios, and it depends on the assumption of efficient envelope recycling in Sect. 4.1.1. If ablated water is retained or rains out to the core, pebble-formed planets are as wet as planetesimal-formed ones — they say exactly this in Sect. 5.2.3. The cited literature is split, so the stress-test note is right to call this a conditional prediction rather than a robust result. Other issues: initial embryo masses differ between the two scenarios, planetesimal sizes are smaller than streaming-instability predictions, and the water-ice ablation factor comes from a private communication that is not independently checkable. No code or data is released, so the numerics are not reproducible as presented.

None of this kills the central formation result. The paper convincingly shows that both accretion modes can form Trappist-1-like systems. What it does not do is break the degeneracy between those modes, except through the recycling-dependent water diagnostic. I would send this to a serious referee, with the main requests being to release the code and data, and to either run a recycling-parameter study or clearly present the water-content result as conditional on that microphysics.

I'd bring it to reading group; it is a useful, honest exploration that will likely be cited once JWST or ARIEL start measuring water fractions for these systems.

Referee Report

4 major / 5 minor

Summary. The paper uses N-body simulations coupled to a 1D viscous disk model, with prescriptions for type-I migration, photoevaporation, and two solid-accretion modes (pebble accretion and planetesimal accretion), to ask whether the Trappist-1 system can be formed in either scenario and whether the two scenarios are distinguishable. A similarity criterion based on planet masses, period ratios, eccentricities, and mass gradients is used to select the most Trappist-like synthetic systems. The authors find that both pebble and planetesimal accretion form resonant chains of low-mass planets matching Trappist-1 in mass, period, and resonance structure, and that the only significant difference is the final water fraction: planetesimal-formed planets are water-rich (~50%), while pebble-formed planets are water-poor (<5%) only when pebble ablation in the envelope is accompanied by full recycling of the envelope with the local disk. Without that recycling, or without ablation, pebble-formed planets are also water-rich. The paper is candid about this dependence, listing more accurate envelope-recycling prescriptions as future work.

Significance. If the central claim holds, the paper provides a genuinely useful result: it shows that Trappist-1-like architectures are not a unique fingerprint of either pebble or planetesimal accretion, and it identifies water content as a potentially decisive discriminator for future observations of low-mass-star planetary systems. The study is broad in parameter coverage (disk mass, pebble fraction, planetesimal size, photoevaporation rate), uses a quantitative similarity criterion rather than eye-balling, and analyzes resonant structures, three-body resonances, and observational biases with commendable detail. The paper also openly discusses limitations, including the planetesimal-size assumption, neglected sublimation, and the recycling uncertainty. The value of the water-fraction result is real but conditional on a microphysical assumption that the authors themselves flag as uncertain, which limits the strength of the paper's headline conclusion.

major comments (4)

[§4.1.1 and §5.2.3]
[§3.1 and §4.2]
[§3.1, §5.2.4, and §6.3(i)]
[§5.1 and Table 2]

minor comments (5)

[Figure 8, bottom right panel caption]
[§4.1.1]
[Table 2 caption]
[§5.2.2]
[§3.2 and §4.3 example simulations]

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: forward formation models are compared with Trappist-1 only as a target, with no parameter fitted to the target; the water-content caveat is explicit.

full rationale

The derivation chain is forward. Initial embryos, discs, migration, photoevaporation, and pebble/planetesimal accretion are prescribed from prior models (Coleman et al. 2017a; Lambrechts & Johansen 2014) and evolved with Mercury-6; observed Trappist-1 masses and periods (Grimm et al. 2018) enter only through the post-hoc similarity criterion (Eqs. 11-13), which does not feed back into the model. No parameter is fitted to reproduce Trappist-1, so there is no fitted-input-called-prediction step. The only observable discriminator, water fraction, is a conditional product of the ablation criterion (Eq. 10) and the full-envelope-recycling assumption in Sect. 4.1.1; the paper explicitly flags this dependence in Sect. 5.2.3 ('For our models, the impact of ablation is dependent on the assumption that the solid material that is ablated is efficiently recycled back into the protoplanetary disc') and lists more accurate envelope recycling as future work (Sect. 6.3(v)). This is an acknowledged microphysical sensitivity, not a definitional equivalence. Self-citations to prior model implementations are independent, earlier developments that do not contain the Trappist-1 result, so they are not load-bearing circularity.

Assumptions & free parameters 6 free parameters · 5 assumptions · 0 invented entities

The central claims rest on several parameter choices and physical assumptions. The most consequential is the full-recycling of planetary envelopes, which determines whether pebble-formed planets are dry or wet. The initial embryo masses and planetesimal sizes are pragmatic choices that could affect the comparison. No new physical entities are introduced.

free parameters (6)

Pebble growth efficiency epsilon_d = 0.05
Free parameter in the pebble production front (Eq. 1), taken from Lambrechts & Johansen (2014). Affects the pebble flux and hence planet growth.
Envelope mass fraction of planets = 0.1% of planet mass
Chosen based on preliminary tests (Sect. 4.1.1). Used to decide whether pebbles ablate; directly controls the water content difference.
Initial embryo mass (planetesimal scenario) = 0.1 Earth masses
Chosen for computational feasibility (Sect. 3.1). Affects early migration and accretion.
Initial embryo mass (pebble scenario) = 0.01 Earth masses
Set to the Bondi-Hill transition mass (Sect. 4.2). Different from planetesimal scenario, potentially biasing comparison.
Pebble fraction of solids = 50-90%
Varied across simulations; affects pebble supply and resulting planet masses (Sect. 4.2).
Planetesimal size = 100 m or 1 km
Varied; smaller than streaming instability predictions (10-150 km), requiring an unmodeled fragmentation mechanism (Sect. 5.2.4).

assumptions (5)

domain assumption Type I migration torque prescriptions of Paardekooper et al. (2010, 2011) are valid for the discs and planets modeled
Used throughout the N-body simulations to compute migration and damping.
ad hoc to paper Planetesimals in the modeled size range (100 m to 1 km) can exist in sufficient numbers; streaming instability must somehow produce or fragment into them
Initial conditions assume 2000 planetesimals of these sizes; the authors note streaming instability produces larger bodies and assume collisional stirring breaks them down (Sect. 5.2.4).
domain assumption Envelopes of sub-terrestrial planets are fully recycled with the disc
Adopted in Sect. 4.1.1 based on Ormel et al. (2015), Lambrechts & Lega (2017), Bethune & Rafikov (2019a); this assumption is the linchpin of the dry pebble-planet prediction.
domain assumption Collisions always result in perfect merging
Stated in Sect. 2; no fragmentation or hit-and-run outcomes are considered.
domain assumption Water content outside the iceline is 50% by mass
Assumed for solids outside the water iceline, based on Lodders (2003); affects computed water fractions.

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Pith. "Pith review of Pebbles versus Planetesimals: The case of Trappist-1." pith.science (2026). https://pith.science/paper/QS4PWDQT

@misc{pith2026190804166,
  author       = {Pith},
  title        = {Pith review of: Pebbles versus Planetesimals: The case of Trappist-1},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/QS4PWDQT}},
  note         = {Machine review of arXiv:1908.04166}
}

read the original abstract

We present a study on the formation of planetary systems around low mass stars similar to Trappist-1, through the accretion of either planetesimals or pebbles. The aim is to determine if the currently observed systems around low mass stars favour one scenario over the other. We ran numerous N-body simulations, coupled to a thermally evolving viscous disc model, including prescriptions for planet migration and photoevaporation. We examine the differences between the pebble and planetesimal accretion scenarios, but also look at the influences of disc mass, planetesimal size, and the percentage of solids locked up within pebbles. When comparing the resulting planetary systems to Trappist-1, we find that a wide range of initial conditions for both accretion scenarios can form planetary systems similar to Trappist-1, in terms of planet mass, periods, and resonant configurations. Typically these planets formed exterior to the water iceline and migrated in resonant convoys to close to the central star. When comparing the planetary systems formed from pebbles to those formed from planetesimals, we find a large number of similarities, including average planet masses, eccentricities, inclinations and period ratios. One major difference was that of the water content of the planets. When including the effects of ablation and full recycling of the planets envelope with the disc, planets formed from pebbles were extremely dry, whilst those formed from planetesimals were extremely wet. If the water content is not fully recycled and instead falls to the planets core, or if ablation of the water is neglected, then the planets formed from pebbles are extremely wet, similar to those formed from planetesimals. Should the water content of the Trappist-1 planets be determined accurately, this could point to a preferred formation pathway for planetary systems, or to specific physics that may be at play.

Figures

Figures reproduced from arXiv: 1908.04166 by the authors.

**Figure 1.** Temporal evolution of planet masses (top), eccentricities (middle) and periods (bottom). The noted letters indicate times of interesting events that are described in the text. lifetime, as it determines when the disc begins to quickly disperse from the inside-out. Finally, we run two versions of each set of parameters, where the planet positions and velocities are initialised using a different random number seed. 3.… view at source ↗

**Figure 2.** Evolution of planet mass versus period for an example planetesimal accretion simulation. Filled black circles represent final masses and periods for surviving planets. The red dots indicates the periods and masses of the Trappist-1 planets with their appropriate error bars (Grimm et al. 2018). mals after ∼ 0.75 Myr disrupts the fragile resonant chain, causing some of the planets to collide, further increasing their… view at source ↗

**Figure 4.** Temporal evolution of planet masses (top), eccentricities (middle) and periods (bottom). The noted letters indicate times of interesting events that are described in the text. beginning of the simulation, the pebble growth front quickly passes the small planetary embryos, allowing them to begin accreting pebbles. As the pebble front advances outwards, a number of planetary embryos reach masses where they begin to si… view at source ↗

Figures from the paper (14 more)

**Figure 5.** Figure 5: Evolution of planet mass versus period for an example simulation. Filled black circles represent final masses and periods for surviving planets. The red dots indicates the periods and masses of the Trappist-1 planets with their appropriate error bars (Grimm et al. 2018…

**Figure 6.** Figure 6: Three body resonant angles for the planets with orbital periods < 20 d in the simulation discussed in Sect. 4.3 and whose evolution is shown in [PITH_FULL_IMAGE:figures/full_fig_p010_6.png]

**Figure 7.** Figure 7: Simulated Trappist-like systems from the planetesimal accretion scenario (top 6 rows) and the pebble accretion scenario (bottom 6 rows). Periods are shown on the x-axis and planet masses are indicated by the symbol size. Resonances are also shown between neighbouring p…

**Figure 8.** Figure 8: Different components contributing to the similarity criterion (eq. 11) for systems formed by pebble accretion (red points) and planetesimal accretion (blue points). The green crosses denote the respective values for the Trappist-1 system. Top left panel: Mean planet ma…

**Figure 9.** Figure 9: The fraction of systems containing n planets within a certain orbital period for the planetesimal accretion scenario (left panel) and for the pebble accretion scenario (right panel). The number of planets is shown by the colour and the minimum planet mass counted was 0…

**Figure 10.** Figure 10: The Probability of observing N planets with periods up to 50 days in a simulated system, formed through either pebble or planetesimal accretion. We show the probabilities for both the transit method, assuming that only planets with masses, mp > 0.1 M⊕ may be observed,…

**Figure 11.** Figure 11: A mass versus period diagram showing the the surviving simulated planets from the planetesimal accretion scenario (left panel), and the pebble accretion scenario (right panel). Marker colour denotes the initial mass of the disc as a percentage of the stellar mass. Red…

**Figure 12.** Figure 12: The mass of solids remaining at the end of each simulation as a function of the initial solid mass. Simulations from the planetesimal accretion scenario are shown in the left panel whilst those from the pebble accretion scenario are shown in the right panel. All solid…

**Figure 13.** Figure 13: The mass of remaining solids locked up in planets with orbital periods less than 20 days, against all remaining solid mass. The left panel is for systems formed through planetesimal accretion, with the right panel being for systems formed through pebble accretion. The…

**Figure 14.** Figure 14: Mass versus period plots showing the the surviving simulated planets from the pebble accretion scenario. Marker colour denotes the final water fraction of the surviving planets. Red markers denote the masses and periods of the observed Trappist1 planets with error ba…

**Figure 15.** Figure 15: Mass versus period plots showing the the surviving simulated planets from the pebble accretion scenario. Marker colour denotes the percentage of solids that is comprised of pebbles. Red markers denote the masses and periods of the observed Trappist1 planets with erro…

**Figure 16.** Figure 16: Mass versus period plot showing the the surviving simulated planets from the planetesimal accretion scenario. Marker colour denotes the planetesimal size in that simulation. Green markers denote the masses and periods of the observed Trappist1 planets with error bar…

**Figure 17.** Figure 17: Cumulative distribution functions of period ratios between neighbouring planets with periods less than 20 days. Blue and red lines show the period ratios for pebble accretion simulations that include/not include the effects of ablation. The yellow and purple lines s…

**Figure 18.** Figure 18: A mass versus period plot showing planetary systems similar to Trappist-1. Observational data was taken from exoplanet.eu. tems around GJ 1132 and GJ 3323, this planet has a period of 232 days, making it completely incompatible with the simulated planetary systems f…

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