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arxiv: 2412.07603 · v1 · submitted 2024-12-10 · 🌌 astro-ph.EP

Two Distinct Populations of Dark Comets Delineated by Orbits and Sizes

Pith reviewed 2026-05-23 07:37 UTC · model grok-4.3

classification 🌌 astro-ph.EP
keywords dark cometsnongravitational accelerationsnear-Earth objectsvolatile outgassingcomet activity continuumsolar system volatilesinactive small bodies
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The pith

Dark comets divide into two populations: larger eccentric ones as comet continuum end members and smaller circular ones as a potential new class.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper identifies nongravitational accelerations on seven additional near-Earth objects previously thought inactive, doubling the known dark comets, with no dust detected in any images. This collection shows a clear split into two groups based on size and orbit. The larger objects on eccentric orbits fit as the least active members of the known comet population, while the smaller ones on near-circular orbits point to an unrecognized group. If accurate, these objects represent stages in the evolution of a hidden volatile-rich population that may have supplied water and organics to Earth.

Core claim

This set of dark comets reveals the delineation between two distinct populations: larger, 'outer' dark comets on eccentric orbits that are end members of a continuum in activity level of comets, and smaller, 'inner' dark comets on near-circular orbits that could signify a new population. These objects may trace various stages in the life cycle of a previously undetected, but potentially numerous, volatile-rich population that may have provided essential material to the Earth.

What carries the argument

The division of dark comets into outer and inner populations based on their sizes and orbital eccentricities, which links the former to the comet activity spectrum and the latter to a distinct source.

Load-bearing premise

The observed accelerations result from asymmetric outgassing of volatiles rather than radiation pressure or other nongravitational forces, and the absence of dust confirms the objects lack comae.

What would settle it

High-resolution imaging that detects a coma or dust around any of these objects, or precise orbit determinations showing accelerations match radiation pressure models instead of outgassing.

Figures

Figures reproduced from arXiv: 2412.07603 by Adina D. Feinstein, Aster G. Taylor, Darryl Z. Seligman, Davide Farnocchia, Henry H. Hsieh, Joseph Masiero, Karen J. Meech, Marco Micheli, Olivier R. Hainaut, Steven R. Chesley.

Figure 1
Figure 1. Figure 1: The orbits of the fourteen currently known dark comets, Jupiter, and the Earth. One orbital period is shown for each object starting on the vernal equinox in 2023 March, and the points represent the corresponding initial positions of the Earth and of Jupiter. The left and right panels highlight the orbits of the outer and inner dark comets respectively. Orbits of the newly discovered dark comets are labele… view at source ↗
Figure 2
Figure 2. Figure 2: Location of all dark comets and near-Earth comets in semimajor axis and eccentricity space. The region where the Tisserand parameter with respect to Jupiter (TJ , Equation 1) satisfies 2 < TJ < 3 is the classical definition of a JFC. The inner dark comets are smaller (larger H mag) than the outer dark comets and unlike the outer dark comets cannot be sourced from the JFC population. comets — smaller object… view at source ↗
Figure 3
Figure 3. Figure 3: Like other small bodies, dark comets show significant nongravitational ac￾celerations. The nuclear brightness is represented by H mag for inactive objects and the M2 comet nuclear magnitude parameter is shown for active objects. Only nongravitational accelerations with > 3 − σ significance are shown. Data for comets and asteroids are from the JPL Small Body Database. Seligman et al. PNAS | December 11, 202… view at source ↗
Figure 4
Figure 4. Figure 4: Images of two dark comets from archival DECam data and our VLT data show no obvious evidence of faint coma either in the images (left) or in the surface brightness profiles compared to field stars (right). The DECam images of 2001 ME1 and 1998 FR11 are from 2022-May-12 08:22 and 2016-Jan-15 05:55 when the objects were at heliocentric distances of ∼ 3.5 and ∼ 4.1 au. The VLT image of 2001 ME1 is a stack of … view at source ↗
Figure 5
Figure 5. Figure 5: ), (ii) measured the median flux per square arcsecond in each wedge, (iii) subtracted the median flux per square arcsecond of all wedges, (iv) normalized the remaining net flux to the brightness of the central stacked source within a 4 ′′ photometry aperture, and (v) plotted these normalized net fluxes per square arcsecond as functions of azimuth angle for (a) - Targeted objects (c) -v Targeted objects (b)… view at source ↗
Figure 6
Figure 6. Figure 6: The net surface brightness fluxes per square arcsecond normalized to the brightness of the central source as functions of azimuthal angle (in degrees counter￾clockwise from the upward vertical direction). The upper and lower panels correspond to the antisolar vector aligned (panels (a) and (b) in [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: The mass-loss rate and timescale for the dark comets (Equations 2 and 3) imply that they were either (i) not always losing mass at this rate or (ii) only recently emplaced on these orbits. The inferred H2O production rate shown on the right y-axis of the lower plot is calculated assuming that all mass-loss is due to H2O sublimation. Hayabusa2# should readily detect this level of activity on 1998 KY26. rati… view at source ↗
Figure 8
Figure 8. Figure 8: Future observability of all of the dark comets between 2024-2034. There will be narrow windows when meaningful observations can be made with large telescopes to search for activity in the dark comets [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: The signed residuals of the astrometric fits for 2012 UR158 for sky position RA (left) and Dec (right). The points indicate the residuals for fits with only gravity (red circles) or with nongravitational accelerations (blue circles). The three sets of panels show different dates, corresponding to apparitions in 2012, 2016, and 2022 respectively. be considered only as order-of-magnitude estimates of the upp… view at source ↗
Figure 10
Figure 10. Figure 10: Comparison of normalized surface brightness profiles of seeing-convolved model nuclei with varying levels of coma ranging from η = 0.00 to η = 0.05 in η = 0.05 intervals (solid, dashed, and dotted pink lines) to that of 2001 ME1 (solid purple line). amounts of additional dust coma to model point sources, (ii) convolving these models with a stellar PSF template created from the same VLT data as our target … view at source ↗
Figure 11
Figure 11. Figure 11: The percentage chance that the object came from the JFC region based on the NEOMOD model (69). The percentage chance is shown in the background color, where warmer colors indicate higher probabilities. The locations of the dark comets are indicated, as in [PITH_FULL_IMAGE:figures/full_fig_p011_11.png] view at source ↗
read the original abstract

Small bodies are capable of delivering essential prerequisites for the development of life, such as volatiles and organics, to the terrestrial planets. For example, empirical evidence suggests that water was delivered to the Earth by hydrated planetesimals from distant regions of the Solar System. Recently, several morphologically inactive near-Earth objects (NEOs) were reported to experience significant nongravitational accelerations inconsistent with radiation-based effects, and possibly explained by volatile-driven outgassing. However, these "dark comets" display no evidence of comae in archival images, which are the defining feature of cometary activity. Here we report detections of nongravitational accelerations on seven additional objects previously classified as inactive (doubling the population) that could also be explainable by asymmetric mass loss. A detailed search of archival survey and targeted data rendered no detection of dust activity in any of these objects in individual or stacked images. We calculate dust production limits of $\sim10$, $0.1$, and $0.1$ kg s$^{-1}$ for 1998 FR$_{11}$, 2001 ME$_{1}$, and 2003 RM with these data, indicating little or no dust surrounding the objects during the observations. This set of dark comets reveals the delineation between two distinct populations: larger, "outer" dark comets on eccentric orbits that are end members of a continuum in activity level of comets, and smaller, "inner" dark comets on near-circular orbits that could signify a new population. These objects may trace various stages in the life cycle of a previously undetected, but potentially numerous, volatile-rich population that may have provided essential material to the Earth.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The paper reports detections of nongravitational accelerations on seven additional near-Earth objects previously classified as inactive, doubling the known sample of 'dark comets.' Archival imaging yields no coma detections and dust production upper limits of ~10, 0.1, and 0.1 kg s^{-1} for three objects. The authors delineate two populations: larger 'outer' dark comets on eccentric orbits interpreted as end-members of a cometary activity continuum, and smaller 'inner' dark comets on near-circular orbits proposed as a distinct new population potentially linked to volatile delivery to Earth.

Significance. If the attribution of the observed accelerations to asymmetric outgassing is robust, the work would double the sample of dark comets and provide evidence for two distinct dynamical classes, with implications for the inventory of volatile-rich small bodies and their role in terrestrial planet volatile delivery. The dust limits and new detections constitute concrete observational contributions.

major comments (2)
  1. [Abstract] Abstract and main text: the central claim that the nongravitational accelerations are 'inconsistent with radiation-based effects' and therefore attributable to volatile outgassing is load-bearing for both the population split and the 'new population' interpretation, yet the manuscript provides no quantitative modeling (e.g., Yarkovsky drift rates as a function of size, spin period, thermal inertia, and obliquity) that excludes radiation-pressure or Yarkovsky contributions across the reported size range, particularly for the smaller inner population.
  2. [Abstract] The delineation into 'outer' and 'inner' populations rests on the assumption that the seven new objects (plus prior ones) exhibit accelerations produced by asymmetric mass loss rather than other mechanisms; without explicit parameter-space exclusion of alternatives for the full sample, the continuum claim for the eccentric group and the novelty claim for the circular group remain interpretive.
minor comments (1)
  1. The dust production limits are reported for only three of the seven new objects; clarifying whether the remaining four have comparable limits or different observational constraints would strengthen the inactivity claim.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed and constructive report. The two major comments both concern the robustness of attributing the observed nongravitational accelerations to outgassing rather than radiation-based effects and the resulting population interpretations. We address each point below and indicate where revisions will be made.

read point-by-point responses
  1. Referee: [Abstract] Abstract and main text: the central claim that the nongravitational accelerations are 'inconsistent with radiation-based effects' and therefore attributable to volatile outgassing is load-bearing for both the population split and the 'new population' interpretation, yet the manuscript provides no quantitative modeling (e.g., Yarkovsky drift rates as a function of size, spin period, thermal inertia, and obliquity) that excludes radiation-pressure or Yarkovsky contributions across the reported size range, particularly for the smaller inner population.

    Authors: We agree that the manuscript does not contain a dedicated quantitative parameter-space exploration of Yarkovsky or radiation-pressure accelerations. The original text relies on order-of-magnitude comparisons to published Yarkovsky models for objects of comparable size and on the fact that the measured accelerations exceed those typically reported for inactive NEOs of similar diameter. To address the referee's concern directly, we will add a new appendix that computes expected Yarkovsky drift rates over a grid of diameters (0.1–2 km), spin periods (2–20 h), thermal inertias (10–1000 J m^{-2} K^{-1} s^{-1/2}), and obliquities (0–180°), using the standard formulation of Vokrouhlický et al. These calculations will show that the observed accelerations lie outside the plausible Yarkovsky range for the reported sizes and orbital distances, particularly for the inner population. The revised manuscript will therefore include this explicit exclusion. revision: yes

  2. Referee: [Abstract] The delineation into 'outer' and 'inner' populations rests on the assumption that the seven new objects (plus prior ones) exhibit accelerations produced by asymmetric mass loss rather than other mechanisms; without explicit parameter-space exclusion of alternatives for the full sample, the continuum claim for the eccentric group and the novelty claim for the circular group remain interpretive.

    Authors: The population division is observationally driven by the clear separation in semimajor axis, eccentricity, and diameter, with the acceleration measurements serving as supporting evidence rather than the sole basis. We acknowledge, however, that a comprehensive exclusion of every conceivable non-outgassing mechanism across the entire sample was not performed. The revised text will add a concise discussion section that (i) reiterates why radiation pressure is ruled out by the magnitude of the accelerations and (ii) notes that other mechanisms (e.g., electrostatic levitation or binary interactions) lack supporting observational signatures in the existing data. We will also qualify the language around the “new population” interpretation to emphasize that it is the most parsimonious explanation given current evidence, while explicitly calling for future detailed dynamical modeling. These changes will be made without altering the core observational results. revision: partial

Circularity Check

0 steps flagged

No circularity; purely observational classification from external data.

full rationale

The paper reports detections of nongravitational accelerations on seven NEOs using standard orbital fits to astrometric data, combined with archival imaging limits on dust production. The two-population delineation follows directly from measured sizes, eccentricities, and semimajor axes without any fitted parameters, self-referential predictions, or load-bearing self-citations that reduce the central claim to its own inputs. No equations or derivations are presented that loop back by construction; the work is an empirical cataloguing exercise against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 1 invented entities

The central claim depends on standard orbital dynamics and the interpretation that accelerations indicate outgassing; the new population is postulated from the data without external confirmation.

axioms (1)
  • standard math Standard models of nongravitational accelerations in solar system dynamics apply to these objects
    Invoked to attribute accelerations to outgassing.
invented entities (1)
  • Inner dark comets as a distinct new population no independent evidence
    purpose: To classify the smaller near-circular objects as potentially novel
    Introduced to organize the observed split; no independent falsifiable prediction outside this study is given.

pith-pipeline@v0.9.0 · 5882 in / 1108 out tokens · 23312 ms · 2026-05-23T07:37:48.045022+00:00 · methodology

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