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The sensitivity of TESS to transiting planets in TOIs with close-in stellar companions

T0 review · 3 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read TESS is far less sensitive to transiting planets in unresolved stellar binaries than around single stars, with the worst sensitivity for planets orbiting the fainter star when the magnitude difference is large.

desk verdict A useful, well-executed quantification of TESS's reduced sensitivity to planets in unresolved binaries, with a real caveat: the lookup tables are conditional on a transit having been detected and need an injection-recovery calibration before use as occurrence-rate corrections. read the letter →

arxiv 2608.13527 v1 pith:SLJOCU5C submitted 2026-08-13 astro-ph.EP astro-ph.SR

Colin Littlefield , Kathryn V. Lester , David R. Ciardi , Steve B. Howell This is my paper · ORCID
classification astro-ph.EPastro-ph.SR
keywords TESStransitingexoplanetsstellarbinariesspeckleinterferometrydetectionsensitivitytransitsignal-to-noiseplanetoccurrenceratesdilution
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

TESS's ability to find transiting exoplanets is substantially degraded when the host star has an unresolved stellar companion, and the loss of sensitivity is most severe for planets that orbit the fainter star. The paper quantifies this by scaling the observed transit signal-to-noise ratios of 2052 single-star and 188 binary TOIs to hypothetical planets of any radius and period. For binaries in which the stars differ by four or more magnitudes, only about half of close-in gas giants and about ten percent of Neptune-sized planets transiting the secondary would pass the detection thresholds of the TESS pipelines. The resulting sensitivity grids give future occurrence-rate studies a way to correct for the bias against small planets in binaries.

What carries the argument

The central tool is the SNR scaling relation $SNR_{\rm hyp} = SNR_{\rm obs}\, (R_{\rm hyp}/R_{\rm obs})^2\, (P_{\rm hyp}/P_{\rm obs})^{-1/3}$ (Eq. 2), which maps each detected planet's catalog signal-to-noise ratio to hypothetical planets of other radii and periods, combined with a dilution-correction factor $X_r$ (Eq. 3) that rescales the planet radius in binaries according to which star is transited. Applied to thousands of TOIs, these relations convert a list of detected planets into grids of detection fractions: the fraction of systems in which a planet of a given radius and period would cross the pipeline threshold (SNR ≥ 7.1 for SPOC, ≥ 9 for QLP).

What would settle it

Run injection-recovery simulations on the actual TESS light curves of the binary TOIs, planting synthetic transits of known depth, period, and duration on the secondary star, and compare the recovered detection fraction per Δmag bin with the grids' predictions; a systematic mismatch of more than about ten percent in any bin would invalidate the SNR-scaling method.

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Extended reading notes

Core claim

Using the transit SNR, radius, and period of every TOI in their speckle-vetted sample, the authors calculate the detection fraction of hypothetical planets as a function of radius and orbital period. They find that TESS sensitivity to transiting planets in unresolved binaries depends sharply on which star hosts the planet: for magnitude differences Δmag ≥ 4, roughly half of close-in gas giants and only about ten percent of Neptune-sized planets would be detected if they transit the fainter star, whereas sensitivity to planets transiting the brighter star is nearly identical to that for single-star hosts. For single stars, sensitivity improves with later spectral type, from about 50 percent for a 5 R⊕ planet in a 10-day orbit around an F dwarf to about 90 percent around an M dwarf.

Load-bearing premise

The analysis assumes that the sample of already-detected TOIs with speckle observations is representative enough that scaling each detected system's catalog transit SNR to hypothetical planets yields the true detection probability of TESS in that system.

Editorial extensions

If this is right

  • Occurrence-rate studies that use TESS planets in binaries must divide observed counts by these sensitivity grids; otherwise they will systematically underestimate the population of small planets around secondary stars.
  • The missing radius valley in binary-host planets reported previously may be partly a detection artifact rather than a purely astrophysical difference.
  • For binaries with Δmag ≥ 4, the practical assumption that the planet transits the primary is justified, but it leaves the planetary content of high-Δmag binaries essentially unknown.
  • The grids can be interpolated to estimate TESS sensitivity for any TOI of known spectral type, radius, and period, not just the sample stars.

Reading between the lines

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

  • The same dilution physics applies to any transit survey with unresolved stellar companions, so the approach and grids could be adapted to Kepler and future missions like PLATO.
  • The sample's lack of near-equal binaries at 0.4–1.2 arcsec separation points to a selection effect in speckle follow-up; a completeness correction for that gap would refine the Δmag<1 grid.
  • The SNR scaling assumes a fixed transit chord, but inclined orbits change transit duration with period; full light-curve injection tests on binary TOIs would test how much this approximation matters.
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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

3 major / 6 minor

Summary. This paper quantifies how unresolved stellar companions affect TESS's ability to detect transiting planets by constructing radius-period sensitivity grids from 2052 single-star TOIs and 188 binary TOIs with speckle observations. For each TOI, the catalog transit SNR is scaled to hypothetical planet radii and periods via Eq. (2), compared against pipeline detection thresholds (7.1 for SPOC, 9 for QLP), and, for binaries, the planet radius is corrected for dilution using Eq. (3). The resulting detection fractions are presented as functions of spectral type for single stars and of magnitude difference for binaries, with the key finding that planets transiting the fainter secondary are strongly suppressed at large Delta-mag. The grids are released as machine-readable lookup tables.

Significance. If the calibration is valid, the paper provides a valuable community resource: it extends earlier injection-recovery work (Ziegler et al. 2021) by explicitly treating secondary-host transits and by delivering interpolable sensitivity grids. The qualitative conclusion—that TESS is substantially less sensitive to small planets in unresolved binaries, especially around the secondary—is physically expected and robust. The paper also draws a useful connection to the observed dearth of secondary-host systems in Lester et al. (2022). However, the absolute detection fractions are conditioned on the sample of already-detected TOIs, which limits their use as occurrence-rate completeness corrections without a target-star denominator.

major comments (3)
  1. [§3.3, Eq. (2); §4.1] The detection fractions are conditional on a TOI already having been detected and vetted in that system. Since all 2052 single and 188 binary systems were selected because they contain a detected transiting planet candidate, the fraction of grids exceeding threshold estimates P(SNR_hyp > threshold | TOI present), not P(planet detectable | star observed by TESS). Stars that were observed but produced no TOI are absent from the denominator, and because unresolved binaries are harder to detect in, the binary subsample is a stronger-selected set than the single-star subsample. Consequently, headline numbers such as ~90% for a 5 R_Earth, 10-day planet around an M dwarf (Fig. 7) and ~10% for Neptune-sized secondary hosts at Delta-mag >= 4 (Fig. 9) are not unbiased sensitivity estimates and are unsafe as occurrence-rate corrections without a target-star denominator. The qualitative conclusion that secondary-host transits are strongly suppressed is physically expected and likely robust, but the calibration of the lookup tables needs an injection-recovery test or a denominator built from the full TESS target list before the grids are released as a community resource. The caveat in §4.1 that the calculations do not account for orbital geometry does not address this conditionality.
  2. [§4.1, Fig. 8] The application in Fig. 8 divides observed radius-period histograms by the sensitivity grids to produce 'sensitivity-corrected' period-radius distributions. This usage treats the grids as standard survey completeness corrections. Given the conditional-on-detection issue above, the corrected distributions inherit the selection bias of the TOI catalog and of the speckle-observed subset (targets selected for follow-up), so they are not a faithful estimate of the underlying period-radius distribution. The authors should either remove this application or clearly reframe it as a relative comparison within the detected TOI population.
  3. [§4.2, Fig. 9] The quantitative detection fractions in Fig. 9 are quoted without uncertainties (e.g., 'only about half' and 'about ten percent' at Delta-mag >= 4, based on N=43 systems, and N=19 for Delta-mag < 1). Since these numbers are central to the paper's quantitative claims and are intended for external use, the grids should be accompanied by uncertainties, for example binomial confidence intervals on the detection fractions or bootstrap resampling over systems. Without such uncertainties, the significance of differences between spectral-type and Delta-mag bins cannot be assessed.
minor comments (6)
  1. [§2] The criterion for a star to be considered 'vetted' with the speckle imagers is not defined (number of observations, detection limits, filters); please specify it so that the sample can be reproduced.
  2. [§2] The DBSCAN clustering parameters (epsilon and min_samples) are not reported; for reproducibility, state them and the distance metric used.
  3. [§3.2] The conversion from speckle Delta-mag to secondary stellar radius through the Pecaut & Mamajek (2013) sequence should state which photometric band is used and whether the sequence is applied iteratively; the current description is ambiguous.
  4. [§3.1, Eq. (2)] Eq. (2) is applied over the full 1-20 R_Earth radius interval even when the observed radius is near one end of this range; extreme extrapolations could be clipped or flagged to avoid giving undue weight to very large scaling factors.
  5. [§3.1, footnote 4] The footnote mentions 14 TOIs with SNR<7.1; please state whether any of these are in the sample and how the detection threshold is applied to them.
  6. [Fig. 4] The two panels have different x-axis scales, which makes the comparison of the primary- and secondary-host X_r distributions less direct; a shared axis or an inset would help the reader.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity; the sensitivity grids are computed from an external SNR scaling relation and fixed pipeline thresholds, not from fitted parameters or self-referential definitions.

full rationale

The paper's central claim—that unresolved binaries, especially faint secondaries, suppress TESS sensitivity to transiting planets—is derived by applying an externally established scaling relation (Eq. 2, Ciardi et al. 2013) to catalog transit SNRs, with fixed pipeline detection thresholds (7.1 for SPOC, 9 for QLP) and a geometric dilution correction (Eq. 3). No parameter is fitted to the outcome being 'predicted'; the detection fractions are direct transforms of measured SNRs and external thresholds. Self-citations to co-authored works supply the scaling law and dilution formalism, but these are parameter-free results with stated assumptions that do not themselves contain the binary-sensitivity conclusion; they constitute independent support rather than circular load-bearing. The chief limitation is statistical, not circular: the sample consists of already-detected TOIs, so the grids estimate detectability conditional on a planet having been found and may be biased for occurrence-rate corrections. That is a correctness and selection-bias concern, not a reduction of the result to its inputs by construction. The qualitative suppression of secondary-host transits follows from the physical dilution model, and the quantitative fractions are computed, not assumed; hence no circular step is exhibited.

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

The central calculation introduces no fitted constants: catalog SNR, radius, period, and magnitude difference are used as measured inputs; scaling exponents are taken from Ciardi et al. (2013); detection thresholds are pipeline values. All additional modeling choices are listed as axioms. The radius-correction factors X_r are computed from magnitude differences and main-sequence relations, not fit to the data.

assumptions (5)
  • domain assumption Transit SNR scales as (R_p/R_star)^2 times P^(-1/3) at fixed noise, per Eq. 2 from Ciardi et al. (2013).
    Invoked in Section 3.1 to convert the measured SNR of each TOI into the SNR of hypothetical planets; assumes unchanged photometric noise and transit chord.
  • domain assumption A transit is detectable if its scaled SNR meets the pipeline threshold, 7.1 for SPOC or 9 for QLP.
    Used in Section 3.3 to convert SNR grids into detection fractions; these thresholds are fixed external pipeline values.
  • domain assumption For binaries, the dilution-corrected planetary radius is X_r = (R_t/R_1) sqrt(F_tot/F_t), with secondary radii estimated from the Pecaut-Mamajek sequence via the speckle magnitude difference.
    The radius correction and the secondary radius estimate are not independently measured for each system; they rest on main-sequence stellar relations and on the assumption that the companion's flux is the only diluting source.
  • domain assumption The single-star classification based on speckle vetting and absence from the ExoFOP companion list is complete enough for the comparison.
    Any unresolved companion below the speckle detection limits would dilute single TOIs as well and could bias the single-star sensitivity grid downward.
  • domain assumption The TOI sample, selected because a transit was already detected and because speckle observations exist, is representative for sensitivity estimation.
    This underlies the entire conversion from detected systems to detection fractions; the authors acknowledge it is not equivalent to occurrence rates in Section 4.1 but do not correct for target selection.

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Pith. "Pith review of The sensitivity of TESS to transiting planets in TOIs with close-in stellar companions." pith.science (2026). https://pith.science/paper/SLJOCU5C

@misc{pith2026260813527,
  author       = {Pith},
  title        = {Pith review of: The sensitivity of TESS to transiting planets in TOIs with close-in stellar companions},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/SLJOCU5C}},
  note         = {Machine review of arXiv:2608.13527}
}
abstract

High resolution imaging with optical speckle interferometry has revealed that many transiting exoplanet host stars possess close-in stellar companions. The objective of this study is to quantify how the presence of these companions impacts the ability of TESS to detect the transits of small planets. We accomplish this by examining 2052 TESS Objects of Interest (TOIs) that appear to be single-star systems based on speckle interferometric observations as well as 188 TOIs in unresolved ($< 1.2\arcsec$) stellar binaries. For each planet, we take its transit signal-to-noise ratio (SNR), radius, and orbital period from the TOI catalog and, for planets in stellar binaries, we correct the radius for dilution by the companion. By applying a scaling relation to the measured transit SNR of each TOI in our sample, we determine the detectability of transits in each TOI as a function of both planet radius and orbital period. When applied to the full sample, this procedure elucidates the sensitivity of TESS to transiting planets as a function of binarity, host-star spectral type, planet radius, and planet orbital period. These sensitivity grids quantify the bias against the detection of small planets in unresolved binaries by TESS and show that there is a particularly low sensitivity to planets transiting secondary stars in unresolved binaries, especially as the magnitude difference between the stars increases. These sensitivity grids are available for download to facilitate their use in other studies.

Figures

Figures reproduced from arXiv: 2608.13527 by the authors.

Figure 1
Figure 1. Effective temperature-magnitude diagram of the single-star and binary TOI hosts in our sample. study by Lester et al. (2021) found that small (R≲ 2R⊕) planets were preferentially detected around single-star TOIs, and the authors attributed this to a bias created by the dilution of the transit signal. In this study, we use speckle observations of host stars of TOIs, in conjunction with the TESS transit signal￾to-nois… view at source ↗
Figure 2
Figure 2. Separations and magnitude differences of the bi￾naries in our sample, subdivided by the detection pipeline. There are no near-equal brightness binaries at wider sepa￾rations, even though they are abundant below 0.4”. The shaded region corresponds to the detection bias discussed by Littlefield et al. (2024). The properties of binary host stars do not correlate with the detection pipeline. Identifying and characterizi… view at source ↗
Figure 3
Figure 3. Representative sensitivity grid. The true period and radius of the planet are marked with an ‘x’ and result in SNR= 349, while the colormap and contours indicate the scaled SNR of hypothetical planets of various periods and radii. TESS Input Catalog which, in turn, can lead to their exclusion from samples. However, as shown in [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figures from the paper (5 more)
Figure 5
Figure 5. Figure 5: Sensitivity curves for an illustrative binary TOI in our sample in which it is currently unknown which of the binary stars is the host of the transiting exoplanet TOI 3353.01. In both panels, the ‘x’ indicates the orbital period and corrected radius of the planet. An E…
Figure 6
Figure 6. Figure 6: Left: Sensitivity of TESS to transits in the full sample of single-star TOIs. Center: The sensitivity for all binary stars in our sample, with all planets presumed to transit the brighter star. Right: As with the top-center panel, except that all planets are assumed to…
Figure 7
Figure 7. Figure 7: Sensitivity grids by spectral type for single TOIs in our sample. These data are available in machine-readable-table format to facilitate their use in other studies. Although the grids are computed up to R = 20 R⊕, we display the grids only to a maximum of 10 R⊕ in ord…
Figure 8
Figure 8. Figure 8: Left column: 2D histograms showing the distribution of observed transiting exoplanets around single TOI hosts of a particular spectral type. fobs is the relative frequency of each bin within the parameter space. Middle column: Sensitivity grid for that spectral type. T…
Figure 9
Figure 9. Figure 9: Effect of ∆mag on TESS sensitivity in binary TOIs. All spectral types are included in each ∆mag bin. The top row shows the sensitivity assuming that the planet orbits the primary while the bottom row assumes that the secondary is the host. The ∆mag range above each pan…

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