REVIEW 3 major objections 6 minor 1 cited by
Free Floating or Merely Detached?
T0 review · 3 major / 6 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read About half of the reported free-floating Neptunes are actually 'detached' planets still bound to their host stars.
desk verdict A credible N-body case for a real population of detached Neptunes that masquerade as free-floating, but the 'about half' headline is an upper bound dressed as a central value. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The argument is carried by the Safronov number, $\Theta = v_{\rm esc}^2/(2 V_{\rm orb}^2)$, which separates ejection-dominated encounters ($\Theta \gg 1$) from collision-dominated ones ($\Theta \ll 1$), showing that Neptune-mass planets outside roughly 2.4 AU around an M-dwarf can eject, and by the angular momentum deficit (AMD) with the AMD-stability criterion. After orbit-crossing, systems undergo diffusive AMD growth followed by an 'AMD cooling' phase in which detached planets torque each other until the remaining planets are AMD-stable, which is why one to three detached planets survive. The quantitative backbone is the ejection timescale $T_{\rm ej} \sim 2.9\times10^7 (m_p/m_{\rm Nep})^{-1.64} P_{1,0}$, which makes Neptune-mass ejections take billions of years and leaves the scattering incomplete in old systems.
What would settle it
A high-angular-resolution imaging campaign of a sample of microlensing events currently classified as free-floating Neptunes: if host stars are detected at roughly 0.1 arcsecond separation for about half the events, the detached-planet claim is confirmed; if essentially no host stars are found, the central claim is falsified.
Extended reading notes
Core claim
The central discovery is that scattering does not simply eject planets; it also creates a previously under-appreciated class of 'detached' planets. In simulations of five equal-mass planets and of one Jupiter with ten Neptunes, an instability typically resolves into one tightly bound inner planet plus one to three detached planets orbiting at tens to hundreds of AU. These detached planets are shielded from further scattering because mutual secular torques raise their pericenters, and they remain bound for the duration of the integration. Since they orbit at 10-100 times the initial semi-major axis (median about 30 times), they lie far beyond the Einstein radius and would appear as free-floating in microlensing surveys. Combining the simulated detached yield with the observed occurrence of ~0.35 bound Neptunes and ~0.06 Jovians per M-dwarf, and assuming all such systems undergo scattering, the paper estimates ~0.9 detached Neptunes per star, about half of the reported ~2 free-floating Neptunes per star.
Load-bearing premise
The 'about half' estimate assumes that all known planetary systems with bound Neptune- or Jupiter-class planets are 'mobilized in scattering', meaning each one undergoes the full dynamical instability simulated here and contributes the computed yield of detached planets.
Editorial extensions
If this is right
- If correct, microlensing-based estimates of free-floating Neptune occurrence drop from about 2 to roughly 1 per star, with the missing planets present as wide-orbit companions.
- Detached planets at ~300 AU (for an initial orbit at 10 AU) would show host stars ~0.1 arcseconds away, detectable by high-contrast imaging or by the star's own lensing signal.
- The scattering scenario predicts that systems with wide-orbit Neptunes should typically have a single inner planet, and that inner planets can be driven to very small pericenters with observable consequences.
- The mass function of true free-floating planets should be skewed toward lower masses than the bound population, a trend consistent with current data but requiring larger samples to confirm.
Reading between the lines
- If even a substantial fraction of bound-Neptune systems never undergo instability, the detached yield of ~0.9 per star would be an upper limit, so measuring the instability fraction is the clearest way to tighten the estimate.
- The same scattering logic applied to lower-mass planets predicts very long ejection times and Safronov numbers below unity, so truly free-floating super-Earths are more likely to have been ejected by higher-mass planets than by their own kind.
- A natural extension is to search for detached planets around stars with known inner super-Earths; the paper's suggestion that cold Jupiters protect inner systems implies a testable anti-correlation between outer instability and inner compactness.
- Future microlensing surveys with higher cadence and adaptive-optics follow-up could directly measure the detached fraction by detecting host stars near FFP events, providing a clean test of the scattering origin.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper uses N-body simulations (REBOUND/ias15) of unstable multi-planet systems—five equal-mass planets at 1, 3, 10, and 30 Neptune masses, plus one Jupiter interacting with ten Neptunes—to study whether planet-planet scattering can explain the observed population of free-floating Neptune-mass planets. It finds that Neptune-mass scatterers eject planets slowly, often requiring more than a gigayear, and that scattering leaves one inner planet plus one to three 'detached' planets on orbits 10–100 times the original semimajor axis, which would appear as free-floating planets in microlensing surveys. Combining detached yields with observed bound-planet occurrence rates (Eq. 10), the paper estimates about 0.9 detached Neptunes per star, or about half of the reported free-floating Neptune rate, and discusses implications for AMD stability and inner planetary systems.
Significance. If the quantitative estimate held, this would be an important correction to microlensing free-floating-planet statistics: roughly half of the reported Neptune-mass FFPs would be bound but detached, with testable signatures such as a host star at ~0.1 arcsec and rare double-lensing events. The dynamical findings are well supported: the integrations use a standard integrator with energy-error checks, the ensembles are scale-free, and the AMD stability criterion provides a physical explanation of the final configurations. The paper is also transparent about missing physics in Section 4.3. The main weakness is that the headline number rests on occurrence-rate assumptions that are not calibrated; the central dynamical result is robust, but the 'about half' estimate requires substantial qualification.
major comments (3)
- [Section 4.2, Eq. (10)] Equation (10) treats the observed per-star planet occurrence rates (0.35 Neptunes and 0.06 Jupiters per star) as if they were rates of scattering systems, and the text explicitly assumes 'all known planetary systems are mobilized in scattering.' This is a strong assumption: if only a fraction of bound-planet systems actually undergo dynamical instability, or if some of the 0.35 Neptunes are members of multi-planet systems (which the paper itself argues is likely in Section 4.1), the number of systems producing detached planets is smaller and the ~0.9 per star estimate is an upper bound rather than a central value. The authors should replace the point estimate with N_detached = 2 f_N (0.35) + 3 f_J (0.06) for the fraction of mobilized systems f_N and f_J, or explicitly label Eq. (10) as a conditional upper bound.
- [Sections 3 and 4.3] The detached yields of 2 per system for the equal-mass ensembles and 3 for the 1J+10N ensemble come from point-mass integrations that ignore collisions and stellar engulfment. Figure 7 shows that inner planets in most simulated systems approach within a stellar radius for the Neptune-mass ensembles, and Section 4.3 states that these effects are not captured and that 'we are at no position to predict the actual dynamics.' Because Eq. (10) multiplies these yields by observed occurrence rates, the headline estimate inherits this idealization. The paper should either demonstrate that the detached-planet yields are insensitive to collisions and tidal capture (for example, by rerunning with a simple collision prescription) or present the estimate as an idealized limiting case with an explicit uncertainty.
- [Section 4.1 vs Section 4.2] There is an internal tension that the authors should resolve. Section 4.1 argues that the observed frequency of multi-planet microlensing systems makes the ejection hypothesis 'less tenable' because it is hard to retain two or more closely-spaced planets after a full-scale dynamical instability, yet Eq. (10) assumes every bound Neptune or Jupiter is the sole survivor of exactly one fully destabilized system. These two statements need to be reconciled; for instance, the authors could estimate the fraction of bound planets that can be retained as inner survivors after scattering and use that to correct Eq. (10) downward.
minor comments (6)
- [Section 4.1] The text contains a typo: 'typically onw planet' should read 'typically one planet.'
- [Figure 1 caption] The caption spells 'Sarfronov number' in the figure label; this should be 'Safronov number' for consistency with the text and Equation (3).
- [Section 3.2 and Figure 8] The typo 'percienter' should be 'pericenter' in the discussion of the 1J+10N evolution.
- [Section 2 and Eq. (4)] The notation 'mNep' is used as a mass unit in Equation (4) but is not explicitly defined as a unit; please write 'mNep = 1 Neptune mass' in the text before first use.
- [Section 4.2] The estimate of the rare double-lensing event rate, '0.35 × 10−4/π ∼ 10−5 per stellar lensing event,' would benefit from a brief derivation of the factor π and the Einstein-radius scaling, as the current one-line statement is difficult to follow.
- [Conclusions and Section 4.2] The claim that the 'about half' result is 'testable' could be strengthened by stating the concrete observable prediction, such as the expected fraction of FFP microlensing events that should show a faint stellar counterpart at ~0.1 arcsec, and how that fraction depends on the assumed host-star mass and distance.
Circularity Check
No significant circularity: the central estimate combines independent microlensing occurrence rates with uncalibrated N-body yields; the 'about half' claim is a conditional model estimate, not a derived tautology.
full rationale
The paper's central quantitative step, Eq. (10), is not circular. The coefficients 2 and 3 are detached-planet yields measured from new N-body integrations, and the rates 0.35 and 0.06 are independently observed bound-planet occurrence rates from Zang et al. (2025). Nothing in the simulations is fitted to the Sumi et al. (2023) free-floating-planet rate, and the 'about half' statement is explicitly conditional: Section 4.2 says 'assuming that all known planetary systems are mobilized in scattering.' The self-citations (Lammers et al. 2024 for choosing initial separations that produce the desired instability-time window, and Hadden & Tremaine 2024 for test-particle scaling) are supporting context and do not carry the final claim; neither paper supplies the detached-planet yields. The authors themselves flag the main uncertainties, including that observed multiplicity 'makes the ejection hypothesis less tenable' (Section 4.1) and that inner-system collisions and engulfment are not modeled (Section 4.3). These are limitations and modeling assumptions, not circular reductions. The only notable self-citation is not load-bearing, so the score is 1 rather than 0.
Assumptions & free parameters
free parameters (1)
- Ejection timescale power law (Tej) =
2.9 x 10^7 (mp/mNep)^-1.64 P1,0
assumptions (5)
- domain assumption Planets are treated as point masses with no physical extent; collisions between planets and with the star are ignored.
- ad hoc to paper All known planetary systems are mobilized in scattering when estimating the detached planet rate.
- domain assumption The simulated initial conditions (five equal-mass planets, or one Jupiter plus ten Neptunes, on circular near-coplanar orbits) are representative of the real planetary systems that produce free-floating planets.
- domain assumption Unstable systems evolve toward a final state with one inner planet plus one to three detached planets, and systems not yet converged at 10^8 orbits continue along this path.
- domain assumption The observed occurrence rates of bound Neptune and Jupiter planets around M-dwarfs are accurate and applicable to the same population as the free-floating planets.
invented entities (1)
-
Detached planet population
independent evidence
Cite this review
Pith. "Pith review of Free Floating or Merely Detached?." pith.science (2026). https://pith.science/paper/RL6RZRCW
@misc{pith2026250708968,
author = {Pith},
title = {Pith review of: Free Floating or Merely Detached?},
year = {2026},
howpublished = {\url{https://pith.science/paper/RL6RZRCW}},
note = {Machine review of arXiv:2507.08968}
}
read the original abstract
Microlensing surveys suggest the presence of a surprisingly large population of free-floating planets, with a rate of about two Neptunes per star. The origin of such objects is not known, neither do we know if they are truly unbound or are merely orbiting at large separations from their host stars. Here, we investigate planet-planet scattering as a possible origin through numerical simulations of unstable multi-planet systems. We find that planet ejection by scattering can be slow, often taking more than billions of years for Neptune-mass scatterers orbiting at a few AU and beyond. Moreover, this process invariably delivers planets to orbits of hundreds of AU that are protected from further scattering. We call these ``detached" planets. Under the scattering hypothesis, we estimate that about half of the reported ``free-floating" Neptunes are not free but merely ``detached".
Figures
Figures from the paper (4 more)
Forward citations
Cited by 1 Pith paper
-
The Dynamics of Planetary Ejection
A review of planetary ejection mechanisms and their predicted free-floating planet demographics, concluding that planet-planet scattering, cluster encounters, and binary instabilities likely dominate FFP production.
Reference graph
Works this paper leans on
-
[1]
, " * write output.state after.block = add.period write newline
ENTRY address archivePrefix author booktitle chapter doi edition editor eprint howpublished institution journal key month number organization pages publisher school series title misctitle type volume year version url label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts ...
-
[2]
write newline
" write newline "" before.all 'output.state := FUNCTION format.url url empty "" new.block "" url * "" * if FUNCTION format.eprint eprint empty "" archivePrefix empty "" archivePrefix "arXiv" = new.block " " eprint * " " * new.block " " eprint * " " * if if if FUNCTION format.doi doi empty "" " " doi * " " * if FUNCTION format.pid doi empty eprint empty ur...
-
[3]
thebibliography [1] 20pt to REFERENCES 6pt =0pt \@twocolumntrue 12pt -12pt 10pt plus 3pt =0pt =0pt =1pt plus 1pt =0pt =0pt -12pt =13pt plus 1pt =20pt =13pt plus 1pt \@M =10000 =-1.0em =0pt =0pt 0pt =0pt =1.0em @enumiv\@empty 10000 10000 `\.\@m \@noitemerr \@latex@warning Empty `thebibliography' environment \@ifnextchar \@reference \@latexerr Missing key o...
work page 2017
-
[4]
Adams, F. C., & Laughlin, G. 2003, Icarus, 163, 290, 10.1016/S0019-1035(03)00081-2
-
[5]
Boley, A. C., Payne, M. J., & Ford, E. B. 2012, , 754, 57, 10.1088/0004-637X/754/1/57
-
[6]
Bonnell, I. A., Smith, K. W., Davies, M. B., & Horne, K. 2001, , 322, 859, 10.1046/j.1365-8711.2001.04171.x
arXiv 2001
-
[7]
Bryan, M. L., Knutson, H. A., Lee, E. J., et al. 2019, , 157, 52, 10.3847/1538-3881/aaf57f
-
[8]
Cai, M. X., Kouwenhoven, M. B. N., Portegies Zwart, S. F., & Spurzem, R. 2017, , 470, 4337, 10.1093/mnras/stx1464
Show all 63 references
-
[9]
Coleman, G. A. L. 2024, , 530, 630, 10.1093/mnras/stae903
2024 doi
-
[10]
Coleman , G. A. L., & DeRocco , W. 2025, , 537, 2303, 10.1093/mnras/staf138
2025 doi
-
[11]
1987, , 94, 1330, 10.1086/114571
Duncan , M., Quinn , T., & Tremaine , S. 1987, , 94, 1330, 10.1086/114571
1987 doi
-
[12]
S., Bennett , D
Gaudi , B. S., Bennett , D. P., Udalski , A., et al. 2008, Science, 319, 927, 10.1126/science.1151947
2008 doi
- [13]
-
[14]
K., Hwang, K.-H., et al
Gould, A., Jung, Y. K., Hwang, K.-H., et al. 2022, Journal of Korean Astronomical Society, 55, 173, 10.5303/JKAS.2022.55.5.173
2022 doi
-
[15]
2022, , 164, 179, 10.3847/1538-3881/ac8d01
Hadden, S., & Tamayo, D. 2022, , 164, 179, 10.3847/1538-3881/ac8d01
2022 doi
-
[16]
2024, , 527, 3054, 10.1093/mnras/stad3478
Hadden, S., & Tremaine, S. 2024, , 527, 3054, 10.1093/mnras/stad3478
2024 doi
-
[17]
S., An , J
Han , C., Gaudi , B. S., An , J. H., & Gould , A. 2005, , 618, 962, 10.1086/426115
2005 doi
-
[18]
C., Ford , E
Hsu , D. C., Ford , E. B., & Terrien , R. 2020, , 498, 2249, 10.1093/mnras/staa2391
2020 doi
-
[19]
2008, , 686, 603, 10.1086/590047
Jurić, M., & Tremaine, S. 2008, , 686, 603, 10.1086/590047
2008 doi
-
[20]
A., Raymond , S
Kaib , N. A., Raymond , S. N., & Duncan , M. 2013, , 493, 381, 10.1038/nature11780
2013 doi
-
[21]
2025, arXiv e-prints, arXiv:2507.01109, 10.48550/arXiv.2507.01109
Kapusta , M., Mroz , P., Ryu , Y.-H., et al. 2025, arXiv e-prints, arXiv:2507.01109, 10.48550/arXiv.2507.01109
2025 doi
-
[22]
V., & Vassiliev, N
Kholshevnikov, K. V., & Vassiliev, N. N. 1999, Celestial Mechanics and Dynamical Astronomy, 75, 67, 10.1023/A:1008384004589
1999 doi
-
[23]
2021, , 162, 15, 10.3847/1538-3881/abfc4a
Kim, H.-W., Hwang, K.-H., Gould, A., et al. 2021, , 162, 15, 10.3847/1538-3881/abfc4a
2021 doi
-
[24]
P., et al
Koshimoto, N., Sumi, T., Bennett, D. P., et al. 2023, , 166, 107, 10.3847/1538-3881/ace689
2023 doi
-
[25]
2024, , 972, 53, 10.3847/1538-4357/ad5be6
Lammers , C., Hadden , S., & Murray , N. 2024, , 972, 53, 10.3847/1538-4357/ad5be6
2024 doi
-
[26]
1997, , 317, L75
Laskar , J. 1997, , 317, L75
1997
-
[27]
Laskar, J., & Petit, A. C. 2017, , 605, A72, 10.1051/0004-6361/201630022
2017 doi
-
[28]
Laughlin, G., & Adams, F. C. 1998, , 508, L171, 10.1086/311736
1998 doi
- [29]
-
[30]
Lin, D. N. C., & Ida, S. 1997, , 477, 781, 10.1086/303738
1997 doi
-
[31]
2011, , 739, 31, 10.1088/0004-637X/739/1/31
Lithwick, Y., & Wu, Y. 2011, , 739, 31, 10.1088/0004-637X/739/1/31
2011 doi
-
[32]
B., & Heggie , D
Malmberg , D., Davies , M. B., & Heggie , D. C. 2011, , 411, 859, 10.1111/j.1365-2966.2010.17730.x
2011
-
[33]
1999, Icarus, 141, 341, 10.1006/icar.1999.6174
Malyshkin, L., & Tremaine, S. 1999, Icarus, 141, 341, 10.1006/icar.1999.6174
1999
-
[34]
1991, , 374, L37, 10.1086/186066
Mao , S., & Paczynski , B. 1991, , 374, L37, 10.1086/186066
1991 doi
-
[35]
Moe , M., & Kratter , K. M. 2021, , 507, 3593, 10.1093/mnras/stab2328
2021 doi
-
[36]
T., Crepp, J
Montet, B. T., Crepp, J. R., Johnson, J. A., Howard, A. W., & Marcy, G. W. 2014, , 781, 28, 10.1088/0004-637X/781/1/28
2014 doi
- [37]
-
[38]
2017, Nature, 548, 183, 10.1038/nature23276
Mróz, P., Udalski, A., Skowron, J., et al. 2017, Nature, 548, 183, 10.1038/nature23276
2017 doi
-
[39]
P., et al
Mróz, P., Udalski, A., Bennett, D. P., et al. 2019, Astronomy & Astrophysics, 622, A201, 10.1051/0004-6361/201834557
2019 doi
-
[40]
2020, , 903, L11, 10.3847/2041-8213/abbfad
Mróz, P., Poleski, R., Gould, A., et al. 2020, , 903, L11, 10.3847/2041-8213/abbfad
2020 doi
-
[41]
D., Pascucci , I., & Apai , D
Mulders , G. D., Pascucci , I., & Apai , D. 2015, , 814, 130, 10.1088/0004-637X/814/2/130
2015 doi
-
[43]
Pham , D., Rein , H., & Spiegel , D. S. 2024, The Open Journal of Astrophysics, 7, 1, 10.21105/astro.2401.02849
2024 arXiv
-
[44]
A., & Ford, E
Rasio, F. A., & Ford, E. B. 1996, Science, 274, 954. https://www.jstor.org/stable/2891291
1996
-
[45]
N., Izidoro, A., & Kaib, N
Raymond, S. N., Izidoro, A., & Kaib, N. A. 2023, , 524, L72, 10.1093/mnrasl/slad079
2023 doi
-
[46]
Rein, H., & Liu, S. F. 2012, , 537, A128, 10.1051/0004-6361/201118085
2012 doi
-
[47]
Rein, H., & Spiegel, D. S. 2015, , 446, 1424, 10.1093/mnras/stu2164
2015 doi
-
[48]
2001, , 122, 432, 10.1086/321121
Reipurth , B., & Clarke , C. 2001, , 122, 432, 10.1086/321121
2001 doi
-
[49]
2021, , 913, 104, 10.3847/1538-4357/abf8a7
Rodet, L., Su, Y., & Lai, D. 2021, , 913, 104, 10.3847/1538-4357/abf8a7
2021 doi
-
[50]
2021, , 161, 126, 10.3847/1538-3881/abd55f
Ryu, Y.-H., Mróz, P., Gould, A., et al. 2021, , 161, 126, 10.3847/1538-3881/abd55f
2021 doi
-
[51]
Safronov , V. S. 1972, Evolution of the protoplanetary cloud and formation of the earth and planets
1972
-
[52]
2023, , 165, 250, 10.3847/1538-3881/acc865
Sikora, J., Rowe, J., Barat, S., et al. 2023, , 165, 250, 10.3847/1538-3881/acc865
2023 doi
-
[53]
2018, , 155, 75, 10.3847/1538-3881/aaa19b
Silsbee, K., & Tremaine, S. 2018, , 155, 75, 10.3847/1538-3881/aaa19b
2018 doi
-
[54]
P., et al
Sumi, T., Kamiya, K., Bennett, D. P., et al. 2011, Nature, 473, 349, 10.1038/nature10092
2011 doi
-
[55]
P., et al
Sumi, T., Koshimoto, N., Bennett, D. P., et al. 2023, , 166, 108, 10.3847/1538-3881/ace688
2023 doi
-
[56]
J., Sheehan , P
Tobin , J. J., Sheehan , P. D., Megeath , S. T., et al. 2020, , 890, 130, 10.3847/1538-4357/ab6f64
2020 doi
-
[57]
X., & McMillan, S
van Elteren, A., Portegies Zwart, S., Pelupessy, I., Cai, M. X., & McMillan, S. L. W. 2019, , 624, A120, 10.1051/0004-6361/201834641
2019 doi
-
[58]
Veras, D., & Raymond, S. N. 2012, , 421, L117, 10.1111/j.1745-3933.2012.01218.x
2012
-
[59]
C., Mustill , A
Veras , D., Wyatt , M. C., Mustill , A. J., Bonsor , A., & Eldridge , J. J. 2011, , 417, 2104, 10.1111/j.1365-2966.2011.19393.x
2011
-
[60]
J., & Marzari, F
Weidenschilling, S. J., & Marzari, F. 1996, Nature, 384, 619, 10.1038/384619a0
1996 doi
-
[61]
P., & Zinnecker , H
Whitworth , A. P., & Zinnecker , H. 2004, , 427, 299, 10.1051/0004-6361:20041131
2004 doi
-
[62]
2024, , 970, 97, 10.3847/1538-4357/ad4f81
Yu, F., & Lai, D. 2024, , 970, 97, 10.3847/1538-4357/ad4f81
2024 doi
-
[63]
K., Yee , J
Zang , W., Jung , Y. K., Yee , J. C., et al. 2025, Science, 388, 400, 10.1126/science.adn6088
2025 doi
-
[64]
2018, , 156, 92, 10.3847/1538-3881/aad22a
Zhu , W., & Wu , Y. 2018, , 156, 92, 10.3847/1538-3881/aad22a
2018 doi
Reviewed August 6, 2026 · model on record in the stance chip above.
Discussion (0). Sign in to comment.