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REVIEW 3 major objections 4 minor 24 references

Why Wide Jupiter-Mass Binary-Objects Cannot Form

T0 review · 3 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read The planet-stripping (SPP) scenario cannot explain the observed JuMBO population in the Trapezium cluster: when ionization of the wide binaries is included, at most one JuMBO is expected at any time.

desk verdict A credible critique of the SPP formation scenario for JuMBOs, but the headline O(1) estimate rests on an under-defended ionization cross-section; the qualitative conclusion likely holds. read the letter →

arxiv 2507.11786 v1 pith:ISXE5IKF submitted 2025-07-15 astro-ph.EP astro-ph.SR

Simon Portegies Zwart , Erwan Hochart (Leiden Observatory , Leiden University) This is my paper · ORCID
classification astro-ph.EPastro-ph.SR
keywords Jupiter-massbinaryobjectsJuMBOsTrapeziumclusterfree-floatingplanetsstar-planet-planetmechanismionizationyoungstellarclustersplanetarydynamics
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

The paper takes the star-planet-planet (SPP) formation scenario for Jupiter-mass binary objects (JuMBOs) and closes a gap in earlier treatments: previous scattering calculations computed how often a passing star strips two planets off their host star, but did not follow the freshly formed binary in the same cluster, where further encounters ionize it again. Treating JuMBOs as soft binaries, the authors find a steady-state population of order one binary in a Trapezium-like cluster, not the roughly forty observed. They conclude that the SPP mechanism cannot explain the observed JuMBOs, and that the discrepancy becomes even worse when the scarcity of sufficiently wide planetary orbits and disks is folded in. The paper therefore argues that if the JuMBOs are confirmed, they must be primordial or produced by ejection of a planet-moon pair, with tight orbits and unequal masses.

What carries the argument

The key machinery is the formation-ionization balance for a soft binary in a background of single stars. The formation side is the SPP production cross-section from the scattering study [2]; the destruction side is the equal-mass binary-single ionization cross-section formula of [7], which gives $\sigma_{\rm ion} \gtrsim 2\times10^7\mathrm{au}^2$ for a semi-major axis $a\gtrsim35\mathrm{au}$. Combining these with a stellar number density $n_\star\simeq5\times10^4\mathrm{pc}^{-3}$ and velocity dispersion $v_{\rm disp}\simeq2\mathrm{km\,s}^{-1}$ gives a lifetime $\tau_{\rm ion}=1/(n_\star\sigma_{\rm ion}v_{\rm disp})\sim20$ kyr, comparable to the formation timescale. The comparison is what carries the argument: when formation and destruction timescales are comparable, the steady-state population is capped at about one, regardless of the peak formation rate. The direct N-body simulations serve as an independent check of this balance.

What would settle it

A direct three-body scattering experiment could settle the claim: send a $1\ M_\odot$ star through an ensemble of equal-mass $1\ M_{\rm Jup}$ binaries with semi-major axes of 35 au and 400 au at relative velocities of $1$-$3\ \mathrm{km\,s}^{-1}$ and count the dissociated systems. If the measured ionization cross-section is near $5.5\times10^5\ \mathrm{au}^2$ rather than $2\times10^7\ \mathrm{au}^2$, the predicted steady-state JuMBO population would be about 30 instead of one, directly contradicting the central claim; conversely, high-contrast spectroscopic follow-up that confirms a substantial fraction of the 40 wide equal-mass candidates would strengthen the paper's conclusion that SPP cannot produce them.

Watch

Extended reading notes

Core claim

The central claim is that the SPP mechanism fails by an order of magnitude: after formation by close stellar encounters, a newly formed wide Jupiter-mass binary is so soft that the same cluster environment ionizes it on about the same timescale as it takes to form one, giving $\tau_{\rm ion}\sim 20$ kyr versus $\tau_{\rm form}\gtrsim 25$ kyr and hence at best $\mathcal{O}(1)$ surviving JuMBO in the Trapezium cluster at any instant. The authors identify why earlier extrapolations overestimated the population: they used formation cross-sections without the ionization term. They also show that the parameter region where SPP production peaks, with an outer planet at roughly ten percent of the encounter velocity and orbital radii of order $2\times10^4$ au, either corresponds to dynamically unstable planet pairs or would require a cluster density of about $10^9$ stars per cubic parsec, far above observed values. Realistic initial conditions give about one JuMBO per $10^4$ free-floating JMOs, and the authors' own direct N-body simulations produce no more than about one JuMBO in a Trapezium-like cluster.

Load-bearing premise

The argument assumes that the ionization cross-section of a wide (about 35 au or larger), equal-mass Jupiter-mass binary is greater than roughly $2\times10^7\ \mathrm{au}^2$; if the true value is the much smaller $5.5\times10^5\ \mathrm{au}^2$ reported in the earlier study, the destruction timescale rises to about 700 kyr, the steady-state count becomes roughly 30, and the central conclusion would be overturned.

Editorial extensions

If this is right

  • If the estimate is correct, the SPP route explains at most one of the roughly forty reported JuMBOs in the Trapezium cluster, so the observed population cannot be attributed to stellar stripping.
  • With realistic initial conditions the JuMBO-to-JMO ratio from SPP is about $10^{-4}$, roughly an order of magnitude below the observed ratio, so the mechanism cannot be rescued by normalizing to free-floating objects.
  • The peak SPP formation rate requires either unstable planetary configurations with separations of only a few mutual Hill radii or cluster densities near $10^9\ \mathrm{stars\,pc}^{-3}$, so no known cluster environment satisfies both stability and high formation efficiency.
  • Any surviving SPP JuMBOs should have small mass ratios and short dynamical lifetimes; if wide equal-mass binaries are confirmed, the observations would point toward primordial formation or planet-moon ejection rather than SPP.
  • The ionization of wide binaries may contribute to the population of free-floating Jupiter-mass objects in the cluster without adding a corresponding binary population.

Reading between the lines

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

  • Editorial inference: the same steady-state argument can be applied to other young clusters, predicting that wide JuMBOs should be essentially absent wherever the ionization timescale is shorter than about a megayear; a systematic search of nearby young clusters could test this prediction.
  • Editorial inference: the decisive uncertain input is the ionization cross-section, so a dedicated set of three-body scattering experiments measuring dissociation of equal-mass $1\ M_{\rm Jup}$ binaries against solar-mass stars at $1$-$3\ \mathrm{km\,s}^{-1}$ would settle the discrepancy with the smaller cross-section value cited in the earlier study.
  • Editorial inference: if confirmed JuMBOs are primordial, their orbital separation and mass-ratio distributions should differ systematically from SPP predictions, which link binary properties to encounter velocities and ejection dynamics.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 4 minor

Summary. The paper argues that the Star-Planet-Planet (SPP) formation scenario, in which a stellar flyby strips two planets from their host star to form a free-floating Jupiter-mass binary (JuMBO), cannot explain the ~40 JuMBOs observed in the Trapezium cluster. The authors combine an analytic steady-state estimate with new N-body simulations. They claim that after accounting for subsequent ionization of the wide, soft JuMBOs in the cluster environment, at most O(1) JuMBO should be present at any time, so the SPP model is ineffective. They further argue that the orbital configurations required for the highest SPP formation efficiency are unrealistically wide and unstable, and that the observed population is therefore either not real, primordial, or produced by a different mechanism such as planet-moon ejection.

Significance. If the central claim is correct, the paper is an important challenge to the leading dynamical explanation for the Trapezium JuMBOs, with consequences for both formation scenarios and the interpretation of the observations. The paper is concise, falsifiable, and ships its simulation scripts, and the numerical experiments usefully extend the previous isolated-encounter calculations to a clustered environment. The significance is conditional, however, because the headline 'at best O(1)' result rests on an unvalidated ionization cross-section and on small-N simulations with no quoted uncertainties.

major comments (3)
  1. [§2, paragraph beginning 'Quantifying this'] The central ionization timescale uses σ_ion ≳ 2×10^7 au², quoted from eq. 5.1 of Hut & Bahcall (1983), but that formula is derived for three equal masses. Here the two binary components have mass ~10^-3 M_sun and the perturbing star has mass ~1 M_sun, a mass ratio for which the equal-mass formula is not established. The manuscript acknowledges that Wang et al. derive 5.5×10^5 au² for this configuration but dismisses their value without a derivation or a numerical check. Because τ_ion scales inversely with σ_ion, the factor-36 difference changes τ_ion from ~20 kyr to ~700 kyr and, under the paper's own steady-state logic, changes the expected number of JuMBOs from ~1 to roughly 30. The statement 'at best O(1) JuMBO is expected' is therefore not supported until this cross-section discrepancy is resolved with scattering experiments or an appropriate analytic treatment for the correct mass ratio.
  2. [§4 and Figs. 1–2] The N-body support is presented without statistical uncertainties. Each grid point is based on only five realizations, and Figures 1 and 2 show integer counts with no error bars, scatter, or confidence intervals. With five runs, a count of '<~1 JuMBO' is indistinguishable from a small-number fluctuation. Please add error bars or a statistical comparison to Wang et al.'s rates, and state the quantitative criterion behind the phrase 'consistent with our finding' rather than leaving it qualitative.
  3. [§1 vs §2] There is an internal tension between the opening statement that Wang et al.'s cross-section calculations agree with the authors' own and the later rejection in §2 of Wang et al.'s ionization cross-section as unexplained. If the agreement concerns only the SPP formation cross-sections, that should be stated explicitly; if it also concerns the ionization cross-section, the factor-36 discrepancy between §2 and Wang et al. must be reconciled. As written, the reader cannot tell whether the authors endorse or reject Wang et al.'s value for the quantity that drives the main conclusion.
minor comments (4)
  1. [§3] The phrase 'mugh tigher' appears to be a typo for 'much tighter', and the text otherwise uses 'JMO's' and 'JuMBOs' inconsistently; please standardize the apostrophes.
  2. [Figs. 1 and 2] The figure captions do not explain the color scale or the meaning of the plotted 'Trapezium' marker and surrounding contours; please add a color bar and clarify what quantity is shown at the marker.
  3. [§4] The statement that the fractal models produce no JuMBOs is not shown in any figure or table; please report this result explicitly or provide a supporting figure.
  4. [§12] In the Source Data section, 'Astrophyiscs' is a misspelling of 'Astrophysics'.

Circularity Check

0 steps flagged · score 0.0 of 10

No load-bearing circularity: the central ionization and N-body arguments depend on external formulas and new simulations, not on the authors' prior definitions.

full rationale

The paper's central claim that the SPP scenario cannot produce the observed Trapezium JuMBO population is not forced by its inputs. The steady-state estimate in Section 2 uses the ionization timescale tau_ion = 1/(n_star * sigma_ion * v_disp) with sigma_ion taken from the external Hut & Bahcall equal-mass scattering formula (eq. 5.1 of [7]), and tau_form is obtained from the observed N_pp = 40 and cluster age of about 1 Myr. The resulting expectation of roughly one JuMBO follows by arithmetic from independently stated quantities; the ionization cross-section is not defined in terms of the JuMBO count the paper seeks to explain. The authors cite their own prior work [3] when stating that Wang et al.'s cross-section calculations agree with their own, but that agreement is also attributed to the external calculation of Yu & Lai [4], and it is used only as a concession before the paper introduces the ionization argument. No load-bearing premise is justified solely by self-citation. The new N-body simulations in Section 4 use explicitly stated initial conditions (2500-star Plummer models, 300 host systems, five mutual Hill radii, aout - ain = 100 au) and compare the resulting counts with Wang et al.; the outputs are not fitted parameters renamed as predictions. The disputed choice between sigma_ion greater than about 2 x 10^7 au^2 and 5.5 x 10^5 au^2 is a quantitative assumption about an external formula, not a circular reduction, and any error there would be a correctness risk rather than a circularity. Overall, the derivation chain is self-contained with respect to the paper's stated inputs, and the self-citations present are minor and corroborated. Therefore the circularity score is 0.

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

The quantitative conclusion (at most one JuMBO) rests on an ionization timescale built from observed cluster density and velocity dispersion, a literature ionization cross-section, and Wang et al.'s peak formation efficiency; each is adopted without in-paper derivation, and the cross-section is contested. The simulations additionally depend on chosen planetary-architecture parameters such as the 100 au separation and 5 Hill radius stability criterion.

free parameters (5)
  • f_peak = 0.04 (4%)
    Peak JuMBO formation efficiency per star from Wang et al. (fig. 6), used in Section 2 to derive the formation timescale tau_form ~ 1 Myr / 40 = 25 kyr. Adopted from the literature, not derived in this paper.
  • f_disk_500 = 0.02 (2%)
    Fraction of Trapezium YSOs with disks extending to 500 au (Vicente & Alves 2005), used in Section 2 to scale the required number of stars to about 50,000.
  • f_planet_sim = 0.12 (300/2500)
    Fraction of stars given two planets in the N-body simulations (Section 4). Hand-chosen to provide a statistically usable sample; higher than observed disk fractions.
  • Delta_a = 100 au
    Orbital separation between the two planets (a_out - a_in) used in the simulations to reproduce observed JuMBO projected distances while keeping the system at 5 mutual Hill radii.
  • Hill_sep = 5 mutual Hill radii
    Separation criterion used in the simulations to define dynamically stable planetary systems, following standard stability practice.
assumptions (5)
  • domain assumption The Trapezium cluster has stellar number density n ~ 5e4 pc^-3 and velocity dispersion v_disp ~ 2 km/s (from Jones & Walker 1988; Hillenbrand & Hartmann 1998).
    Used in Section 2 to compute the ionization timescale tau_ion = 1/(n*sigma*v). The cluster environment is assumed uniform at these values.
  • domain assumption Hut & Bahcall (1983) eq. 5.1 gives the correct ionization cross-section for a 1+1 MJup binary with a > 35 au against 1 M_sun stars.
    The formula is for equal-mass three-body scattering and is extrapolated to a very wide, low-mass, unequal-mass system. Wang et al. report a cross-section about 36 times smaller.
  • domain assumption Wang et al.'s peak 4% formation efficiency is the correct rate for the SPP mechanism in an idealized system (400 and 500 au circular coplanar orbits).
    The paper uses this to derive tau_form ~ 25 kyr for the Trapezium, despite later arguing that the required wide orbits are rare (disk fraction 2%).
  • domain assumption The observed JuMBO population (40 objects) is taken at face value for the initial estimate, though the paper later questions the identification.
    Section 3 states 'Here we assume that the identified population of 40 JuMBOs is confirmed', then the Verdict suggests the number may drop.
  • domain assumption The cluster is well represented by a virialised Plummer sphere for the simulations.
    Simulations in Section 4 use 2500 stars in a virialised Plummer sphere; fractal initial conditions are mentioned but no figure is provided.

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Cite this review

Pith. "Pith review of Why Wide Jupiter-Mass Binary-Objects Cannot Form." pith.science (2026). https://pith.science/paper/ISXE5IKF

@misc{pith2026250711786,
  author       = {Pith},
  title        = {Pith review of: Why Wide Jupiter-Mass Binary-Objects Cannot Form},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/ISXE5IKF}},
  note         = {Machine review of arXiv:2507.11786}
}
abstract

The discovery of $N_{\rm pp} = 40$ Jupiter-mass binary objects (JuMBOs) alongside $N_{\rm p} = 500$ free-floating Jupiter-mass objects (JMOs) in the Trapezium cluster's central portion raises questions about their origin \cite{2023arXiv231001231P}. \citet{2024NatAs...8..756W} argue that the rate at which two planets orbiting the same star are stripped by a close encounter can explain about half the observed JuMBOs in the Trapezium cluster. Although, their cross-section calculations agree with our own \citep{2024ScPA....3....1P}, one cannot extrapolate their results into clustered environments because it ignores the dissociation of JuMBOs due to subsequent encounters in the clustered environment. The inability of forming JuMBOs via the proposed scenario either calls for another formation mechanism, or the observed JuMBOs require thorough confirmation.

Discussion (0). Continue with ORCID to comment.

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