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Upper Limits to the Proper Motions of JuMBO 24, a Jupiter-Mass Binary Object Candidate in Orion

T0 review · 2 major / 4 minor · reviewed 2026-08-16 · deepseek-v4-flash

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Pith's one-line read Combining archival and new VLA radio detections, this paper finds that JuMBO 24—the only radio-detected Jupiter-mass binary object candidate in Orion—has no significant proper motion, with a 3-sigma upper limit of about 6 km/s on its…

desk verdict Useful but modest follow-up on JuMBO 24's radio astrometry; the ~6 km/s proper-motion limit is credible, but a frequency-dependent centroid bias and a weak statistical link to formation scenario need attention before publication. read the letter →

arxiv 2504.15519 v1 pith:JH74RJDF submitted 2025-04-22 astro-ph.EP astro-ph.SR

Luis F. Rodriguez , Laurent Loinard , Luis Zapata , Gisela N. Ortiz-Leon This is my paper · ORCID
classification astro-ph.EPastro-ph.SR
keywords Jupiter-massbinaryobjectsJuMBO24OrionNebulaClusterradioastrometrypropermotionfree-floatingplanetsphotoerosionformationVLAobservations
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 tries to establish that JuMBO 24, the only radio-detected Jupiter-mass binary object candidate in Orion, is nearly stationary in the plane of the sky. Combining four earlier VLA radio positions with two new 10 GHz detections from late 2024 and early 2025, the authors derive proper-motion components consistent with zero and translate the 3-$\sigma$ level into a transverse-velocity upper limit of about 6 km s$^{-1}$ at the adopted 388 pc distance. After subtracting the average motion of the radio stars in Orion's core, JuMBO 24 moves at the same level as those stars rather than running away from the cluster. If this result is right, it favors formation from a stationary contracting core, as in star formation, over ejection at high speed in a dynamical encounter. The radio emission is steady, unresolved on very long baselines, and lacks circular polarization, which the authors read as evidence against a non-thermal origin.

What carries the argument

The load-bearing mechanism is phase-referenced radio astrometry: VLA observations that measure a source's position relative to nearby calibrator sources, allowing centroid shifts over a decade to be compared at sub-milliarcsecond precision. The authors fit a linear position-versus-epoch model in right ascension and declination to the six data points, then subtract the known average proper motion of radio-emitting stars in the Orion core ($\mu_\alpha\cos\delta = +1.07\pm 0.09$ mas yr${}^{-1}$, $\mu_\delta = -0.84\pm 0.16$ mas yr${}^{-1}$) to place the motion in the cluster frame. The adopted distance of 388 pc converts the 3$\sigma$ proper-motion bound into the transverse-velocity limit of about 6 km s${}^{-1}$. A second piece of machinery is the Gaussian deconvolution of the 10 GHz image, whose angular size and orientation match the JWST binary, indicating comparable radio emission from both components.

What would settle it

A future astrometric epoch taken at least five years after 2025, with VLA or JWST positions tied to background extragalactic sources and per-epoch uncertainty below about 1 mas, that shows a drift larger than roughly 3 mas yr${}^{-1}$ (more than 6 km s${}^{-1}$ at 388 pc) would falsify the paper's upper limit; a drift consistent with zero would corroborate it.

Watch

Extended reading notes

Core claim

On its own terms, the paper's central claim is that JuMBO 24 shows no significant proper motion: $\mu_\alpha\cos\delta = +1.01\pm 0.94$ mas yr${}^{-1}$ and $\mu_\delta = +0.66\pm 1.03$ mas yr${}^{-1}$ from a single linear fit to the six radio epochs. At the adopted distance of 388 pc, the 3$\sigma$ bound of about 3 mas yr${}^{-1}$ corresponds to a plane-of-sky speed below $\simeq 6$ km s${}^{-1}$. Registering the measurement to the Orion cluster rest frame by subtracting the average proper motion of radio stars in the core leaves a residual consistent with zero, so the object is not moving at large velocity relative to its environment. The same data show steady flux near 50 $\mu$Jy, no circular polarization above roughly 20%, and no 5 GHz detection on baselines longer than 5,000 km above an 18 $\mu$Jy limit; the paper takes these as not favoring a non-thermal, gyro-synchrotron origin. The paper also notes that a high radial velocity is not excluded by these measurements.

Load-bearing premise

The 6 km s$^{-1}$ limit rests on the assumption that the six radio positions taken in different years and at two frequencies point to the same spot on the sky with no hidden offset from how the coordinates were tied together; if any such offset is larger than the quoted errors, the apparent drift and the limit could be wrong.

Editorial extensions

If this is right

  • If the limit holds, JuMBO 24 cannot have been ejected from a stellar encounter with a plane-of-sky speed above about 6 km s${}^{-1}$; in the cluster frame its residual motion is consistent with zero.
  • The radio source's angular size and orientation matching the JWST binary implies that both components contribute comparably to the radio emission, so higher-resolution imaging could measure each component separately.
  • The absence of 5 GHz emission on very long baselines rules out a single 50 $\mu$Jy non-thermal source, although two 25 $\mu$Jy non-thermal components remain possible.
  • The observed velocity upper limit is consistent with formation from a stationary contracting core, whether by direct collapse or by photoerosion of a prestellar core, and is in tension with the 6-9 km s${}^{-1}$ velocity dispersions expected if JuMBOs are dynamically ejected free-floating planets.

Reading between the lines

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

  • Editorial extension: a radial-velocity measurement of JuMBO 24's two components could still reveal a large three-dimensional speed; if it did, the ejection scenario would be back on the table despite the low sky-plane motion.
  • Editorial extension: comparing 6 GHz and 10 GHz positions tied to background extragalactic sources would test whether the null proper motion is contaminated by frequency-dependent centroid shifts, a check the present data cannot perform.
  • Editorial extension: if stationary formation is the right picture, JuMBO-like binaries should also be found in other massive star-forming regions with low velocity dispersions relative to their clusters; measuring proper motions of those objects would confirm the channel without relying on a single source.
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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

2 major / 4 minor

Summary. The manuscript reports new VLA 10 GHz observations of the radio counterpart of JuMBO 24, a candidate Jupiter-mass binary object in Orion, together with an HSA 5 GHz non-detection. Combining two new 10 GHz positions with four earlier 6 GHz positions from Rodriguez et al. (2024), the authors derive proper motions mu_alpha cos(delta) = +1.01 +/- 0.94 mas/yr and mu_delta = +0.66 +/- 1.03 mas/yr. Interpreting the 3-sigma level of about 3 mas/yr as a transverse-velocity upper limit of about 6 km/s at 388 pc, and after subtracting the mean proper motion of radio stars in the Orion cluster core from Dzib et al. (2017), they argue that JuMBO 24 is not moving at large velocity relative to the cluster. They further report steady radio flux, lack of circular polarization, and a marginally resolved morphology aligned with the JWST binary, and they conclude that the kinematics favor a star-like, stationary contracting-core origin over a high-velocity ejection origin.

Significance. If the measured proper-motion limit is robust, this is the first direct kinematic constraint on a JuMBO candidate and is a useful step toward distinguishing formation scenarios. The paper is concise, uses public VLA data, quotes explicit numbers, and its central quantitative claim is falsifiable. The radio detection and the multi-epoch position series are valuable in themselves. However, the discriminating power of a single 3-sigma upper limit against a predicted ejection-velocity dispersion of 6-9 km/s is modest, and the conclusion relies on an unquantified assumption about the centroid stability of a resolved binary source between frequencies. The HSA non-detection and polarization limits are useful but not decisive.

major comments (2)
  1. [Section 3.1 and Table 1] The proper-motion fit combines 6 GHz positions from 2012, 2018, and 2022 with 10 GHz positions from 2024/2025 in a single linear fit, implicitly assuming that the measured radio position is the same physical point at both frequencies. The source is resolved and believed to be a ~100 mas binary, and Section 3.2 argues that the radio emission comes from both components in comparable proportions without quantifying the flux ratio or its frequency dependence. If the component flux ratio differs between 6 and 10 GHz by 20-50%, the flux-weighted centroid shifts by roughly 5-10 mas between the two frequency groups. Over the ~7.6 yr mean time separation between the 6 and 10 GHz groups, this mimics a proper-motion bias of ~0.6-1.3 mas/yr, comparable to or larger than the quoted ~1 mas/yr uncertainties. This systematic could therefore change the 3-sigma upper limit and the inferred 6 km/s bound. I ask the authors to quantify the per-component flux ratio at each frequency from their images, to test for a frequency-dependent centroid shift, or otherwise to include an explicit systematic term in the error budget before the upper limit is presented as robust.
  2. [Section 3.1] The conclusion that the 6 km/s upper limit favors a star-like origin is statistically underpowered given the adopted ejection model. The authors compare a single 3-sigma bound of about 6 km/s with the 6-9 km/s velocity dispersion expected from van Elteren et al. (2019). Even if the ejection model were correct, a substantial fraction of ejected objects would have plane-of-the-sky velocities below 6 km/s: for a one-dimensional Gaussian with sigma = 6 km/s, about 68% of sources have |v| < 6 km/s, and for the two-dimensional speed distribution about 39% would still satisfy the bound. For sigma = 9 km/s the corresponding two-dimensional fraction is about 20%. A single non-detection of large motion therefore has weak discriminating power, and the statement that the result 'favors' a stationary contracting-core origin overstates the evidence. The authors should either soften the conclusion or provide a quantitative Bayesian or frequentist comparison of the two hypotheses.
minor comments (4)
  1. [Section 2.1] The text says observations were made on six epochs, but Table 1 lists only two mean epochs, with the last five concatenated. Please clarify in the text that the first epoch is treated separately and the remaining five are combined, so that the reader does not count six independent data points in the proper-motion fit.
  2. [Table 1, column 6] Table 1 labels the circular-polarization limits as '4-sigma upper limit' while the text in Section 3.3 refers to 'about 20%' without specifying the sigma level. Please make the quoted significance consistent between the table and the text.
  3. [Section 3.3] The sentence 'another frequent properties on non-thermal radiation' contains a grammatical error and should read 'another frequent property of non-thermal radiation'.
  4. [Figure 1 caption] The caption states that the cross marks the position of JuMBO 24 precessed to the average epoch of the radio image, but it does not give that average epoch. Adding the epoch would make the comparison with the radio position more transparent.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the proper-motion upper limit is derived from independent VLA position measurements, and the comparison frames are external observational or simulation results.

full rationale

The paper's central result, an upper limit of about 6 km/s on the plane-of-the-sky velocity of JuMBO 24, is obtained by a least-squares fit to six VLA positions (Section 3.1, Figure 2). Four of these positions come from the authors' prior paper (Rodriguez et al. 2024) and two from new 10 GHz observations; these are independent observational data points, not outputs of the present derivation. The fit itself is a standard measurement, and the conversion to a transverse velocity uses an adopted distance (Kounkel et al. 2017) with no equation that presupposes the velocity limit. The rest-frame comparison subtracts the average proper motion of Orion radio stars measured by Dzib et al. (2017); even though several authors overlap, that catalog is an external, published observational quantity rather than a result derived in this paper, and the mean is not constructed so as to force the target's residual to zero. The ejection-velocity comparison relies on an external simulation (van Elteren et al. 2019) and is used only as an interpretive benchmark. The skeptical concern about frequency-dependent centroid shifts between the 6 and 10 GHz epochs is a plausible systematic-error critique, but it is not a circularity: it challenges the accuracy of the astrometric measurement without claiming that any fitted parameter is being renamed as a prediction or that any equation reduces to its own input by construction. No self-definitional, fitted-input-called-prediction, self-citation-chain, or imported-uniqueness step was found; consequently the paper is self-contained with respect to the circularity criteria and receives a score of 0.

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

No free parameters are introduced in the sense of ad hoc constants; the proper-motion slopes are measured from data. The main external inputs are distance and cluster rest-frame motion. The weakest unstated premise is the astrometric frame stability across epochs and frequencies. No new physical entities are postulated.

assumptions (5)
  • domain assumption The radio source detected by the VLA is physically associated with JuMBO 24.
    The association rests on positional coincidence with a random-match probability of 0.00012 from Rodriguez et al. (2024); without this association the proper-motion limit would not constrain the JuMBO.
  • domain assumption VLA astrometric positions from 2012, 2018, 2022 (6 GHz) and 2024/2025 (10 GHz) share a common absolute reference frame with errors given by the quoted fit uncertainties.
    A single linear fit in Section 3.1 combines all epochs; no tie to background astrometric standards or discussion of frequency-dependent centroid offsets is provided.
  • domain assumption The distance to the Orion Nebula Cluster is 388 pc.
    Adopted from Kounkel et al. (2017) to convert the proper-motion limit of about 3 mas/yr into 6 km/s; the distance uncertainty is not propagated.
  • domain assumption The mean proper motion of radio stars in the Orion core defines the cluster rest frame.
    Section 3.1 subtracts Dzib et al. (2017) values to conclude JuMBO 24 is not moving fast relative to stars.
  • domain assumption Ejection velocity statistics for single free-floating planets from van Elteren et al. (2019) apply to free-floating binary planets.
    The paper explicitly assumes this in Section 3.1 when it compares the 6 km/s limit to the predicted 6 to 9 km/s dispersion; if binary ejection differs, the interpretation weakens.

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Cite this review

Pith. "Pith review of Upper Limits to the Proper Motions of JuMBO 24, a Jupiter-Mass Binary Object Candidate in Orion." pith.science (2026). https://pith.science/paper/JH74RJDF

@misc{pith2026250415519,
  author       = {Pith},
  title        = {Pith review of: Upper Limits to the Proper Motions of JuMBO 24, a Jupiter-Mass Binary Object Candidate in Orion},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/JH74RJDF}},
  note         = {Machine review of arXiv:2504.15519}
}
abstract

Using JWST near-infrared data of the inner Orion Nebula, \citet{Pearson_McCaughrean_2023} detected 40 binary systems they proposed to be Jupiter-Mass Binary Objects (JuMBOs) -- although their actual nature is still in debate. Only one of the objects, JuMBO\,24, was detected in the radio continuum. Here, we report on new radio continuum (10 GHz) Karl G. Jansky Very Large Array (VLA) detections of the radio counterpart to JuMBO\,24, and on an unsuccessful search for 5 GHz continuum emission with the High Sensitivity Array (HSA). From our new VLA detections and adopting a distance to the region, we set an upper limit of $\simeq 6$~km~s$^{-1}$ to the velocity of the radio source in the plane of the sky. This upper limit favors an origin for this source similar to that of stars, that is, from a stationary contracting core. The nature of the radio emission remains uncertain but the lack of strong variability (all VLA observations are consistent with a steady flux of $\sim$50 $\mu$Jy), of detection on long HSA baseline, and of detectable circular polarization in VLA data do not favor a non-thermal origin.

Figures

Figures reproduced from arXiv: 2504.15519 by the authors.

Figure 1
Figure 1. VLA image of the 10 GHz emission from JuMBO 24, made concatenating the last five epochs of project 24B-082. The contours are -3, 3, 4, 5, 6, and 8 times 4 µJy beam−1 , the rms of the image. The cross indicates the position of JuMBO 24 as reported by Pearson & McCaughrean (2023): RA(J2000) = 05h 35m19.s 50616, DEC(J2000) = −05◦ 23′ 39.′′7303. The synthesized beam of the radio map is shown at the bottom left of the im… view at source ↗
Figure 2
Figure 2. Radio position of JuMBO 24 as a function of time. The dashed lines indicate the least-squares linear fit to the data [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Can planet-planet binaries survive in star-forming regions?

    astro-ph.SR 2025-05 conditional novelty 5.0 of 10

    N-body simulations show that 50 to 90 percent of wide planet-planet binaries in dense star-forming regions are destroyed within a few million years, so many more than the observed 42 JuMBOs must have formed.

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