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A Far-Infrared Search for Planet Nine Using AKARI All-Sky Survey

T0 review · 4 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read The paper claims that two far-infrared sources in the AKARI all-sky survey, each seen twice within a day and missing from scans six months later, are viable Planet Nine candidates with fluxes matching a 6-12 Earth-mass planet at 300-900 au.

desk verdict A genuinely new far-infrared search with two follow-up-worthy candidates, but the motion inference rests on an unquantified six-month non-detection. read the letter →

arxiv 2506.12854 v1 pith:7KSFPZNN submitted 2025-06-15 astro-ph.EP astro-ph.IMastro-ph.SR

Amos Y.-A. Chen , Tomotsugu Goto , Issei Yamamura , Takao Nakagawa , Cossas K.-W. Wu , Terry Long Phan , Tetsuya Hashimoto , Yuri Uno
show 2 more authors
Simon C.-C. Ho Seong Jin Kim
This is my paper · ORCID
classification astro-ph.EPastro-ph.IMastro-ph.SR
keywords PlanetNinefar-infraredastronomyAKARIsurveymovingobjectsearchtrans-NeptunianobjectsKuiperbeltparallax
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

This paper claims that a dedicated search of the AKARI far-infrared all-sky survey has found two Planet Nine candidates: point-like sources detected twice within 24 hours and absent from scans six months later, as expected for a distant object whose apparent position shifts by parallax. The search works in the thermal far-infrared rather than reflected optical light, where a planet's signal falls off as distance squared instead of distance to the fourth power, allowing a wider volume to be probed. After excluding known sources, cirrus contamination, cosmic-ray artifacts, and fast-moving objects, the remaining two sources have 90-micron fluxes of 0.5-1.6 Jy, consistent with a 6-12 Earth-mass planet at 300-900 au from the Sun with a temperature of 28-53 K. The paper argues these are the strongest far-infrared Planet Nine candidates to date and that they warrant follow-up observations, though it stops short of claiming a confirmed detection.

What carries the argument

The central mechanism is the AKARI-FIS Single Scan Detection List (FISSSDL), a source list built from individual satellite scans without the standard requirement that a source be confirmed in multiple scans separated by months, a requirement that would automatically reject a moving object like Planet Nine. To exploit this list, the authors use the fact that a planet at 300-900 au shifts by 10-25 arcminutes over six months due to parallax, well above the 3.5-arcsecond positional accuracy, while its shift in one hour is only about 0.23 arcseconds. Candidates must therefore be detected at least twice within 24 hours, meaning they are stationary on the short timescale, but must have no detection at the same position six months later, meaning they move on the long timescale. Flux measurements use the most sensitive AKARI band, WIDE-S at 90 microns, with quality cuts FLUX90/FERR90 > 5 and background strength BG90 < 0.2 to reject cirrus contamination, followed by visual inspection of detection probability maps to eliminate cosmic-ray artifacts.

What would settle it

Re-analyze the AKARI detection probability maps at the positions of FISSSDL J0250422-150114 and FISSSDL J0301112-164240 in the six-month-separated scans, injecting a fake 0.5 Jy point source to measure the local detection limit; if such a stationary source would be confidently detected, the absence supports motion, and if not, the candidates could be stationary objects.

Watch

Extended reading notes

Core claim

On its own terms, the paper's central discovery is the identification of two infrared sources, FISSSDL J0250422-150114 and FISSSDL J0301112-164240, in the AKARI Single Scan Detection List that pass every filter designed to select a moving outer Solar System planet. Both were detected twice within 24 hours in the 90-micron WIDE-S band with no counterpart at the same position in scans six months later, which the authors interpret as the signature of a planet's six-month parallax. Their flux densities, measured per scan, lie between 0.5 and 1.6 Jy, matching the black-body emission predicted for the mass, distance, and temperature ranges taken from the Millholland-Laughlin 2017 simulation and Cowan-Holder-Kaib 2016 thermal model. The paper presents these as viable candidates consistent with theoretical predictions, not as a confirmed detection, and recommends follow-up to determine orbits and confirm the Planet Nine hypothesis.

Load-bearing premise

The paper treats a source's absence from scans six months later as evidence that it moved, but it never shows that a stationary source of the same brightness would have been detected in those later scans, so a missed stationary object could masquerade as a moving candidate.

Editorial extensions

If this is right

  • If either candidate is confirmed by follow-up, it would be the first direct detection of Planet Nine and would support the dynamical explanation for the clustering of extreme Kuiper belt objects.
  • The search probes distances up to roughly 800 au for a 53 K planet, beyond the reach of previous optical surveys that exclude orbits brighter than V~21.
  • The two candidates are within reach of a few pointings with the Subaru Hyper Suprime-Cam, which can detect a 26th-magnitude point source, so confirmation or rejection is observationally feasible.
  • If both candidates are ruled out, the survey's non-detections still place upper limits on the 90-micron flux of Planet Nine in the ML17 sky region, constraining its allowed mass and distance.

Reading between the lines

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

  • The paper's 'no detection after six months' criterion is used as evidence of motion, but the later-epoch detection probability maps at the candidate positions are never calibrated; a stationary 0.5 Jy source might have been missed, so a re-analysis with injected fake sources would test this directly.
  • The two candidates show flux variations of a factor of about 2.5-2.6 between their two detections within 24 hours; if these variations are real rather than noise, they are unusual for a distant black-body source and could indicate a variable or transient object instead.
  • The same single-scan selection applied to the full FISSSDL, not just the ML17 region, might reveal additional moving far-infrared sources, including previously unrecognized asteroids or other distant planets.
  • If either candidate is a real planet, the far-infrared thermal approach would become a viable complement to optical reflected-light searches, since thermal flux falls as distance squared rather than to the fourth power, extending the detectable volume.
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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 / 5 minor

Summary. This paper presents a search for Planet Nine in the AKARI all-sky survey using the FIS Single Scan Detection List, which, unlike the standard FIS Bright Source Catalogue, does not require confirmation across multiple scans. The authors select sources in the Millholland and Laughlin (2017) sky region (30°<R.A.<50°, –20°<Dec.<20°), cross-match against nine optical and infrared catalogues, apply flux and background cuts (FLUX90/FERR90>5, BG90<0.2), require two detections within 24 hours and no detection after six months, and visually inspect detection probability maps to reject cosmic rays and edge artifacts. After this pipeline they identify two candidate sources with per-scan 90 µm fluxes between 0.5 and 1.6 Jy, at positions within the selected region, and argue that their fluxes are consistent with a 6–12 Earth-mass planet at 400–900 au with an effective temperature of 28–53 K. The paper estimates contamination from astrophysical transients and noise fluctuations, reports expected counts of about 0.014 transients and 0.029 noise overlaps, and proposes Subaru HSC follow-up observations.

Significance. If the two candidates are genuine moving objects, this would be the first far-infrared detection of a plausible Planet Nine and would provide strong motivation for targeted follow-up. The paper also makes a methodological contribution by exploiting the AKARI FISSSDL, which is not subject to the moving-source rejection of the standard catalogue. The claimed result is, however, not yet fully supported: the key evidence of motion—the six-month non-detection—is not quantified in terms of sensitivity, and the selection pipeline relies on several post hoc thresholds and subjective image inspection. The transient and noise contamination estimates are useful but do not cover all possible false-positive branches, so the significance of the two candidates remains uncertain pending a quantitative demonstration that the non-detections are meaningful.

major comments (4)
  1. [Sections 3.6–3.7, Table 2] The central claim that the two candidates are moving objects rests on the statement that they were 'confirmed to have no appearances before six months or disappear after six months' (Section 3.7). However, the paper never quantifies the sensitivity of the six-month-separated scans at the candidate positions. It does not report the number of later-epoch scans, the local detection-probability-map values, or the background and noise level at the candidate coordinates. Without this information, a non-detection could equally be explained by a stationary unresolved source that was not covered by, or fell below the detection threshold of, the later scans. The authors should provide, for each of the two candidates, the detection-probability-map value or an equivalent flux upper limit at the six-month epoch, and demonstrate that a source with the observed first-epoch flux would have been detected had it remained stationary.
  2. [Section 5] The contamination estimate in Section 5 accounts for astrophysical transients (CCSNe, SNe Ia, novae, TDEs) and for overlapping noise fluctuations, but it does not include the branch in which a stationary far-infrared source is detected in the first epoch and missed in the later epoch because of coverage gaps or sensitivity variations. Since the six-month non-detection is the only evidence that the candidates moved, the authors should either quantify how many stationary sources would be expected to pass the full selection (for example, by injecting artificial stationary sources into the FISSSDL and running the selection pipeline) or explicitly add this branch to the false-positive budget.
  3. [Section 3.7] The description of the candidate selection after the one-year check is ambiguous. The text states that only 8 out of 10 candidates have detection maps in the subsequent or preceding year, and then all 8 of those are removed, leaving two candidates. This implies that the final two candidates are among those without one-year detection maps. The authors should clarify, for each final candidate, which epochs were actually observed, whether the six-month-separated scans covered the candidate position, and how the 'no appearances before six months or disappear after six months' confirmation was performed if the one-year maps are absent. A table listing the scan epochs and the detection-probability-map values at the candidate positions for both the initial and the follow-up epochs would remove this ambiguity.
  4. [Sections 3.5 and 3.7] Several selection thresholds (BG90 < 0.2, detection-probability-map value > 21, and the two-detections-within-24-hours requirement) are chosen after inspecting the data and are not accompanied by a robustness test. Because the final candidate list shrinks from 393 to 2 through these cuts, the authors should demonstrate that the two surviving candidates are not an artifact of the specific threshold choices—for example, by varying each threshold over a reasonable range and reporting how the candidate list changes, or by estimating the effective number of trials. This is particularly important given that the visual rejection of cosmic rays and edge detections is subjective.
minor comments (5)
  1. [Title page] The header contains the placeholder-like text 'Cambridge Large T wo(2024)' and the running title should be checked for typographical errors before final submission.
  2. [Figure 2 caption] The caption states 'The X-axis shows the number of sources in each bin' for the right panel; the number of sources is plotted on the y-axis, not the x-axis. In addition, the sentence 'The dark blue histogram shows ... with the same y-axis as the left panel' is unclear because the two panels have different y-axis quantities.
  3. [Section 3.7] The text refers to an 'arbitral unit' for the detection probability map; this should be 'arbitrary unit'. Also, the sentence describing the selection of 165 candidates could be reworded to clarify the difference between 'clearly detected in all scans within 24 hours' and 'confirmed to have no appearances before six months or disappear after six months'.
  4. [Section 5] The transient time-window factor is written as '(T – 1)/365'; if the requirement is two detections within one day, the probability of catching a transient of duration T in a one-year survey is not simply (T–1)/365. Please explain the derivation of this factor.
  5. [Table 2] The table lists per-scan fluxes for the two candidates but does not provide flux uncertainties. Given that the fluxes vary by factors of 2–3 between the two scans of each candidate, the authors should report the photometric errors to allow the reader to judge whether the variability is significant.

Circularity Check

1 steps flagged · score 1.0 of 10

No significant circularity; the only by-construction element is the positional consistency statement, which is non-load-bearing.

  1. self definitional [Abstract; Section 3.3 (Position Selection)]
    "The dynamical simulation from ML17 suggests that the probability of finding Planet Nine is higher in the region 30◦ < R.A. < 50◦, –20 ◦ < Dec. < 20◦. There are 50,033 sources in this area out of 5,274,338 sources in FISSSDL. ... Our analysis reveals two possible Planet Nine candidates whose positions and flux are within the theoretical prediction ranges."

    All candidates are drawn from the ML17 sky box, so their right ascension and declination are inside that box by construction. The abstract's statement that the candidates' positions are 'within the theoretical prediction ranges' is therefore a restatement of the Section 3.3 selection cut, not an independent test of the ML17 prediction. The flux part of the sentence is not forced because the pipeline does not apply a FLUX90 window; the selection uses FLUX90/FERR90 and BG90 cuts. This positional tautology is not load-bearing for the final candidacy, which rests on the two-detection/no-six-month-confirmation motion filter and the transient/noise estimates.

full rationale

The paper's derivation chain is otherwise self-contained. The expected 90 µm flux is computed from literature inputs (ML17 mass range, Cowan, Holder, and Kaib 2016 temperature range, an assumed Neptune/Uranus-like density) via the black-body formula in Section 3.1, and the expected parallax is computed from Cowan, Holder, and Kaib 2016 in Section 3.2; no parameter is fitted to the two surviving detections and then renamed a prediction. The candidate-selection filters (9-catalogue cross-match, BG90 and S/N cuts, two detections within 24 hours, no monthly confirmation, CR/edge/asteroid rejection, one-year-separation checks) are fixed data-quality and motion criteria applied before the final candidates are identified. The transient-rate and noise-overlap estimates in Section 5 are independent contamination checks. There is no load-bearing self-citation: Phan et al. 2025 is cited only as a related search with different data and distance coverage, and Yamamura et al. 2010 is the standard FISBSC catalogue reference. The unquantified sensitivity of the six-month-separated non-detections is a real completeness/validity concern, but it is not a circular reduction—it concerns whether absence implies motion, not whether a result is equivalent to its input. Hence the only circular element is the trivial positional consistency statement, which is minor and non-load-bearing, giving a score of 1.

Assumptions & free parameters 5 free parameters · 6 assumptions · 0 invented entities

The paper introduces no new theoretical entities. The two candidates are observational detections, not postulated objects. The main burden of assumptions lies in the completeness of the single-scan catalog, the reliability of visual artifact rejection, and the interpretation of non-detection as motion.

free parameters (5)
  • Assumed density of Planet Nine = 1.454 g/cm3
    Average of Neptune and Uranus densities, used in Equation 2 to convert mass to radius and compute expected flux (Section 3.1).
  • BG90 threshold = 0.2
    Selected to exclude cirrus-contaminated sources after inspecting the FLUX90-BG90 distribution, where the source count peaks at BG90 = 0.3 (Section 3.5).
  • FLUX90/FERR90 threshold = > 5
    Chosen to select sources with reliable flux (Section 3.5); the value is not derived from first principles.
  • Detection probability map threshold = > 21
    Adopted after the authors noticed that detections below 21 were not reliable during image inspection (Section 3.7).
  • Mass and temperature ranges for Planet Nine = 6-12 Earth masses, 28-53 K
    Taken from ML17 and Cowan et al. 2016; these ranges set the broad expected flux band used for the consistency check (Sections 3.1).
assumptions (6)
  • domain assumption Planet Nine exists with mass 6-12 Earth masses and lies in the ML17 region 30 < RA < 50 and -20 < Dec < 20.
    The entire search region and mass range come from ML17 (Section 3.3); if Planet Nine is not there, the search cannot find it.
  • domain assumption A source with FLUX90 above the detection limit would produce a detection in a single AKARI scan.
    Section 2 says FISSSDL includes any source detected at least once, but the completeness of single-scan detections is not characterized.
  • domain assumption Non-detection in six-month-separated scans implies the source moved.
    Sections 3.6 and 3.7 treat MCONF90 = 0 and absence in later detection maps as evidence of motion, without quantifying the sensitivity of the later maps.
  • domain assumption Cross-matching with nine external catalogs removes all known stationary sources.
    Section 3.4 relies on the completeness of the external catalogs and on the calibrated cross-match radii.
  • domain assumption Cosmic-ray hits can be reliably identified by their image morphology.
    Section 3.7 uses visual inspection of detection probability maps to reject about half of the candidates as cosmic-ray artifacts.
  • domain assumption The blackbody model with assumed density and temperature describes Planet Nine's far-infrared emission.
    Section 3.1 computes expected flux from a blackbody with a constant density assumption and the temperature range of Cowan et al. 2016.

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Cite this review

Pith. "Pith review of A Far-Infrared Search for Planet Nine Using AKARI All-Sky Survey." pith.science (2026). https://pith.science/paper/7KSFPZNN

@misc{pith2026250612854,
  author       = {Pith},
  title        = {Pith review of: A Far-Infrared Search for Planet Nine Using AKARI All-Sky Survey},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/7KSFPZNN}},
  note         = {Machine review of arXiv:2506.12854}
}
abstract

An unusual orbital element clustering of Kuiper belt objects (KBOs) has been observed. The most promising dynamic solution is the presence of a giant planet in the outer Solar system, Planet Nine. However, due to its extreme distance, intensive searches in optical have not been successful. We aim to find Planet Nine in the far-infrared, where it has the peak of the black body radiation, using the most sensitive all-sky far-infrared survey to date, AKARI. In contrast to optical searches, where the energy of reflected sunlight decreases by $d^{4}$, thermal radiation in the infrared decreases with the square of the heliocentric distance $d^{2}$. We search for moving objects in the AKARI Single Scan Detection List. We select sources from a promising region suggested by an N-body simulation from Millholland and Laughlin 2017: $30^{\circ}<$ R.A. $<50^{\circ}$ and $-20^{\circ}<$ Dec. $<20^{\circ}$. Known sources are excluded by cross-matching AKARI sources with 9 optical and infrared catalogues. Furthermore, we select sources with small background strength to avoid sources in the cirrus. Since Planet Nine is stationary in a timescale of hours but moves on a monthly scale, our primary strategy is to select slowly moving objects that are stationary in 24 hours but not in six months, using multiple single scans by AKARI. The selected slowly moving AKARI sources are scrutinised for potential contamination from cosmic rays. Our analysis reveals two possible Planet Nine candidates whose positions and flux are within the theoretical prediction ranges. These candidates warrant further investigation through follow-up observations to confirm the existence and properties of Planet Nine.

Figures

Figures reproduced from arXiv: 2506.12854 by the authors.

Figure 1
Figure 1. Flux histograms of FISSSDL sources. Four histograms correspond [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Left: Estimated 90 µm flux of Planet Nine. We calculate the expected Planet Nine’s 90 µm flux in Section 3.1 and plot it with 4 com￾binations of 2 parameters: mass and temperature in the left panel. The x-axis is the heliocentric distance of Planet Nine. The mass range of 6 to 12 M⊕ was predicted by ML17. Cowan, Holder, and Kaib 2016 suggested a temperature range of 28 K to 53 K. Right: A histogram of FLUX90. The da… view at source ↗
Figure 3
Figure 3. Anticipated proper motion and parallax of Planet Nine within half [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figures from the paper (5 more)
Figure 4
Figure 4. Figure 4: Separation distribution of matched sources. There are 50 bins on a linear scale in each subplot. The stars show the separation of matched pairs of [PITH_FULL_IMAGE:figures/full_fig_p005_4.png]
Figure 5
Figure 5. Figure 5: Candidates remained after each step on the 90 [PITH_FULL_IMAGE:figures/full_fig_p006_5.png]
Figure 6
Figure 6. Figure 6: Each scan of FISSSDL J0250422-150114, one of the Planet Nine candidates. The image size is [PITH_FULL_IMAGE:figures/full_fig_p008_6.png]
Figure 7
Figure 7. Figure 7: Fake detection caused by CRs when AKARI pass through SAA. These images are selected from different sources but with similar features. We reject the candidates contaminated by the CRs [PITH_FULL_IMAGE:figures/full_fig_p008_7.png]
Figure 8
Figure 8. Figure 8: Work flow of this work. Orange blocks are steps we applied to select candidates. Blue blocks show the remaining sources after each step. [PITH_FULL_IMAGE:figures/full_fig_p009_8.png]

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Works this paper leans on

5 extracted references · 3 canonical work pages

  1. [30]

    https://dx.doi.org/10

    https://doi.org/10.3847/1538-4365/aaf bea. https://dx.doi.org/10. 3847/1538-4365/aaf bea. Scholtz, Jakub, and James Unwin. 2020. What if planet 9 is a primordial black hole? Phys. Rev. Lett. 125 (5): 051103. https://doi.org/10.1103/ PhysRevLett.125.051103. https://link.aps.org/doi/10.1103/PhysRevLett. 125.051103. Sedgwick, Chris, and Stephen Serjeant. 202...

    work page doi:10.3847/1538-4365/aaf 2020

  2. [444]

    arXiv: 2011.08751 [astro-ph.HE]

    https://doi.org/10.1146/annurev-astro-112420-114502. arXiv: 2011.08751 [astro-ph.HE]. Cowan, Nicolas B., Gil Holder, and Nathan A. Kaib. 2016. Cosmologists in search of planet nine: the case for cmb experiments. The Astrophysical Journal Letters 822, no. 1 (April): L2. https://doi.org/10.3847/2041- 8205/822/1/L2. https://dx.doi.org/10.3847/2041-8205/822/1...

    arXiv 2011

  3. [707]

    https://dx.doi.org/10.1086/ 300738

    https://doi.org/10.1086/300738. https://dx.doi.org/10.1086/ 300738. Robin, A. C., C. Reylé, S. Derrière, and S. Picaud. 2003. A synthetic view on structure and evolution of the Milky Way. Astron. Astrophys. 409 (October): 523–540. https://doi.org/10.1051/0004-6361:20031117. Rowan-Robinson, Michael. 2021. A search for Planet 9 in the IRAS data. Monthly Not...

    doi:10.1086/300738 2003

  4. [2021]

    Searching for Planet Nine with Coadded WISE and NEOWISE-Reactivation Images

    The catwise2020 catalog. The Astrophysical Journal Supplement Se- ries 253, no. 1 (February): 8. https://doi.org/10.3847/1538-4365/abd805. https://dx.doi.org/10.3847/1538-4365/abd805. Masterson, Megan, Kishalay De, Christos Panagiotou, Erin Kara, Iair Arcavi, Anna-Christina Eilers, Danielle Frostig, et al. 2024. A new population of mid-infrared-selected t...

    work page Pith review arXiv

  5. [8711]

    OSSOS VI. Striking Biases in the detection of large semimajor axis Trans-Neptunian Objects

    https://doi.org/10.1093/mnras/stac2044. eprint: https://academic. oup.com/mnras/article- pdf /515/4/4828/45475230/stac2044.pdf . https://doi.org/10.1093/mnras/stac2044. Shankman, Cory, J. J. Kavelaars, Michele T. Bannister, Brett J. Gladman, Samantha M. Lawler, Ying-Tung Chen, Marian Jakubik, et al. 2017. OSSOS. VI. Striking Biases in the Detection of Lar...

    work page Pith review arXiv

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