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Disks around young free-floating planetary-mass objects: Ultradeep Spitzer imaging of IC348

T0 review · 2 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read Six of 13 free-floating planetary-mass objects in IC348 host disks.

desk verdict A careful, small-sample stacking analysis gives a 6/13 disk fraction for planetary-mass objects in IC348; the result is plausible and useful, though the paper should address how representative those 13 objects are of the full M9+ census. read the letter →

arxiv 2501.15930 v1 pith:YADRW5BJ submitted 2025-01-27 astro-ph.SR astro-ph.EPastro-ph.GA

Holly Hanbee Seo , Aleks Scholz (SUPA , University of St Andrews) This is my paper · ORCID
classification astro-ph.SRastro-ph.EPastro-ph.GA
keywords free-floatingplanetary-massobjectsprotoplanetarydisksdiskfractionIC348browndwarfsinfraredexcessSpitzerIRAClifetime
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 uses stacked Spitzer infrared images of the young cluster IC348 to measure fluxes at 3.6 and 4.5 microns for 13 free-floating planetary-mass objects with spectral type M9 or later. By comparing their dereddened infrared colors and full spectral energy distributions with photospheric templates, the authors identify six objects with circumstellar disks, a disk fraction of 6/13 or 46% with a 1-sigma range of 34-59%. Combining this measurement with disk fractions in other young regions, the paper argues that planetary-mass objects keep their disks for at least 3-4 Myr, comparable to more massive brown dwarfs and low-mass stars. If correct, this implies that objects with masses comparable to giant planets form and evolve along the same disk-bearing path as stars.

What carries the argument

The method that carries the argument is the stacking of 38 archival Spitzer/IRAC time-series images per band, reaching a combined exposure of 456 seconds per filter and a depth about two magnitudes better than a single epoch. Disk identification uses the dereddened K-[4.5] colour compared with photospheric colour relations for late M and early L dwarfs, followed by spectral energy distribution fitting with BT-Settl model atmospheres. An object counts as having a disk only when the excess appears in at least two bands, with longer-wavelength detections at 5.8 and 8.0 microns serving as confirmation where available.

What would settle it

A complete infrared census of all 23 M9-or-later members of IC348, for example with JWST, that measures a disk fraction outside the 34-59% range would show the 13-object subsample was biased.

Watch

Extended reading notes

Core claim

The central claim is that free-floating planetary-mass objects in IC348, at an age of roughly 3-5 Myr, host disks at a rate of about 46%, statistically indistinguishable from the disk fractions of more massive brown dwarfs and low-mass stars in the same cluster. Six of the 13 objects with valid IRAC measurements show infrared excess in the K-[4.5] colour and in multi-band SED fits; three more are ambiguous and treated as non-detections. The paper concludes that disk fractions for planetary-mass objects stay in the 40-50% range for at least 3-4 Myr before dropping, so the disk lifetime at planetary masses is not strongly mass-dependent.

Load-bearing premise

The 13 planetary-mass objects with valid measurements are assumed to be representative of all 23 M9-or-later members of IC348; if the ten objects without clean measurements preferentially have or lack disks, the 46% fraction would not be the population value.

Editorial extensions

If this is right

  • The six disk-bearing planetary-mass objects in IC348 become specific targets for higher-resolution infrared and submillimetre follow-up aimed at measuring disk masses and structures at the lowest mass end.
  • The comparison across five star-forming regions implies that disk dissipation timescales are roughly constant from low-mass stars down to a few Jupiter masses.
  • A disk fraction near 50% in a 3-5 Myr cluster supports the picture in which many free-floating planetary-mass objects form like stars, rather than as ejected planets stripped of their disks.
  • Long-lived disks around planetary-mass objects raise the possibility that bodies resembling moons or sub-planets could assemble around objects that are themselves comparable to giant planets.

Reading between the lines

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

  • A natural test the paper does not run is to search for accretion signatures in the six disk hosts; if those disks accrete at measurable rates, they are actively evolving rather than passive remnant rings.
  • The representativeness of the 13 measured objects for the full 23-member census could be checked with a complete JWST survey; if the uncovered members differ systematically in extinction or binarity, the 46% value could be off.
  • If the disk fraction is truly flat across two orders of magnitude in mass, the clock for disk dissipation may be set more by the cluster environment than by the central object, a hypothesis the current data cannot yet distinguish.
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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 / 5 minor

Summary. The paper stacks 38 epochs of Spitzer/IRAC 1 and 2 time-series imaging of IC348 to produce deep 3.6 and 4.5 micron images, measures photometry for a sample of 13 planetary-mass candidate members (spectral type M9 or later) selected from the Luhman et al. (2016) census, dereddens the measurements using Wang & Chen (2019), and identifies infrared excess relative to BT-Settl photospheric templates. Six of the 13 objects are classified as disk-bearing, yielding a disk fraction of 6/13 = 46% with a binomial 1-sigma interval of 34-59%. The paper then compares this fraction with literature values for IC348 and other star-forming regions and concludes that free-floating planetary-mass objects retain disks for several million years, at rates comparable to brown dwarfs and low-mass stars.

Significance. If the measurement is taken at face value, it is a useful new data point in a sparsely sampled regime: disk fractions around objects below or near the deuterium-burning limit. The photometry is carefully described, the dereddening is explicitly documented, the SED fitting uses external templates, and the authors state that individual images were inspected for neighbors and edge effects. The paper also benefits from being a direct follow-up to Scholz et al. (2023), providing a consistency check across two clusters. The main caveats are the small sample size, the fact that one disk classification (LRL5231) is marginal, and the lack of a demonstrated representative relation between the 13 measured objects and the full 23-object planetary-mass census. Because the central claim is a population-level disk fraction and an age-evolution comparison, the selection issue is not cosmetic.

major comments (2)
  1. [Sections 2.2 and 3.4] The population-level disk fraction is computed as 6/13 from the 13 M9+ objects with both IRAC1 and IRAC2 measurements, but this is a subsample of the 23 M9+ members in the Luhman et al. (2016) census. The paper does not test whether the 13 are representative of the full planetary-mass population. Excluded objects include four that are only covered in IRAC1 and sources outside the stacked footprints or near edges. The quoted 34-59% interval is binomial sampling within the 13 and does not include this selection uncertainty. The sensitivity is large: if the 10 missing objects are all disk-free, the population fraction would be 6/23 = 26%; if all have disks, it would be 16/23 = 70%. Please compare the included and excluded subsamples in K magnitude, A_J, and spatial position, and either restrict the population-level claim to the 13-object sample or propagate the selection uncertainty into the quoted disk fraction.
  2. [Section 3.3, LRL5231] The classification of LRL5231 as disk-bearing rests on marginal excess in IRAC1 and substantial excess in IRAC2 relative to a 2000 K template, with photometric errors of 0.22-0.23 mag, and it is the one object for which the authors' classification disagrees with Luhman et al. (2016). Removing this object changes the central result from 6/13 to 5/13, i.e., from 46% to 38%. Please quantify the significance of the excess for this object (for example, the measured color relative to the template color plus its uncertainty) and state explicitly how the disk fraction changes if LRL5231 is excluded.
minor comments (5)
  1. [Abstract and Section 3.4] The abstract reports the disk fraction as both '46% (34-59%)' and '46±13 12%' in different places; please unify the notation for the uncertainties.
  2. [Table 1] The table lists two entries with the same coordinates J034449.33+320949.4, one with spectral type M9 and no LRL number and one with LRL40023 and M9.5; please verify whether these are indeed two distinct sources or whether one coordinate is a typo.
  3. [Figure 3] The x-axis label appears as 'log( ( m))' in the provided version; it should read as wavelength in microns, e.g., log(lambda/micron).
  4. [Throughout] There are several typographical errors, including 'Adadditional' in Section 2.3, 'An trend' in Section 3.4, and the header 'MNRAS000, 1–7 (20252024)' with mismatched years; these should be corrected.
  5. [Section 3.2] The statement that the disk identification outcome does not depend on the specific choice of model temperature is plausible but not demonstrated; please state the range of effective temperatures tested and the resulting spread in the number of disk classifications.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the 46% disk fraction is derived from new Spitzer photometry and external photospheric templates, not from a fitted parameter or self-citation.

full rationale

The claimed derivation chain is: stack archival Spitzer IRAC images (Section 2.1); select M9+ members from Luhman et al. (2016) (Section 2.2); measure [3.6] and [4.5] fluxes (Section 2.3); compare dereddened K-[4.5] colors and SEDs to external photospheric templates (Sanghi et al. 2023; BT-Settl-AGSS models) (Sections 3.1-3.2); classify excess sources individually (Section 3.3); and count 6/13 = 46% (Section 3.4). None of these steps defines the disk fraction in terms of itself. The photospheric colors are taken from published polynomial relations (Sanghi et al. 2023) and model spectra (Allard et al. 2007), not fitted to the IC348 disk sample; the template scaling to J/H photometry is standard normalization and does not encode the IRAC excess that determines disk status. The binomial interval (34-59%) is a posterior derived from the observed count, not a fitted parameter. The paper does cite the authors' prior NGC1333 study (Scholz et al. 2023) as a methodological follow-up and as a comparison data point in Figure 5, but the IC348 measurement itself is new, independent photometry, and the Figure 5 trend also uses external regions (Luhman et al. 2008; Scholz & Jayawardhana 2008; Luhman & Mamajek 2012). The main caveat - that the 13 objects with two-band IRAC coverage are a subset of the 23 M9+ members and representativeness is not tested - is a statistical completeness issue, not a circular derivation. Therefore no circular step can be exhibited.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

The measurement rests on external photospheric templates, model atmospheres, extinction laws, and the assumption that the 13 observed M9+ objects represent the census. No new physical entities are introduced.

free parameters (1)
  • Effective temperature of photospheric template per object = 1900-2200 K
    Chosen by eye to match the dereddened J- and H-band fluxes in Section 3.2, but disk classification is stated to be robust to this choice.
assumptions (5)
  • domain assumption BT-Settl-AGSS model atmospheres represent the photospheres of young M9-L3 objects in IC348
    Used in Section 3.2 to define the photospheric SED; if the models are off, the inferred excesses shift.
  • domain assumption The Sanghi et al. (2023) WISE color relations, mapped to IRAC2, predict the photospheric K-[4.5] color for late M and early L objects
    Validated with Upper Scorpius data in Section 3.1; if wrong, the initial excess selection is biased.
  • domain assumption Spectral type M9 corresponds to mass around or below the deuterium burning limit at IC348 age
    Section 2.2; underpins the planetary-mass object label.
  • domain assumption The covered 13-object sample represents the 23-object PMO census
    Section 3.4; selection is by field coverage and edge proximity, assumed uncorrelated with disks.
  • domain assumption Wang and Chen (2019) extinction law applies to IC348 lines of sight
    Used in Sections 2.3 and 3.2 for dereddening.

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Cite this review

Pith. "Pith review of Disks around young free-floating planetary-mass objects: Ultradeep Spitzer imaging of IC348." pith.science (2026). https://pith.science/paper/YADRW5BJ

@misc{pith2026250115930,
  author       = {Pith},
  title        = {Pith review of: Disks around young free-floating planetary-mass objects: Ultradeep Spitzer imaging of IC348},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/YADRW5BJ}},
  note         = {Machine review of arXiv:2501.15930}
}
read the original abstract

Protoplanetary disks have been found around free-floating objects with masses comparable to those of giant planets. The frequency and properties of these disks around planetary-mass objects are still debated. Here we present ultradeep mid-infrared images for the young cluster IC348, obtained through stacking of time series images from Spitzer. We measure fluxes at 3.6 and 4.5 microns for known free-floating planetary-mass objects (FFPMOs, spectral type M9 or later) in this cluster. By comparing the observed infrared spectral energy distributions with photospheric templates, we identify six planetary-mass objects with disks, plus three which may or may not have a disk. This corresponds to a disk fraction of 46% (34-59%). The disk fraction among planetary-mass objects is comparable to more massive brown dwarfs. We show the disk fraction among free-floating planetary-mass objects as a function of age, demonstrating that these objects retain disks for several million years, similar to low-mass stars and brown dwarfs.

Figures

Figures reproduced from arXiv: 2501.15930 by the authors.

Figure 1
Figure 1. Spatial distribution of brown dwarfs in IC348, defined as spectral type M6 or later. In red and orange, we show the approximate coverage of the fields in IRAC1 and IRAC2 for the stacked image used in this paper. We mark the planetary-mass objects with spectral type M9 or later. The median photometric error for our sample is 0.07 mag. For faint objects, the photometric error can be significantly larger; for the plane… view at source ↗
Figure 2
Figure 2. 𝐾 − 𝐼𝑅𝐴𝐶 infrared colours vs spectral type for all objects with valid measurements. In this paper we primarily focus on the objects with spectral types M9 or later, on the right side of the red dash-dotted line. For 𝐾 − 𝐼𝑅𝐴𝐶2 the separation between objects with/without disks is apparent; for this panel we also overplot the photospheric colours. For more details, see text. MNRAS 000, 1–7 (20252024) [PITH_FULL_IMAGE:… view at source ↗
Figure 3
Figure 3. Spectral energy distributions for planetary-mass brown dwarfs with potential excess emission due to disks. The black, connected datapoints are based on dereddened photometry, with the datapoints at 3.6 and 4.5 𝜇𝑚 from this current study. The yellow shaded region indicates the error; here we adopt a conservative error of 5% for the JHK photometry. The spectrum plotted in blue comes from models used as templates. When… view at source ↗
Figures from the paper (1 more)
Figure 5
Figure 5. Figure 5: Disk fraction among free-floating planetary-mass objects as a function of age. Included are datapoints for NGC1333 (1 Myr Scholz et al. 2023), Chamaeleon-I (2 Myr Luhman et al. 2008), IC348 (3 Myr, this paper), 𝜎 Orionis (4 Myr Scholz & Jayawardhana 2008) and Upper Sco…

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