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REVIEW 4 major objections 4 minor 38 references

Three fast-rotating Jovian Trojans identified by TESS set new population density limits

T0 review · 4 major / 4 minor · reviewed 2026-08-09 · deepseek-v4-flash

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Pith's one-line read TESS light curves show the Jovian Trojan (13383) spins once every 2.926 hours, implying a bulk density near 1.6 g/cm3 and pushing the population's accepted density limit upward.

desk verdict Two confirmed fast Trojans are a solid result; the (13383) period that drives the new density limit is plausible but not independently confirmed, and the paper is honest about that. read the letter →

arxiv 2501.18440 v1 pith:ANGNFVQE submitted 2025-01-30 astro-ph.EP

Csaba Kiss , Nóra Takács , Csilla E. Kalup , Róbert Szakáts , László Molnár , Emese Plachy , Krisztián Sárneczky , Róbert Szabó
show 3 more authors
Gyula M. Szabó Attila Bódi András Pál
This is my paper · ORCID
classification astro-ph.EP
keywords JovianTrojansasteroidrotationperiodsTESSlightcurvesrubble-piledensitylimitcohesioncollisionsDrucker-Pragercriterionsmallsolarsystembodies
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 reports the fastest reliably classified Jovian Trojans found so far, using TESS light curves. One of them, (13383), rotates every 2.926 hours, a period short enough that a strengthless rubble-pile model requires a bulk density near 1.6 g/cm3, well above the ≤1 g/cm3 limit usually quoted for this population. The other two, (38615) and (228155), rotate in 4.26 and 4.75 hours, just under the previously accepted ~5-hour breakup limit. If these periods hold, the population-level density limit for Jovian Trojans must be raised, and bodies like (13383) either are denser than typical Trojans or need a few kilopascals of internal cohesion. The authors connect the fast spin and relatively high albedo of (13383) to a possible recent energetic collision.

What carries the argument

The load-bearing tool is the standard amplitude–period relation for strengthless rubble piles, which converts a measured rotation period and light-curve amplitude into a lower bound on bulk density, together with the Drucker–Prager failure criterion that estimates the cohesion needed if the density is lower. The observed double-peaked light curves fix the rotation periods; the amplitudes then set the axial ratio and hence the density/cohesion limits.

What would settle it

A resolved shape model of (13383) from stellar occultation, radar, or spacecraft imaging that shows its projected axis ratio is closer to 1 than the light-curve amplitude implies, or a spin axis far from equatorial, would invalidate the inferred 1.6 g/cm3 density.

Watch

Extended reading notes

Core claim

The paper establishes that three Jovian Trojans—(13383), (38615), and (228155)—rotate with periods of 2.926, 4.259, and 4.749 hours, making (13383) the fastest reliably classified Trojan and the first fast rotator found in the ~20 km size range of this population. For the two slower targets the periods sit just below the previously accepted ~5 h breakup limit; for (13383) the period is so short that a strengthless rubble-pile model requires a bulk density of about 1.6 g/cm3, above the ≤1 g/cm3 limit assumed for Jovian Trojans. If the body is instead as porous and low-density as other Trojans, then it must have internal cohesion of a few kilopascals, more than typical lunar regolith. The authors also note that (13383)'s relatively high albedo and fast spin could both stem from an energetic collision that spun up the body and exposed brighter material.

Load-bearing premise

The paper relies on the assumption that each asteroid is a loose pile of rocks with no internal strength, seen from near its equator, so that the measured brightness variation directly reflects the body's shape and sets the density.

Editorial extensions

If this is right

  • The accepted upper density limit for Jovian Trojans (~0.9–1 g/cm3) must be revised upward to at least ~1.6 g/cm3 if (13383)'s rotation period and rubble-pile assumption hold.
  • Fast rotation at the ~20 km scale implies that some Jovian Trojans are not the low-density, highly porous aggregates commonly assumed; they are either denser or possess cohesion of a few kilopascals.
  • The fastest reliably classified Trojan is no longer (187463) with its 4.84 h period, and fast rotation is confirmed at larger sizes than previously documented.
  • The light-curve amplitudes and periods rule out contact-binary structure for these three targets, since that would require implausibly high densities of about 5–13 g/cm3.
  • If the collision hypothesis is correct, fast spin and higher albedo may be correlated markers of recent energetic impacts among Jovian Trojans.

Reading between the lines

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

  • A testable extension the paper does not pursue: apply the same TESS-plus-ZTF period-finding pipeline to Hildas and Neptune Trojans to see whether their population density limits also rise when fast rotators are searched for with equal sensitivity.
  • If the few-kPa cohesion required for a low-density (13383) is real, it gives a directly usable lower bound on the tensile strength of primitive icy rubble-pile material, relevant to collision and disruption models beyond Trojan asteroids.
  • The suggested fast-spin/high-albedo correlation, if confirmed over a larger sample, would offer a remote-sensing marker of recent energetic collisions in the outer solar system, independent of colour surveys.
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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

4 major / 4 minor

Summary. The paper analyzes TESS light curves of three Jovian Trojans and reports rotation periods of 2.926 h for (13383), 4.259 h for (38615), and 4.749 h for (228155), the latter two being moderately below the previously accepted ~5 h Trojan breakup limit. Using the Pravec and Harris (2000) strengthless-rubble-pile relation and the Drucker-Prager criterion (Holsapple 2007), the authors derive density estimates of ~1.6, ~0.7, and ~0.8 g cm^-3 and argue that (13383) exceeds the generally accepted Jovian Trojan density limit, requiring cohesion of a few kPa if its density is lower. ZTF data are used as an independent check; the periods of (38615) and (228155) are confirmed, but the period of (13383) is not unambiguously recovered. The paper also discusses albedo and possible collisional resurfacing of (13383).

Significance. If the reported periods are correct, the paper provides valuable new constraints on the rotational and physical properties of Jovian Trojans, and the density inference for (13383) would be the first strong evidence that at least some Trojans in the ~20 km size range have densities above the commonly adopted ~1 g cm^-3 limit. The work uses established observational methods and published models rather than introducing free parameters, and the confirmation of two periods by ZTF is an important strength. The main significance, however, rests on the least-secure measurement, the 2.926 h period of (13383), so the population-level conclusion should be regarded as provisional until the period ambiguity is resolved.

major comments (4)
  1. [Abstract; Sect. 3; Fig. 1] The headline result for (13383) is not yet established at the confidence implied. The independent ZTF check fails for this object ('could not be unambiguously recovered', Sect. 3), and the only internal test against the quadruple-peaked alternative P4 = 5.852 h yields p≤0.25, which is not a rejection at conventional significance. Because the Pravec-Harris density scales as P^{-2}, a period of 5.852 h would lower the density estimate from ~1.6 to ~0.4 g cm^-3 and remove the claimed density limit. Please provide a quantitative model comparison (e.g., a bootstrap of the asymmetry statistic or an AIC/BIC comparison of the two periods) and, ideally, independent time-resolved photometry before the population-level conclusion is drawn.
  2. [Sect. 4, Holsapple paragraph] The zero-cohesion density values are listed in an order that contradicts the rest of the paper: '(0.65, 0.8 and 1.6 g cm−3 for (13383), (38615) and (228155), respectively)' conflicts with the earlier critical densities of ~0.7, 0.8, and 1.5 g cm−3 assigned to (38615), (228155), and (13383), and with Fig. 3. This must be corrected, since it directly concerns the quantitative conclusions.
  3. [Abstract; Sect. 4, Pravec-Harris and Drucker-Prager models] The abstract presents ρ≈1.6 g cm−3 as a density estimate, but it is a model-dependent quantity obtained under specific assumptions: strengthless rubble-pile structure, equator-on viewing, b/c = 1, and adopted friction angles. Relaxing any of these changes the derived value; the authors themselves note that a larger b/a or non-equatorial aspect would alter the cohesion bound. The wording should be qualified to 'density estimate under the stated assumptions' or 'model-dependent density limit'.
  4. [Sect. 3; Fig. 1] For (38615) and (228155), the period assignment from single prominent periodogram peaks relies on 'slight asymmetries' in the two halves of the folded light curve, with no quantitative significance reported. The ZTF confirmation mitigates this concern, and the ambiguity is in the direction of even shorter periods rather than longer ones, but a brief quantitative test or a statement of the adopted criterion would strengthen the reliability of both periods.
minor comments (4)
  1. [Sect. 2 and Sect. 3] The asteroid is referred to as (288155) in the text but as (228155) in Table 1 and the abstract; please use a consistent identifier.
  2. [Table 1] The column header 'pV (km)' is incorrect: pV is unitless; the diameter column should be labeled 'D (km)' and the albedo column 'pV'.
  3. [Sect. 4, friction angle sentence] The sentence 'We assume an angle of friction ϕ = 45◦ that corresponds to a slope parameter of s = 0.356 (Holsapple 2007), as well ϕ = 45◦ (s = 0.315) used in Polishook et al. (2016)' appears to contain a typo; the second value should presumably be ϕ = 40◦, consistent with Fig. 3.
  4. [References] The entries Pirani et al. 2019a and 2019b list identical journal, volume, and article number; please verify that two distinct works are intended and correct the citations if not.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the reported density limits follow from measured TESS periods and amplitudes through published external models (Pravec & Harris 2000; Holsapple 2004/2007), with no fitted parameter or load-bearing self-citation doing the claimed work.

full rationale

The paper's derivation chain is: TESS light curves are reduced (Pál et al. 2020 pipeline), rotation periods and amplitudes are measured directly from those light curves, and bulk-density or cohesion limits are then computed using the published Pravec & Harris (2000) strengthless-rubble-pile relation and the Holsapple (2004/2007) Drucker-Prager failure criterion. No parameter is fitted to the target data to produce the headline density; the period and amplitude are observables, and the density relations are external, stated assumptions (equator-on viewing, b/c ratio from amplitude, chosen friction angles). The albedo and size for the two fainter targets use a population-average value from Grav et al. (2012), but the density estimate from period and amplitude does not depend on those sizes, and the Holsapple cohesion curves use only the amplitude-derived shape and assumed friction angle. The ZTF non-recovery for (13383) and the weak p<=0.25 rejection of the quadruple-peaked alternative are data-quality and model-ambiguity caveats, not circularity: they weaken the observational support but do not reduce the conclusion to its own inputs. The self-citations (Pál et al. 2020 data reduction, Pál et al. 2016 statistical test, Kalup et al. 2021 on YORP versus collisions) are methodological or contextual, not the load-bearing justification for the new density limit. Therefore no circular step is present and the derivation is self-contained relative to the stated external models.

Assumptions & free parameters 3 free parameters · 6 assumptions · 0 invented entities

The central density claim uses no fitted constants beyond the measured periods and amplitudes. The free parameters listed are literature values or shape assumptions that influence the cohesion curves, not the zero-cohesion density lower bound. The main burden is therefore the external validity of the rubble-pile model and the reliability of the period identifications.

free parameters (3)
  • Geometric albedo pV for (38615) and (228155) = 0.07 (population average, no uncertainty quoted)
    Adopted from Grav et al. (2012) to convert H_V to diameter; the size enters the Drucker-Prager pressure calculation for the cohesion estimates.
  • Friction angle phi = 40 and 45 degrees
    Chosen from Holsapple (2007) and Polishook et al. (2016) as representative of lunar regolith; these values set the slope parameter s and thus the cohesion-density curves.
  • Shape axis ratio b/c = 1 (assumed)
    Set equal to unity following Holsapple (2007); makes the asteroid a triaxial ellipsoid with a > b = c, changing the stress state used for the rupture limit.
assumptions (6)
  • domain assumption The three targets are stable Jovian Trojans.
    Population assignment is based on Greenstreet et al. (2024); the paper shows no orbital elements or membership analysis of its own (Section 4).
  • domain assumption The Lomb-Scargle periods are the true rotation periods.
    Section 3: for (38615) and (228155) only the double-frequency/half-period signal is significant and the double-peaked period is accepted from 'slight asymmetries'; for (13383) the alternate quadruple-peaked period is rejected at p<=0.25 and ZTF does not recover the period.
  • domain assumption Pravec-Harris strengthless rubble-pile relation applies.
    Section 4: the critical density is computed from the rotation period and amplitude using Pravec and Harris (2000), requiring a cohesionless, gravity-bound body.
  • domain assumption Equator-on viewing and b=c ellipsoid shape.
    Section 4: 'assuming that we see the system equator-on' and that b and c axes are equal; this maps the amplitude to the a/b ratio and the authors state it yields an upper limit on cohesion.
  • domain assumption Drucker-Prager criterion with regolith friction angles applies.
    Section 4, following Holsapple (2004, 2007); the friction-angle values come from lunar regolith and may not represent JT material.
  • domain assumption TESS and ZTF photometric reductions are accurate.
    Section 2: data reduction follows the TSSYS pipeline and FITSH; the paper does not provide independent validation of the photometry.

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Cite this review

Pith. "Pith review of Three fast-rotating Jovian Trojans identified by TESS set new population density limits." pith.science (2026). https://pith.science/paper/ANGNFVQE

@misc{pith2026250118440,
  author       = {Pith},
  title        = {Pith review of: Three fast-rotating Jovian Trojans identified by TESS set new population density limits},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/ANGNFVQE}},
  note         = {Machine review of arXiv:2501.18440}
}
abstract

Here we report on the identification of the three fastest rotating Jovian Trojans with reliable population assignment, using light curve data from the Transiting Exoplanet Satellite Survey mission, also confirmed by Zwicky Transient Facility data. For two of our targets the rotation periods are moderately below the previously accepted ~5 h Jovian Trojan breakup limit (4.26 and 4.75 h), however, the rotation period of (13383) was found to be P = 2.926 h, leading to a density estimate of $\rho$ $\approx$ 1.6 $g cm^{-3}$, higher than the generally accepted $\leq$ 1 $g cm^{-3}$ density limit of Jovian Trojans. If associated with lower densities, this rotation rate requires considerable cohesion in the order of a few kPa. The relatively high albedo (pV $\approx$ 0.11) and fast rotation suggest that (13383) may have undergone an energetic collision that spun up the body and exposed bright material to the surface.

Figures

Figures reproduced from arXiv: 2501.18440 by the authors.

Figure 1
Figure 1. Top: TESS light curves folded with the rotation periods. Grey points are individual observations. We combined the data into 36 phase bins and show the binned rotation curve with large red points. Middle: power spectra. For (13383) both the rotation frequency and its double are present. For (38615) and (228155), only the double-frequency (half-period) signal is significant, due to their symmetric rotation curves. Bot… view at source ↗
Figure 2
Figure 2. Distribution of basic rotational properties of the inner solar sys￾tem asteroid populations. Top panel: rotational frequency versus abso￾lute magnitude. The dashed horizontal line marks the breakup limit of main belt asteroids, corresponding to a rotational period of 2.2 h (Pravec & Harris 2000). Bottom panel: light curve amplitude versus rotational frequency. Dashed curves correspond to constant critical densities,… view at source ↗
Figure 3
Figure 3. Cohesion versus density curves for our three targets obtained using the Drucker-Prager criterion of failure. Red and blue curves cor￾respond to (38615) and (228155), respectively. Dashed and solid curves correspond to friction angles of ϕ = 40 and 45◦ , respectively. The shaded areas represent the cohesion curves of (13383) allowed by the uncertain￾ties in absolute magnitude and albedo determination (striped: ϕ = 40… view at source ↗

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