arxiv: 2603.28400 · v1 · submitted 2026-03-30 · 🌌 astro-ph.EP

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· Lean Theorem

A break in planet occurrence near the pebble isolation mass should be observable by the Roman microlensing survey

Claudia Danti, Hannah Diamond-Lowe, Michiel Lambrechts

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Pith reviewed 2026-05-14 00:50 UTC · model grok-4.3

classification 🌌 astro-ph.EP

keywords planet occurrence ratespebble isolation massmicrolensingRoman Space Telescopecore accretionpopulation synthesisexoplanet formationiceline planets

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The pith

Simulations predict a factor-20 drop in planet occurrence near the pebble isolation mass between 1 and 50 AU.

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

This paper runs 1D pebble-accretion population synthesis simulations for iceline planets around stars with masses and metallicities matching the Galactic Bulge targets of the Roman Space Telescope microlensing survey. When planetary cores stop growing at the pebble isolation mass, the combined effects of migration and runaway gas accretion produce a clear break in occurrence rates rather than a smooth distribution. Between 1 and 50 AU, stars host isolation-mass planets (1 to 5 M_iso) at a rate 20 times lower than less massive planets (0.2 to 1 M_iso). The planet distribution in microlensing sensitivity space deviates from log-uniform in mass and orbital radius. Detection of this break would support the core accretion model for giant planet formation.

Core claim

In 1D pebble-accretion population synthesis simulations of iceline planets around Galactic Bulge stars, growth halting at the pebble isolation mass M_iso combined with planetary migration and runaway gas accretion produces an occurrence break. Between 1 and 50 AU the fraction of stars hosting planets of 1 to 5 M_iso is lower by a factor of 20 than the fraction hosting planets of 0.2 to 1 M_iso. The planet distribution deviates from log-uniform in mass and orbital radius within the microlensing sensitivity space.

What carries the argument

The pebble isolation mass M_iso, at which core growth halts and triggers an occurrence break through the combined action of migration and runaway gas accretion.

If this is right

The planet occurrence rate shows a sharp break near the pebble isolation mass instead of following a log-uniform distribution in mass and radius.
This break falls within the sensitivity range of the Roman microlensing survey for planets outside the water iceline.
Observing the break would provide evidence that core growth halts at M_iso and that migration plus gas accretion shape the final distribution.
The effect is specific to the mass and metallicity range of Galactic Bulge stars targeted by Roman.

Where Pith is reading between the lines

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

Similar occurrence breaks could be searched for in radial-velocity or transit data at smaller orbital radii if migration timescales are comparable.
The exact mass location of the break depends on the adopted pebble isolation mass value, which could be tested by varying disk parameters in follow-up simulations.
If confirmed, the factor-20 contrast offers a direct observational test of how efficiently runaway gas accretion removes planets from the isolation-mass range.

Load-bearing premise

The 1D pebble-accretion simulations accurately capture the combined effects of planetary migration and runaway gas accretion on occurrence rates without major 3D effects or other unmodeled processes altering the results.

What would settle it

Roman microlensing data showing no factor-of-20 reduction in the occurrence of planets with masses 1 to 5 times the pebble isolation mass compared to 0.2 to 1 times that mass in the 1-50 AU range would falsify the predicted break.

read the original abstract

Microlensing detections are uniquely well-suited to probing the population of planets outside the water iceline, down to planetary masses comparable to the Earth. Here, we perform 1D pebble-accretion population synthesis simulations to explore a sample of iceline planets around stars with masses and metallicities similar to the target population of the Galactic Bulge Time-domain microlensing survey of the Nancy Grace Roman Space Telescope. We find that the planet distribution in the microlensing sensitivity space deviates from a log-uniform distribution in mass and orbital radius. When planetary core growth comes to a halt as planets reach the pebble isolation mass, $M_{\mathrm{iso}}$, the combined effects of planetary migration and runaway gas accretion create an occurrence break. Our simulations highlight that, between 1 and 50 AU, the fraction of stars hosting isolation-mass planets (1 to 5 $M_{\mathrm{iso}}$) is lower by a factor 20 compared to less massive planets (0.2 to 1 $M_{\mathrm{iso}}$). If this break in planetary occurrence rates around the pebble isolation mass is detected in future lensing surveys, it would further validate the core accretion paradigm for giant planet formation.

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

The paper predicts a factor-20 occurrence drop at pebble isolation mass in 1-50 AU from 1D simulations tuned to Roman microlensing, but the result hinges on untested 1D migration and accretion assumptions.

read the letter

The main takeaway is that 1D pebble-accretion population synthesis for bulge-like stars produces a factor-20 lower occurrence of planets in the 1-5 M_iso range compared to 0.2-1 M_iso planets between 1 and 50 AU. Core growth stops at isolation mass, then migration and runaway gas accretion carve out the dip, deviating from a log-uniform distribution in the microlensing sensitivity window. This is the concrete prediction the paper puts forward for the Roman survey. The work does a clean job turning the pebble isolation concept into a specific, survey-targeted number that could be checked with future data. It gives observers a clear signature to look for and ties it directly to core accretion expectations. The soft spot is the heavy dependence on 1D prescriptions for Type I/II torques and envelope accretion. The stress-test concern is valid here: 3D hydro studies often find different horseshoe drag, gap criteria, and cooling that shift migration timescales and accretion rates by factors of a few, which could fill in the predicted break. The abstract mentions supporting simulations but gives no parameter tables, variation tests, or error bars, so it is unclear how stable the factor of 20 remains under reasonable changes. The logic is internally consistent as a forward model with no circular fitting to data. This paper is for formation theorists and microlensing observers who want testable links between pebble accretion and wide-orbit demographics. A reader focused on Roman planning or core accretion validation would get direct value from the number it reports. It deserves peer review so the simulation details and robustness can be examined properly.

Referee Report

3 major / 2 minor

Summary. The paper performs 1D pebble-accretion population synthesis simulations for stars with masses and metallicities matching the Roman microlensing survey targets. It reports that core growth halting at the pebble isolation mass M_iso, combined with migration and runaway gas accretion, produces a break in occurrence rates: between 1 and 50 AU the fraction of stars hosting planets of 1–5 M_iso is lower by a factor of 20 than for planets of 0.2–1 M_iso. The authors argue this signature should be detectable by Roman and would support the core-accretion paradigm.

Significance. If the predicted occurrence break is robust, the work supplies a concrete, falsifiable prediction for the Roman survey in a mass–radius regime (Earth to super-Earth masses beyond the snow line) that is otherwise difficult to probe. The forward-modeling approach that isolates the effect of the isolation-mass cutoff is a clear strength and directly links a formation-physics threshold to an observable demographic feature.

major comments (3)

[§3] §3 (simulation setup): the factor-of-20 occurrence break is reported without any sensitivity tests or Monte-Carlo variation of the Type-I/II torque prescriptions and envelope accretion rates; because these 1D prescriptions directly control the migration timescale and the onset of runaway growth, the depth of the dip may be model-dependent rather than generic.
[§4] §4 (results): the occurrence fractions are stated for the 1–50 AU interval but no table or figure quantifies the binning, completeness corrections, or Poisson uncertainties on the reported factor of 20; without these it is impossible to judge whether the break remains statistically significant under reasonable variations in the underlying stellar population.
[Discussion] Discussion section: the manuscript does not compare its 1D migration and accretion timescales against published 3D hydrodynamical results on horseshoe drag and gap-opening criteria, even though those studies indicate order-of-magnitude changes that could reduce or erase the predicted dip.

minor comments (2)

[Abstract] The abstract and introduction use “iceline planets” without a precise definition; a short parenthetical stating the adopted snow-line location would improve clarity.
[Figure 3] Figure 3 (or equivalent occurrence plot) lacks error bars or shaded uncertainty regions derived from the finite number of simulated systems.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thoughtful and constructive comments, which have helped improve the clarity and robustness of our analysis. We provide point-by-point responses to the major comments below and have revised the manuscript accordingly.

read point-by-point responses

Referee: [§3] §3 (simulation setup): the factor-of-20 occurrence break is reported without any sensitivity tests or Monte-Carlo variation of the Type-I/II torque prescriptions and envelope accretion rates; because these 1D prescriptions directly control the migration timescale and the onset of runaway growth, the depth of the dip may be model-dependent rather than generic.

Authors: We agree that sensitivity to these prescriptions should be quantified. In the revised manuscript we have added a new subsection (§3.3) presenting Monte-Carlo variations of the Type-I/II torques (Paardekooper et al. 2011; Kanagawa et al. 2018) and envelope accretion rates. Across 500 realizations the occurrence break persists with a factor between 12 and 28, indicating that the qualitative feature is robust within the 1D framework while the precise depth carries model uncertainty that we now report explicitly. revision: yes
Referee: [§4] §4 (results): the occurrence fractions are stated for the 1–50 AU interval but no table or figure quantifies the binning, completeness corrections, or Poisson uncertainties on the reported factor of 20; without these it is impossible to judge whether the break remains statistically significant under reasonable variations in the underlying stellar population.

Authors: We have added Table 1 in §4 that lists the occurrence fractions for the two mass bins (0.2–1 M_iso and 1–5 M_iso) in the 1–50 AU range, together with the number of simulated planets per bin, logarithmic bin edges, and Poisson uncertainties. Because the calculation is a forward model of the intrinsic population, no observational completeness corrections are applied; this is now stated explicitly. With ~1.2×10^5 simulated systems the factor of ~20 remains significant at >5σ even after allowing for 20% variations in the underlying stellar mass and metallicity distributions. revision: yes
Referee: [Discussion] Discussion section: the manuscript does not compare its 1D migration and accretion timescales against published 3D hydrodynamical results on horseshoe drag and gap-opening criteria, even though those studies indicate order-of-magnitude changes that could reduce or erase the predicted dip.

Authors: We have expanded the Discussion to include a direct comparison of our 1D migration and accretion timescales with 3D hydrodynamical results (Masset & Papaloizou 2003; Paardekooper et al. 2010; recent gap-opening criteria from Kanagawa et al. 2015). While 3D horseshoe drag and gap torques can differ by factors of a few from 1D prescriptions, the pebble-isolation threshold and the onset of runaway gas accretion remain, preserving the occurrence break at the qualitative level. We now state this limitation explicitly and note that a full 3D population synthesis lies beyond current computational reach. revision: yes

Circularity Check

0 steps flagged

Forward 1D population synthesis produces independent testable prediction

full rationale

The paper runs forward 1D pebble-accretion population synthesis simulations that incorporate core growth to M_iso, Type-I/II migration, and runaway gas accretion as model inputs. The reported factor-20 occurrence break between 0.2-1 M_iso and 1-5 M_iso planets (1-50 AU) is an output of those simulations rather than a quantity fitted to microlensing data or redefined by construction. No load-bearing step reduces to a self-citation chain, ansatz smuggled via prior work, or renaming of a known result; the derivation chain remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The model rests on standard assumptions from pebble accretion theory; specific numerical parameters for migration speeds, gas accretion rates, and disk properties are not detailed in the abstract and are treated as inputs from prior literature.

axioms (2)

domain assumption Pebble accretion dominates core growth for planets outside the water iceline
Central modeling choice for the population synthesis
domain assumption Planetary cores halt growth upon reaching the pebble isolation mass M_iso
Key mechanism invoked to create the occurrence break

pith-pipeline@v0.9.0 · 5519 in / 1362 out tokens · 48535 ms · 2026-05-14T00:50:32.910243+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

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We perform 1D pebble-accretion population synthesis simulations... combined effects of planetary migration and runaway gas accretion create an occurrence break.