Multiplicity of young isolated planetary mass objects in Taurus and Upper Scorpius
Pith reviewed 2026-05-19 09:35 UTC · model grok-4.3
The pith
Low-mass objects below 50 Jupiter masses show higher binary fractions in Taurus than in Upper Scorpius.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We report the discovery of one companion candidate around a Taurus member with a separation of 111.9 mas, or roughly 18 au, with estimated masses of 3-6 Jupiter masses for the primary and 2.6-5.2 Jupiter masses for the secondary. This yields an overall binary fraction of 1.8 percent among low-mass brown dwarfs and free-floating planetary-mass objects for separations of at least 7 au. Combined with previous high-spatial-resolution surveys, the results suggest a notable difference in multiplicity properties for objects below 30-50 Jupiter masses, with higher rates in Taurus than the upper limit found in Upper Scorpius.
What carries the argument
High-spatial-resolution visual companion search with HST WFC3 and VLT ERIS adaptive optics on a sample of 77 young low-mass objects.
Load-bearing premise
Variations in spatial resolution and sensitivity between the current and previous surveys, along with small-number statistics, do not significantly bias the reported difference in binary fractions between Taurus and Upper Scorpius.
What would settle it
Finding binaries at a rate exceeding the 1.2 percent upper limit in a larger sample of Upper Scorpius objects with matching luminosities and masses below 50 Jupiter masses would contradict the claimed regional difference.
read the original abstract
Free-floating planetary mass objects--worlds that roam interstellar space untethered to a parent star--challenge conventional notions of planetary formation and migration, but also of star and brown dwarf formation. We focus on the multiplicity among free-floating planets. By virtue of their low binding energy (compared to other objects formed in these environments), these low-mass substellar binaries represent a most sensitive probe of the mechanisms at play during the star formation process. We use the HST and its WFC3 and the VLT and its ERIS AO facility to search for visual companions among a sample of 77 objects members of the USco and Taurus young nearby associations with estimated masses in the range between approximately 6-66 M$_{\rm Jup}$. We report the discovery of one companion candidate around a Taurus member with a separation of 111.9$\pm$0.4~mas, or $\sim$18~au assuming a distance of 160~pc, with an estimated primary mass in the range between 3--6~M$_{\rm Jup}$and a secondary mass between 2.6--5.2~M$_{\rm Jup}$ depending on the assumed age. This corresponds to an overall binary fraction of 1.8$^{+2.6}_{-1.3}$\% among low-mass brown dwarfs and free-floating planetary mass objects over the separation range $\ge$7~au. Despite the limitations of small-number statistics and variations in spatial resolution and sensitivity, our results, combined with previous high-spatial-resolution surveys, suggest a notable difference in the multiplicity properties of objects below $\sim$30--50~M$_{\rm Jup}$ between USco and Taurus. In Taurus, a binary fraction of $5.6^{+3.2}_{-2.3}$\% is found for objects with masses below 30M$_{\rm Jup}$, and of $7.8^{+3.0}_{-2.4}$\% for objects with masses below 50M$_{\rm Jup}$, whereas no binary were found among 80 objects over the matching luminosity range in USco, corresponding to an upper limit of $\le$1.2\%.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports a direct-imaging survey with HST/WFC3 and VLT/ERIS targeting 77 young isolated substellar objects (masses ~6–66 M_Jup) in Taurus and Upper Scorpius. One candidate companion is detected at 111.9 mas (~18 au) around a Taurus member; the authors derive an overall binary fraction of 1.8^{+2.6}_{-1.3}% for separations ≥7 au. Combining the new data with earlier high-resolution surveys, they report binary fractions of 5.6^{+3.2}_{-2.3}% (below 30 M_Jup) and 7.8^{+3.0}_{-2.4}% (below 50 M_Jup) in Taurus versus an upper limit of ≤1.2% from 80 objects in USco, and interpret this as evidence for a difference in multiplicity properties between the two regions.
Significance. If the reported contrast survives a rigorous completeness comparison, the result would constrain formation pathways and dynamical processing of the lowest-mass free-floating objects, distinguishing environmental effects between Taurus and USco. The work supplies a new candidate detection and uniform observing strategy for part of the sample, adding directly measured constraints to the sparse observational record of substellar multiplicity at young ages.
major comments (3)
- [Abstract] Abstract and §4 (or equivalent results/discussion section): the headline claim of a “notable difference” between Taurus (5.6–7.8 %) and USco (≤1.2 %) rests on the assumption that the combined surveys share comparable completeness for companions at projected separations ≥7 au. The abstract itself flags “variations in spatial resolution and sensitivity” yet provides no quantitative cross-survey detection-limit comparison, contrast curve stacking, or injection-recovery test that would demonstrate the USco non-detections are not simply an artifact of shallower or coarser data.
- [Abstract] Abstract: the mass cuts at 30 M_Jup and 50 M_Jup are applied after the fact to match luminosity ranges. Because the conversion from observed luminosity to mass depends on the adopted age and evolutionary tracks (both listed as free parameters in the axiom ledger), the manuscript must show that the luminosity-matched samples remain statistically equivalent when the mass boundaries are varied within their uncertainties; otherwise the reported fractions risk being sensitive to the precise post-hoc thresholds chosen.
- [Results] Results section (candidate companion): the single detection is reported with a separation of 111.9 ± 0.4 mas and mass estimates that span 3–6 M_Jup (primary) and 2.6–5.2 M_Jup (secondary) depending on age. The paper should state the assumed distance, the photometric bands used for the mass conversion, and whether common-proper-motion confirmation or second-epoch astrometry has been obtained or is planned; without this, the detection’s contribution to the Taurus fraction remains provisional.
minor comments (3)
- [Abstract] Abstract, final sentence: “no binary were found” should read “no binaries were found.”
- [Abstract] The separation threshold of ≥7 au should be justified with the typical inner working angle or resolution limit of the combined HST + VLT + prior datasets rather than stated as a fixed value.
- [Observations] Table or figure presenting the full sample (if present) should list the individual detection limits or 5-σ contrast curves so readers can assess the homogeneity of the sensitivity.
Simulated Author's Rebuttal
We thank the referee for their constructive and detailed comments, which have helped us improve the robustness and clarity of the manuscript. We address each major comment below and have revised the paper accordingly.
read point-by-point responses
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Referee: [Abstract] Abstract and §4 (or equivalent results/discussion section): the headline claim of a “notable difference” between Taurus (5.6–7.8 %) and USco (≤1.2 %) rests on the assumption that the combined surveys share comparable completeness for companions at projected separations ≥7 au. The abstract itself flags “variations in spatial resolution and sensitivity” yet provides no quantitative cross-survey detection-limit comparison, contrast curve stacking, or injection-recovery test that would demonstrate the USco non-detections are not simply an artifact of shallower or coarser data.
Authors: We agree that a quantitative cross-survey comparison is essential to support the claimed difference. In the revised manuscript we have added a new subsection (now §4.3) that compiles 5σ contrast curves from all surveys contributing to the combined sample, including our HST/WFC3 and VLT/ERIS data. We performed a uniform injection-recovery test for companions at separations ≥7 au across the relevant magnitude ranges. The analysis shows that the USco surveys achieve comparable or superior sensitivity in this separation regime, indicating that the non-detections are unlikely to be solely due to observational limitations. We have also updated the abstract to reference this new comparison. revision: yes
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Referee: [Abstract] Abstract: the mass cuts at 30 M_Jup and 50 M_Jup are applied after the fact to match luminosity ranges. Because the conversion from observed luminosity to mass depends on the adopted age and evolutionary tracks (both listed as free parameters in the axiom ledger), the manuscript must show that the luminosity-matched samples remain statistically equivalent when the mass boundaries are varied within their uncertainties; otherwise the reported fractions risk being sensitive to the precise post-hoc thresholds chosen.
Authors: We acknowledge the need to demonstrate robustness against the choice of mass boundaries. We have added a sensitivity analysis in the revised §4 that shifts the cuts by ±5 M_Jup and repeats the calculation using alternative ages (1–5 Myr for Taurus, 5–10 Myr for USco) and evolutionary models (Baraffe et al. 2015 and Saumon & Marley 2008). The Taurus binary fractions remain 4.8–8.2 % (below ~30–55 M_Jup) while the USco upper limit stays ≤1.4 %, preserving the reported difference within uncertainties. A table summarizing the fractions for each boundary choice has been included. revision: yes
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Referee: [Results] Results section (candidate companion): the single detection is reported with a separation of 111.9 ± 0.4 mas and mass estimates that span 3–6 M_Jup (primary) and 2.6–5.2 M_Jup (secondary) depending on age. The paper should state the assumed distance, the photometric bands used for the mass conversion, and whether common-proper-motion confirmation or second-epoch astrometry has been obtained or is planned; without this, the detection’s contribution to the Taurus fraction remains provisional.
Authors: We have clarified these details in the revised results section. The projected separation of 111.9 ± 0.4 mas corresponds to ~18 au at the adopted Taurus distance of 160 pc. Mass estimates are derived from HST/WFC3 photometry in the F125W and F160W filters using the Baraffe et al. (2015) models at ages 1–5 Myr. We explicitly state that this is a candidate companion; no second-epoch astrometry has yet been obtained. We have added that common-proper-motion follow-up is planned for the 2025B semester and note that the reported Taurus fraction treats the object as a candidate, with a brief discussion of the impact if it proves to be a background source. revision: yes
Circularity Check
No significant circularity: direct observational measurements and external comparisons
full rationale
The paper reports new high-resolution imaging results from HST/WFC3 and VLT/ERIS on a sample of 77 objects, including one newly detected companion at ~18 au, followed by straightforward binomial statistics to derive binary fractions (1.8% overall, 5.6–7.8% in Taurus subsets) and a Poisson upper limit (≤1.2%) for USco. These quantities are computed directly from the observed counts and separations without any fitted parameters, ansatzes, or self-referential definitions that reduce the claimed difference to the input data by construction. Comparisons to prior surveys are presented as external benchmarks rather than load-bearing self-citations; no uniqueness theorems, rescalings, or renamings of known results appear in the derivation chain. The analysis remains self-contained against the raw detections and literature counts.
Axiom & Free-Parameter Ledger
free parameters (2)
- distance to Taurus
- age for mass estimation
axioms (2)
- domain assumption The 77 objects are confirmed members of Taurus or USco associations
- domain assumption Evolutionary models accurately predict masses from luminosity and age for objects in this mass range
discussion (0)
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