Close-in Exoplanets as Candidates of Strange Quark Matter Objects

Abudushataer Kuerban; Hang Gong; Hong-shi Zong; Jin-Jun Geng; Yong-Feng Huang

REVIEW 4 major objections 5 minor 116 references

Close-in Exoplanets as Candidates of Strange Quark Matter Objects

T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Close-in exoplanets with orbital periods under 6,100 seconds are most plausibly strange quark matter planets, and four pulsar companions already meet this criterion.

desk verdict A useful candidate list, but the orbital-period screen is a necessary condition, not a test that distinguishes SQM planets from brown dwarfs or degenerate cores. read the letter →

arxiv 1908.11191 v1 pith:E4PNL6IN submitted 2019-08-29 astro-ph.HE

Abudushataer Kuerban , Jin-Jun Geng , Yong-Feng Huang , Hong-Shi Zong , Hang Gong This is my paper · ORCID

classification astro-ph.HE PACS 97.60.Gb97.60.Jd04.30.-w

keywords strangequarkmatterpulsarplanetstidaldisruptionorbitalperiodgravitationalwaveswhitedwarfcompactobjectsexoplanets

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 argues that if strange quark matter is the true ground state of hadronic matter, then some pulsars are strange stars and some very close-in companions are strange quark matter planets rather than ordinary planets. It uses a tidal-disruption criterion: an ordinary planet with density up to 30 g cm$^{-3}$ would be shredded inside an orbital radius of $5.6\times10^{10}$ cm, corresponding to an orbital period of about 6100 s, while an SQM planet can survive there. Applying this criterion to cataloged exoplanets, the authors find four pulsar companions that fully satisfy the period cutoff and five white-dwarf companions with periods under 0.1 day that may also be SQM planets. They further calculate that persistent gravitational waves from three of these systems should be detectable by LISA, and merger bursts should be detectable by advanced LIGO and the Einstein Telescope, making the SQM hypothesis observationally testable.

What carries the argument

The load-bearing object is the tidal-disruption radius, $r_{\rm td}\approx (6M/\pi\rho)^{1/3}$, combined with Kepler's law to convert radius into a period threshold. An SQM planet with density $\sim4\times10^{14}$ g cm$^{-3}$ is disrupted only at $\sim2.37\times10^6$ cm, essentially at the stellar surface, whereas a normal planet capped at 30 g cm$^{-3}$ is disrupted at $5.6\times10^{10}$ cm. The period-density relation $\rho_{\min}\approx 3\pi/(0.4623\,G P_{\rm orb}^2)$ then gives a minimum density for each observed companion, and the criterion $P_{\rm orb}<6100$ s becomes a simple observational filter for exotic matter.

What would settle it

A radius measurement showing any of the four pulsar companions has a mean density below roughly 30 g cm$^{-3}$, for example through an X-ray eclipse or optical detection revealing a large low-density body, would break the tidal-disruption criterion and remove the need for strange quark matter. Alternatively, a gravitational-wave burst from a merger whose matter signature is incompatible with quark matter would settle the question.

Watch

Extended reading notes

Core claim

The central claim is that extreme closeness alone can identify exotic compact planets: a planet observed with $P_{\rm orb}<6100$ s (orbital radius $a<5.6\times10^{10}$ cm) must have a mean density exceeding the normal-matter ceiling of roughly 30 g cm$^{-3}$, so it is most likely a strange quark matter object. Among known pulsar planets, XTE J1807-294 b (2404 s), XTE J1751-305 b (2545 s), PSR 0636 b (5789 s), and PSR J1807-2459A b (6048 s) fully meet the criterion, while PSR 1719-14 b (7837 s) and PSR J2051-0827 b (8563 s) are close enough to be potential candidates. Among white-dwarf companions, GP Com b, V396 Hya b, and J1433 b have orbital periods of 2765, 3888, and 4666 s and minimum densities of 187.5, 94.8, and 65.8 g cm$^{-3}$, respectively, which the paper argues makes them very good SQM planet candidates. The paper thereby converts the SQM hypothesis into a concrete list of observable targets.

Load-bearing premise

The argument assumes that an orbital period under 6100 s forces a mean density higher than ordinary matter can provide, so the companion must be exotic; if a crystallized C/O dwarf or a brown dwarf can be that dense and that close, the SQM conclusion does not follow.

Editorial extensions

If this is right

Any future planet found in a circular orbit with $P_{\rm orb}<6100$ s around a pulsar or white dwarf would, by this criterion, be classified as an SQM candidate rather than an ordinary planet.
The four listed pulsar companions are concrete follow-up targets: measuring their radii or detecting their surfaces would directly test whether they are ordinary matter.
Persistent gravitational waves from GP Com b, V396 Hya b, and J1433 b should lie above LISA's one-year sensitivity curve, so LISA data can be searched for these predicted signals.
Merger bursts from close-in SQM planets are predicted to exceed the sensitivity of advanced LIGO and the Einstein Telescope, offering a gravitational-wave channel to test the SQM hypothesis.
Confirmation of even one candidate would support the conjecture that millisecond pulsars in low-mass X-ray binaries are strange stars, linking accretion-induced deconfinement to the presence of exotic companions.

Reading between the lines

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

The same period-density logic could be applied to future ultra-short-period planets around white dwarfs found by wide-field surveys: any object with $P_{\rm orb}<6100$ s and a minimum density above 30 g cm$^{-3}$ would automatically be flagged as an SQM candidate even without a pulsar host.
The 30 g cm$^{-3}$ ceiling and the assumed circular orbit are the tunable parameters; allowing eccentric orbits or higher-density ordinary cores, such as crystallized C/O dwarfs, would shift the period cutoff and could change which near-threshold candidates count.
A targeted LISA search for the three above-threshold white-dwarf systems could serve as a clean test: detecting monochromatic emission at $f=2/P_{\rm orb}$ would confirm a very compact companion without needing an electromagnetic radius measurement.
If follow-up rules out all eleven candidates, the absence of ultra-short-period normal planets around pulsars would itself constrain the abundance of SQM planets and the strange-star fraction of millisecond pulsars.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

This paper is a candidate-screening exercise, and it does a real service: it compiles all known pulsar and white-dwarf companions with P_orb under 0.1 days, applies the period-density argument, and flags four pulsar companions and five white-dwarf companions as SQM planet candidates. That list is new relative to earlier work by the same group, which had singled out PSR J1719-14 b. The GW calculations are standard and mostly correct, and the paper is honest about the uncertainties in classification. It also cites the alternative interpretations in the original sources, which helps the reader see what is at stake.

The central soft spot is that the P_orb < 6100 s criterion is only a necessary condition, not a distinguishing test. The period-density relation gives a lower bound on mean density required to underfill the Roche lobe, and any compact object—a crystallized C/O core, a degenerate He core, an irradiated brown dwarf—can clear that bar. The paper acknowledges these alternatives in the text but then sets them aside too quickly. The two copper-sample objects, XTE J1807-294 b and XTE J1751-305 b, are explicitly described in the cited literature as possible C/O dwarf cores, and the paper's mass argument does not rule that out; the PSR J1719-14 b case shows the same issue. The white-dwarf companions are up to 57 Mjup, which is well into brown-dwarf territory, and the claim that they cannot be brown dwarfs because their minimum densities are high assumes a picture of brown dwarfs that is too loose. The internal inconsistency in the density formula also matters: the printed formula rho_min ~ 3pi/(0.4623 G P^2) does not reproduce Table 3; the table values use 0.4623^3. That is a typo, but it matters because PSR 0636 b and PSR J1807-2459A b drop below the 30 g/cm^3 threshold if the printed formula is used.

I do not think the central claim that these are likely SQM objects holds up. The paper is better read as a useful target set: a list of systems worth following up with photometry, spectroscopy, or GW observations to test the SQM hypothesis. The GW detectability calculations, while not unique to SQM, are a legitimate addition.

Who is this for? Researchers working on SQM phenomenology or on compact companions around pulsars and white dwarfs. It deserves a serious referee, because the candidate list is useful and the argument, though incomplete, is coherent enough to engage. With the density formula fixed and the classification discussion sharpened, this could be a solid screening paper.

Yes, send it to peer review.

Referee Report

4 major / 5 minor

Summary. The paper argues that normal-matter planets with densities below about 30 g/cm^3 would be tidally disrupted if their orbital radius is below about 5.6e10 cm or their orbital period is below about 6100 s, whereas strange quark matter planets could survive at such close separations. Using this criterion, the authors compile a sample of 19 pulsar-planet candidates from public exoplanet catalogs, identify four pulsar companions (XTE J1807-294 b, XTE J1751-305 b, PSR 0636 b, PSR J1807-2459A b) as completely meeting the criterion and five white-dwarf companions with P_orb < 0.1 day as additional candidates, and compute minimum densities as well as persistent and merger gravitational-wave signals for these systems, concluding that several are detectable by LISA, advanced LIGO, and the Einstein Telescope.

Significance. If the strange quark matter hypothesis is correct and the listed candidates are genuine SQM planets, the paper would provide a useful target list and a potentially falsifiable gravitational-wave search strategy. The strengths of the manuscript are its systematic use of public catalogs, its transparent gold/silver/copper classification of pulsar companions, and its quantitative use of standard Peters-Mathews formulas for gravitational-wave emission. However, the central screening criterion is only a necessary condition based on mean density; ordinary degenerate cores and dense brown dwarfs can also satisfy it. The paper would therefore be better framed as a candidate-screening study than as an identification of SQM objects, and it needs additional quantitative treatment of the competing interpretations before its central claim can be fully supported.

major comments (4)

[Sec. 4.1, Table 3] The four pulsar companions that are said to 'completely meet' the SQM criteria are not robustly identified as SQM planets, because the period-density relation gives only a necessary lower bound on mean density. The text itself notes in Sec. 4.1.3 that XTE J1807-294 b and XTE J1751-305 b may be crystallized C/O dwarf cores (Deloye & Bildsten 2003), and the counter-argument based on masses below roughly 100 M_jup does not exclude such cores, which can have masses well below that value in stripped configurations (the PSR J1719-14 b case cited by the authors is a precedent). Similarly, PSR 0636 b and PSR J1807-2459A b, discussed in Sec. 4.1.1, are compact enough to be degenerate cores rather than SQM planets. The paper should either provide additional diagnostics that distinguish SQM planets from C/O degenerate cores, or explicitly weaken the claim from 'good candidates' to 'objects that pass a necessary-condition screen'.
[Sec. 4, density formula] The printed formula rho_min approximately 3 pi / (0.4623 G P^2) is missing the cube on 0.4623; the values in Table 3 are consistent with 3 pi / (0.4623^3 G P^2), since the Roche-lobe radius enters as 0.4623 a (m/M)^(1/3) and therefore contributes a volume factor of 0.4623^3. With the formula as printed, PSR 0636 b and PSR J1807-2459A b would have rho_min values around 9 and 8 g/cm^3, below the 30 g/cm^3 threshold used to justify their SQM candidacy. Please correct the equation and state explicitly which normalization was used to produce Table 3.
[Sec. 4.2, Table 2] The brown-dwarf alternative is not quantitatively excluded for the five white-dwarf companions. The masses of GP Com b, V396 Hya b, J1433 b, WD 0137-349 b, and SDSS J1411+2009 b are in the range 18 to 57 M_jup, which substantially overlaps the brown-dwarf mass range, and the text acknowledges that several of these objects have been suggested to be irradiated brown dwarfs. The assertion in Sec. 4.2 that the densities of the three shortest-period objects are 'so high that they can hardly be normal brown dwarfs' is not backed by a mass-radius calculation for old, irradiated brown dwarfs, which can contract to mean densities well above 30 g/cm^3. A quantitative comparison of expected brown-dwarf and C/O-core mean densities at these masses is needed before these systems can be promoted to SQM candidates.
[Sec. 5.1] The claim that gravitational-wave observation 'would be a unique tool to search for SQM candidates' is not justified. The persistent gravitational-wave strain in Eqs. (3) and (4) depends only on the chirp mass, orbital period, and distance; it is the same for an SQM planet, a C/O degenerate core, and a brown dwarf at identical orbital parameters. The statement that a close-in planet-like object discovered via gravitational waves 'must be an SQM planetary system' is therefore too strong, since gravitational waves alone do not measure the companion density or radius. The persistent signal can be part of a multi-messenger identification only when combined with an independent compactness measurement.

minor comments (5)

[Table 1, Sec. 4.1] Object naming is inconsistent: the text uses 'PSR J0636 b' and 'PSR J1719-14 b', while Table 1 lists 'PSR 0636 b' and 'PSR 1719-14 b'. Please unify names to the standard J2000 pulsar designations.
[Sec. 6] The phrase 'cooper sample' should be 'copper sample'.
[Eq. (5), Sec. 2.2] The symbol M is used for the host mass in Eqs. (1) and (2) but then defined as the total mass M + m in the text following Eq. (5); this notational clash should be fixed.
[Sec. 3, Figure 4] Figure 4 labels 'PSR 1719-14 b' and 'PSR 1807-2459A b' differ from the names used in Table 1; please align the figure labels with the table.
[Sec. 3] For reproducibility, please state the date on which the exoplanet databases were queried and the version of each catalog used, since the number of listed candidates depends on catalog updates.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity: candidate selection and GW predictions use external catalogs and standard physics; self-citations are minor and not load-bearing.

full rationale

The paper's candidate list is not a fitted output. The selection rule P_orb < 6100 s is derived in Sec. 2.1 from the standard tidal-disruption radius (Hills 1975) with an assumed normal-density ceiling of 30 g/cm^3; this is a stated physical assumption, not a parameter calibrated to the candidate objects. The minimum densities in Table 3 use the period-density relation with catalog orbital periods, and the GW estimates in Sec. 5 use the standard Peters & Mathews (1963) formulas with external masses, periods, and distances. No predicted quantity is equal by construction to an input fitted value. The self-citations (Huang & Yu 2017 for the 6100 s threshold; Geng et al. 2015 for GW burst templates) are present but not load-bearing, because the same formulas are independently stated and attributed to standard references. The paper's own caveats that copper-sample objects may be crystallized C/O dwarf cores and that WD companions may be brown dwarfs, plus the apparent typo in the printed period-density relation relative to Table 3, affect the soundness of the inference to SQM, but they are correctness concerns, not circularity. No step in the derivation reduces to its own input by definition.

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

The paper's central claim rests on the SQM hypothesis, the standard Roche density framework, and a permissive planet mass classification that is not independently established. No new free parameters are fitted in this paper; the threshold values (30 g/cm^3, 6100 s) are taken from prior work by the same group. The most fragile entries are the mass classification and the period-density relation, whose printed form is inconsistent with the table.

assumptions (6)

domain assumption Strange quark matter is the true ground state of hadronic matter and strange stars exist.
The entire candidate search presupposes the SQM hypothesis; if false, the candidates are ordinary compact objects. Introduced in Section 1 and 2.1.
standard math The tidal disruption radius of a planet is given by r_td = (6 M / (pi rho))^(1/3).
Standard fluid Roche limit formula from Hills 1975, used in Section 2.1 to set the SQM criteria.
domain assumption Normal matter planets have density no higher than 30 g/cm^3.
Adopted from Huang & Yu (2017) to define the close-in criterion; no independent justification is given in this paper.
domain assumption The period-density relation provides a lower limit on the companion mean density.
Used in Section 4 to infer minimum densities; the printed formula in the paper is inconsistent with the tabulated values, and the inferred lower limits are what drive the SQM classification.
domain assumption Companions with masses below about 60 Mjup can be classified as planets.
The paper relies on generous upper mass limits from the literature to call 18 to 57 Mjup objects planets, despite the deuterium-burning boundary near 13 Mjup that the paper itself acknowledges.
standard math Kepler's laws and the Peters-Mathews gravitational wave formulas hold.
Used throughout Sections 2.2 and 5 to compute radii, strains, luminosities, and coalescence times.

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Cite this review

Pith. "Pith review of Close-in Exoplanets as Candidates of Strange Quark Matter Objects." pith.science (2026). https://pith.science/paper/E4PNL6IN

@misc{pith2026190811191,
  author       = {Pith},
  title        = {Pith review of: Close-in Exoplanets as Candidates of Strange Quark Matter Objects},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/E4PNL6IN}},
  note         = {Machine review of arXiv:1908.11191}
}

abstract

Since the true ground state of the hadrons may be strange quark matter (SQM), pulsars may actually be strange stars rather than neutron stars. According to this SQM hypothesis, strange planets can also stably exist. The density of normal matter planets can hardly be higher than 30 g cm$^{-3}$. As a result, they will be tidally disrupted when its orbital radius is less than $\sim 5.6\times10^{10} \rm \, cm $, or when the orbital period ($P_{\rm orb}$) is less than $ \sim \rm 6100\, s $. On the contrary, a strange planet can safely survive even when it is very close to the host, due to its high density. The feature can help us identify SQM objects. In this study, we have tried to search for SQM objects among close-in exoplanets orbiting around pulsars. Encouragingly, it is found that four pulsar planets (XTE J1807-294 b, XTE J1751-305 b, PSR 0636 b, PSR J1807-2459A b) completely meet the criteria of $P_{\rm orb} < \rm 6100\, s $, and are thus good candidates for SQM planets. The orbital periods of two other planets (PSR J1719+14 b and PSR J2051-0827 b) are only slightly higher than the criteria. They could be regarded as potential candidates. Additionally, we find that the periods of five white dwarf planets (GP Com b, V396 Hya b, J1433 b, WD 0137-349 b, and SDSS J1411+2009 b) are less than 0.1 days. We argue that they might also be SQM planets. It is further found that the persistent gravitational wave emissions from at least three of these close-in planetary systems are detectable to LISA. More encouragingly, the advanced LIGO and Einstein Telescope are able to detect the gravitational wave bursts produced by the merger events of such SQM planetary systems, which will provide a unique test for the SQM hypothesis.

Figures

Figures reproduced from arXiv: 1908.11191 by the authors.

**Figure 1.** Orbital periods versus masses for all the 1638 exoplanets with data available from the EU web site (http: //www.exoplanet.eu/catalog/). The red stars represent candidate pulsar planets. The blue points correspond to the five close-in WD planets with Porb < 0.1 days, and the black dots represent other 1616 exoplanets. iron material (typically with a density of 1−10 g cm−3 ). If these objects are planets but not smal… view at source ↗

**Figure 2.** Power of gravitational wave emission versus orbital radius for the planetary systems of our sample. The red stars are pulsar planets and the blue points are WD planets with Porb < 0.1 day. The vertical green dashed line markes the critical tidal disruption radius of a = 5.6 × 1010 cm for normal matter planets. 10 4 10 3 10 2 10 1 10 0 / Hz 10 25 10 24 10 23 10 22 10 21 10 20 h (strain) Lisa sensitivity Pulsar plane… view at source ↗

**Figure 3.** GW strain amplitude versus frequency for the planetary systems of our sample. The blue points represent WD planets with Porb < 0.1 day and the red stars represent pulsar planets. The black dashed line represents the sensitivity curve of LISA with an one year integration time. For a similar plot, please also see Cunha et al. (2018) and Wong et al. (2018). GW emissions from the candidate SQM systems in our sample and… view at source ↗

Figures from the paper (2 more)

**Figure 4.** Figure 4: Strain spectral amplitude of the GW bursts for coalescing SS and SQM planet systems. The sensitivity curves of the advanced LIGO and Einstein Telescope are also plotted. 10 3 10 4 10 5 Porb (s) 10 7 10 8 10 9 10 10 10 11 10 12 10 13 10 14 t c o (y r) m = 10 2M m = 10 …

**Figure 5.** Figure 5: Coalescence timescale versus the initial orbital period for the candidate SQM planetary systems in our sample. Red stars correspond to pulsar planets and blue points correspond to white dwarf planets. The three straight lines illustrate the coalescence timescale for t…

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