Secure Identification of Free-Floating Planets

Cheongho Han

REVIEW

Secure Identification of Free-Floating Planets

Cheongho Han

Astrophysics · Submitted 2 Mar 2006 · 22 references

Pith's verdict: A sound idea with the right physical instinct, but the paper's own numbers are internally inconsistent and need a careful re-derivation.

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arXiv IDastro-ph/0603040 (version 1)

Pith ID656NUT3R, Pith's permanent record of this paper and its review

CitationCheongho Han. "Secure Identification of Free-Floating Planets." arXiv:astro-ph/0603040 (2006). https://pith.science/paper/656NUT3R

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Editor's note

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Cheongho Han has the right physical idea here: astrometric microlensing decays as u^-1 while photometric decays as u^-4, so a wide-separation planet's host star should leave a detectable centroid shift even when its photometric signal is negligible. That is a genuinely useful way to distinguish bound wide-separation planets from free-floating ones, and it is more complete than the photometric methods in Han et al. (2005) or the bright-primary method in Bennett & Rhie (2002). The paper is clearly written, the basic formalism is standard, and the application to this degeneracy is new as far as I know. No fitted parameters, no tuning, so no circularity concerns.

But the paper's quantitative feasibility demonstration is internally inconsistent, and the stress-test note is correct. Equation (12) gives δ ≈ 40 μas at r_perp = 10 AU, but with the author's adopted r_E = 2.45 AU and θ_E = 410 μas, Eq. (7) in the u >> 1 limit gives δ ≈ θ_E r_E / r_perp ≈ 100 μas at 10 AU. The factor 2.5 discrepancy looks like a typo (r_E = 1 AU instead of 2.45), but it matters because the claimed ~100 AU reach depends on it. Eq. (13) similarly does not follow from the SIM specification quoted just above it: at V = 19 and δ_th = 4 μas it evaluates to ~50 min, not <10 min, and it contradicts the stated σ = 0.4 μas in 0.5 min at V = 12 (which would give ~3 min for a 4 μas threshold at V=19). So as written, the paper does not give a consistent demonstration of the headline numbers.

The encouraging part is that fixing these errors strengthens the method rather than weakening it. A corrected Eq. (12) gives larger shifts at a given separation, so the reach is greater (roughly 250 AU for a 4 μas floor), and a corrected exposure budget is shorter than the text's <10 min. The physical principle survives intact; the conclusions are qualitatively right, but the quantitative claims need to be re-derived.

I would send this to peer review. The idea is worth having on record, and the errors are straightforward to fix. It is aimed at microlensing practitioners and astrometry missions; readers outside the subfield may skip it. I'd cite it once the numbers are corrected, and it could be a useful reading-group discussion on how easy it is to propagate minor arithmetic mistakes into feasibility claims.

Referee report

Significance

If the quantitative feasibility is correctly established, the paper would make a valuable contribution to microlensing planet detection methodology. The physical principle is sound: astrometric lensing has a much longer spatial reach than photometric lensing, and this asymmetry is a natural way to probe wide-separation companions. The method has clear advantages over earlier proposals: it does not depend on primary brightness, it measures the angular Einstein radius, and it is in principle applicable to nearly all separations within a large range. The paper also makes a falsifiable prediction, namely a primary-induced centroid shift in isolated planetary events. However, the central demonstration depends on two equations that are internally inconsistent with each other and with the standard single-lens formalism, so the headline feasibility numbers are not currently supported. The conceptual idea merits publication after a careful and complete correction of the quantitative derivation.

Major objections (3)

[§4, Eq. (12)] Equation (12) is inconsistent with the standard single-lens astrometric shift given by the paper's own Eq. (7). For the adopted parameters (r_E = 2.45 AU, theta_E = 410 microas), the u >> 1 limit of Eq. (7) is delta ~ theta_E / u = theta_E * r_E / r_perp, which gives delta ~ 100 microas at r_perp = 10 AU and ~10 microas at r_perp = 100 AU. Equation (12) instead states 40 microas and 4 microas, respectively, a factor ~2.5 discrepancy. This is not a minor rounding issue: the threshold delta_th = 4 microas used in Figure 2 and in the exposure-time argument is exactly the value from Eq. (12). Please correct Eq. (12) or provide a derivation that reconciles it with Eq. (7).
[§4, Eq. (13) and text] The exposure-time formula and the accompanying claim 'texp <~ 10 min for V ~ 19' are mutually inconsistent. Evaluating Eq. (13) at V = 19 and delta_th = 4 microas gives texp ~ 0.08 * 10^{0.4*7} = 50.5 min, not <~ 10 min. Moreover, Eq. (13) does not follow from the SIM specification quoted immediately before it: with sigma ~ 0.4 microas at V = 12 in 0.5 min, the time needed to reach sigma = delta_th = 4 microas at V = 12 would be 0.005 min, not 0.08 min (the coefficient 0.08 corresponds to sigma ~ 1 microas, not 4). The authors should recompute the exposure time from the stated SIM noise model and then re-examine the condition 'V <~ 19, texp <~ 10 min' with the corrected delta.
[§4, Figure 2 and reach claim] The threshold delta_th = 4 microas defines the claimed reach of r_perp ~ 100 AU through Eq. (12). Because Eq. (12) is incorrect, the quantitative reach is unsupported as written. With the correct scaling from Eq. (7), the primary-induced shift at 100 AU is ~10 microas rather than 4 microas, which would make the threshold easier to reach; however, the exposure-time estimate must also be corrected consistently (see previous comment). The authors should recompute the maximum separation reachable under the stated observational conditions and report the corrected numbers in the abstract, §4, and Figure 2.

Minor objections (4)

[References] The reference 'Miral-Escude, J. 1996, A&A, 470, L113' is missing a syllable; the correct name is Miralda-Escudé, as spelled in the text of §4.
[Throughout] The instrument is referred to as the 'Space Interferometric Mission' (SIM) in §4; the standard name is the 'Space Interferometry Mission'.
[§4, notation] In the paragraph after Eq. (9), s is described as a 'position vector of the planet from the star' but is used as a scalar separation elsewhere (e.g., Eq. (8) and the text 's = 10, 20, 30'). Please clarify the notation to avoid confusion.
[Header] The manuscript header shows 'DRAFT VERSION SEPTEMBER 28, 2018', which appears to be an artifact of the submission pipeline. This should be cleaned up if the paper is revised.

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The paper

The paper claims

Free-floating planets can be securely identified by astrometric follow-up of isolated microlensing events.

The reading

Microlensing is the only proposed way to detect free-floating planets, but isolated planetary lensing events can also be caused by planets on wide orbits around host stars too faint to show up in photometry. This paper establishes that astrometric follow-up observations can settle the question: the host star's gravitational deflection creates a centroid shift that decays only as $1/u$, so it remains measurable at projected separations of tens to roughly 100 AU, long after any photometric signal has vanished. Working through the geometry for typical bulge events, the paper shows that a few observations of $V \lesssim 19$ sources with high-precision interferometry should detect the primary's roughly 4 microarcsecond shift. If correct, this gives a nearly complete way to distinguish bound wide-separation planets from genuinely free-floating ones, independent of the primary's brightness.

What carries the argument

The load-bearing object is the astrometric centroid shift of a lensed source image, $\delta = u\,\theta_E/(u^2+2)$ for a single lens, whose large-separation limit is $\delta \to \theta_E/u$. Photometric magnification approaches unity as $1/u^4$, but the centroid shift falls only as $1/u$, so when a source passes through a planet's tiny Einstein ring while remaining far from the primary's much larger Einstein ring, the primary's astrometric signature is still substantial. The discriminant is therefore the trajectory of the image centroid: a nearly circular, primary-dominated motion with a short elliptical perturbation when the source crosses the planetary caustic region.

Extended reading notes

Core claim

The paper claims that isolated microlensing events can be unambiguously classified by astrometric follow-up: a wide-separation planet's host star produces a detectable centroid shift in the source image even when the photometric perturbation is negligible, because astrometric lensing decays as $\delta \to \theta_E/u$ while photometric magnification falls as $A \to 1+2/u^4$. For a typical bulge lens with mass $0.5\,M_\odot$, $D_L=6$ kpc, $D_S=8$ kpc, and $\theta_E=410$ microarcseconds, the primary-induced shift at projected separation $r_\perp$ is approximately $\delta \simeq 40\,\mu\mathrm{as}\,(\theta_E/410\,\mu\mathrm{as})(r_\perp/10\,\mathrm{AU})^{-1}$. Since shifts near 4 microarcseconds persist out to about 100 AU, a handful of exposures with the Space Interferometry Mission on sources brighter than $V\sim 19$ should reveal the primary's slow circular centroid motion, cleanly separating bound wide-separation planets from true free-floating planets and also yielding a measurement of the primary's angular Einstein radius.

Load-bearing premise

The whole identification scheme rests on the assumption that a centroid shift of about 4 microarcseconds from a host star at roughly 100 AU can actually be measured for $V\sim 19$ sources within the quoted short exposures, so that the host's astrometric signal remains visible out to that separation.

Editorial extensions

If this is right

Bound wide-separation planets with projected separations up to roughly 100 AU can be identified, far beyond the roughly 20 AU reach of the previously proposed photometric methods.
The method does not depend on the primary's brightness, so it can find planets around faint or even dark host stars.
Measuring the primary-induced centroid shift yields the angular Einstein radius $\theta_E$, which does not depend on the lens-source transverse speed, thereby tightening constraints on the primary mass and hence the planet mass.
Astrometric follow-up does not need to be prompt because the centroid motion is slow when the source is far from the primary, easing scheduling for interferometric observations.
If an isolated event shows no detectable primary-induced shift after astrometric follow-up, it can be classified as a genuine free-floating planet rather than remaining ambiguous.

Reading between the lines

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

The same astrometric-tail argument should transfer to any high-precision astrometric instrument operating alongside the next-generation photometric surveys, so the method is not tied to one specific mission.
The technique could be extended to reveal faint stellar or substellar companions at separations far beyond the lensing zone, not only planetary-mass companions, because the $1/u$ astrometric tail is mass-independent in form.
Archived astrometric measurements that happen to cover an isolated event could be re-examined after the photometric event is identified, since the primary-induced signal persists and drifts slowly on the sky.

Original abstract

Among the methods proposed to detect extrasolar planets, microlensing is the only technique that can detect free-floating planets. Free-floating planets are detected through the channel of short-duration isolated lensing events. However, if a seemingly isolated planetary event is detected, it is difficult to firmly conclude that the event is caused by a free-floating planet because a wide-separation planet can also produce an isolated event. There were several methods proposed to break the degeneracy between the isolated planetary events produced by the free-floating and wide-separation planets, but they are incomplete. In this paper, we show that free-floating planets can be securely identified by conducting astrometric follow-up observations of isolated events to be detected in future photometric lensing surveys by using high-precision interferometers to be operated contemporarily with the photometric surveys. The method is based on the fact that astrometric lensing effect covers much longer range of the lens-source separation than the photometric effect. We demonstrate that several astrometric follow-up observations of isolated planetary events associated with source stars brighter than $V\sim 19$ by using the {\it Space Interferometry Mission} with an exposure time of $\lesssim 10 {\rm min}$ for each observation will make it possible to measure the centroid shift induced by primaries with projected separations up to $\sim 100 {\rm AU}$. Therefore, the proposed method is far more complete than previously proposed methods that are flawed by the limited applicability only to planets with projected separations $\lesssim 20 {\rm AU}$ or planets accompanied by bright primaries.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 6,516 input tokens.

What the abstract itself states, as probabilities: claims first 0.11 · states limitation 0.55 · human participants 0.01 · controlled comparison 0.11 · randomized assignment 0.01 · states quantitative result 0.96 · compares with prior results 0.97 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; show every role.

Among the methods proposed to detect extrasolar planets, microlensing is the only technique that can detect free-floating planets. Free-floating planets are detected through the channel of short-duration isolated lensing events. However, if a seemingly isolated planetary event is detected, it is difficult to firmly conclude that the event is caused by a free-floating planet because a wide-separation planet can also produce an isolated event. There were several methods proposed to break the degeneracy between the isolated planetary events produced by the free-floating and wide-separation planets, but they are incomplete. In this paper, we show that free-floating planets can be securely identified by conducting astrometric follow-up observations of isolated events to be detected in future photometric lensing surveys by using high-precision interferometers to be operated contemporarily with the photometric surveys. The method is based on the fact that astrometric lensing effect covers much longer range of the lens-source separation than the photometric effect. We demonstrate that several astrometric follow-up observations of isolated planetary events associated with source stars brighter than $V\sim 19$ by using the {\it Space Interferometry Mission} with an exposure time of $\lesssim 10 {\rm min}$ for each observation will make it possible to measure the centroid shift induced by primaries with projected separations up to $\sim 100 {\rm AU}$. Therefore, the proposed method is far more complete than previously proposed methods that are flawed by the limited applicability only to planets with projected separations $\lesssim 20 {\rm AU}$ or planets accompanied by bright primaries.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	Among the methods proposed to detect extrasolar planets, microlensing is the only technique that can detect free-floating planets.	background 1.00	0.05	0.03	asserted 1.00
s1	Free-floating planets are detected through the channel of short-duration isolated lensing events.	background 0.99	0.06	0.03	asserted 1.00
s2	However, if a seemingly isolated planetary event is detected, it is difficult to firmly conclude that the event is caused by a free-floating planet because a wide-separation planet can also produce an isolated event.	background 0.58	0.05	0.02	asserted 0.74
s3	There were several methods proposed to break the degeneracy between the isolated planetary events produced by the free-floating and wide-separation planets, but they are incomplete.	gap 0.86	0.04	0.02	asserted 1.00
s4	In this paper, we show that free-floating planets can be securely identified by conducting astrometric follow-up observations of isolated events to be detected in future photometric lensing surveys by using high-precision interferometers to be operated contemporarily with the photometric surveys.	contribution 0.88	0.70	0.94	asserted 0.99
s5	The method is based on the fact that astrometric lensing effect covers much longer range of the lens-source separation than the photometric effect.	method 0.77	0.18	0.05	asserted 1.00
s6	We demonstrate that several astrometric follow-up observations of isolated planetary events associated with source stars brighter than $V\sim 19$ by using the {\it Space Interferometry Mission} with an exposure time of $\lesssim 10 {\rm min}$ for each observation will make it possible to measure the centroid shift induced by primaries with projected separations up to $\sim 100 {\rm AU}$.	method 0.57	0.63	0.10	asserted 0.72
s7	Therefore, the proposed method is far more complete than previously proposed methods that are flawed by the limited applicability only to planets with projected separations $\lesssim 20 {\rm AU}$ or planets accompanied by bright primaries.	interpretation 0.76	0.42	0.29	asserted 1.00

What kind of paper the classifier reports this is: method or tool 1.00 · evidence from argument 0.67.

What was examined, and what was not

sources read

arXiv listing abstract v1

not examined

full text
figures and tables
references and citing sentences
relations to other papers

levels run

P abstract

not run

S sections
T body sentences
C citations
R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

References

Works cited (22)

[1]

Albrow, M., et al. 1998, ApJ, 509, 687

View on Pith 1998
[2]

Alcock, C., et al. 1997, ApJ, 491, 436

View on Pith 1997
[3]

Beaulieu, J.-P., et al. 2006, Nature, 439, 437

View on Pith 2006
[4]

Bennett, D. P., & Rhie. S. H. 2002, ApJ, 574, 985

View on Pith 2002
[5]

Bond, I. A., et al. 2001, MNRAS, 327, 868

View on Pith 2001
[6]

Bond, I. A., et al. 2004, ApJ, 606, L155

View on Pith 2004
[7]

Boden, A. F., Shao, M., & Van Buren, D. 1998, ApJ, 502, 538

View on Pith 1998
[8]

Gould, A. 2001, PASP, 113, 903

View on Pith 2001
[9]

Gould, A. 2005, New Astronomy Reviews, 49, 424

View on Pith 2005
[10]

Gould, A., & Loeb, A. 1992,ApJ, 396, 104

View on Pith 1992
[11]

Han, C., Chung, S.-J., Kim, D., Park, B.-G., Ryu, Y .-H., Kang, S., & Lee, D. 2004, ApJ, 604, 372

View on Pith 2004
[12]

Han, C., & Kang, Y . W. 2003, ApJ, 596, 1320

View on Pith 2003
[13]

Han, C., & Kim, T.-W. 1999, MNRAS, 305, 795

View on Pith 1999
[14]

Han, C., Gaudi, B. S., An, J. H., & Gould, A. 2005, ApJ, 618, 962 Høg, E., Novikov, I. D., & Polnarev, A. G. 1995, A&A, 294, 287

View on Pith 2005
[15]

Mao, S., & Paczy´nski, B. 1991, ApJ, 374, L37

View on Pith 1991
[16]

Miral-Escude, J. 1996, ApJ, 470, L113

View on Pith 1996
[17]

Miyamoto, M., & Yoshii, Y . 1995, AJ, 110, 1427 Paczy´nski, B. 1986, ApJ, 304, 1 Paczy´nski, B. 1998, ApJ, 494, L23

View on Pith 1995
[18]

Udalski, A., et al. 2005, ApJ, 628, L109

View on Pith 2005
[19]

Udalski, A., Szyma´nski, M., Kału˙ zny, J., Kubiak, M., Mateo, M., Krzemi´nski, W., & Paczy´nski, B. 1994, Acta Astron., 44, 227

View on Pith 1994
[20]

Walker, M. A. 1995, ApJ, 453, 37

View on Pith 1995
[21]

Witt, H. J. 1990, A&A, 236, 311

View on Pith 1990
[22]

Yoo, J., et al. 2004, ApJ, 616, 1204

View on Pith 2004

Discussion

Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.

[1] [1]

Albrow, M., et al. 1998, ApJ, 509, 687

View on Pith 1998

[2] [2]

Alcock, C., et al. 1997, ApJ, 491, 436

View on Pith 1997

[3] [3]

Beaulieu, J.-P., et al. 2006, Nature, 439, 437

View on Pith 2006

[4] [4]

Bennett, D. P., & Rhie. S. H. 2002, ApJ, 574, 985

View on Pith 2002

[5] [5]

Bond, I. A., et al. 2001, MNRAS, 327, 868

View on Pith 2001

[6] [6]

Bond, I. A., et al. 2004, ApJ, 606, L155

View on Pith 2004

[7] [7]

Boden, A. F., Shao, M., & Van Buren, D. 1998, ApJ, 502, 538

View on Pith 1998

[8] [8]

Gould, A. 2001, PASP, 113, 903

View on Pith 2001

[9] [9]

Gould, A. 2005, New Astronomy Reviews, 49, 424

View on Pith 2005

[10] [10]

Gould, A., & Loeb, A. 1992,ApJ, 396, 104

View on Pith 1992

[11] [11]

Han, C., Chung, S.-J., Kim, D., Park, B.-G., Ryu, Y .-H., Kang, S., & Lee, D. 2004, ApJ, 604, 372

View on Pith 2004

[12] [12]

Han, C., & Kang, Y . W. 2003, ApJ, 596, 1320

View on Pith 2003

[13] [13]

Han, C., & Kim, T.-W. 1999, MNRAS, 305, 795

View on Pith 1999

[14] [14]

Han, C., Gaudi, B. S., An, J. H., & Gould, A. 2005, ApJ, 618, 962 Høg, E., Novikov, I. D., & Polnarev, A. G. 1995, A&A, 294, 287

View on Pith 2005

[15] [15]

Mao, S., & Paczy´nski, B. 1991, ApJ, 374, L37

View on Pith 1991

[16] [16]

Miral-Escude, J. 1996, ApJ, 470, L113

View on Pith 1996

[17] [17]

Miyamoto, M., & Yoshii, Y . 1995, AJ, 110, 1427 Paczy´nski, B. 1986, ApJ, 304, 1 Paczy´nski, B. 1998, ApJ, 494, L23

View on Pith 1995

[18] [18]

Udalski, A., et al. 2005, ApJ, 628, L109

View on Pith 2005

[19] [19]

Udalski, A., Szyma´nski, M., Kału˙ zny, J., Kubiak, M., Mateo, M., Krzemi´nski, W., & Paczy´nski, B. 1994, Acta Astron., 44, 227

View on Pith 1994

[20] [20]

Walker, M. A. 1995, ApJ, 453, 37

View on Pith 1995

[21] [21]

Witt, H. J. 1990, A&A, 236, 311

View on Pith 1990

[22] [22]

Yoo, J., et al. 2004, ApJ, 616, 1204

View on Pith 2004